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| Understanding Our Planet Through Chemistry
Understanding Our Planet Through Chemistry
Understanding Our Planet Through
This U.S.Geological Survey site shows how chemists and
geologists use analytical chemistry to: determine the age of the Earth; show
that an extraterrestrial body collided with the Earth; predict volcanic
eruptions; observe atmospheric change over millions of years; and document
damage by acid rain and pollution of the Earth`s surface.
II. Understanding the Earth
IIa. History recorded in chemistry. How old is the
III. Mapping the chemistry of the Earth`s surface
IIIa. Assessment of public lands
IV. Can we depend on chemical analyses?
IVa. Measuring quality
This document describes the role of chemistry in issues
vital to our economy, health, and well-being. When we are analyzing a sample of
the Earth, we never ask if a specific element is present. Virtually every sample
of the Earth contains every natural element at some amount. The more appropriate
questions are: How much of it is present? Is there enough to be mined
profitably? In the environment, is it dangerous at this level or in this form?
And after we`ve identified the issues that we need to solve about our planet, we
then need to ask, What clues can we find that will give us the answer?
We will show you how many geologic problems are solved
using routine analyses of the major components of rocks. We will also show you
the complexity of analyzing trace amounts of common components in extremely
small samples, such as rare samples of air from more than 100 million years ago,
tiny samples of ore-forming fluids that were entombed in minerals 300 million
years ago, or small amounts of naturally-occurring radioactive isotopes that are
as old as the Earth. Because some elements in our environment are hazardous at
trace levels, they must be analyzed down to those low levels. The impact of
quality control on analyses will also be discussed, as well as the production of
standard reference materials that are distributed internationally to Federal and
As the primary Federal Earth-Science Agency, the USGS
studies and provides solutions to questions concerning our planet, assesses the
mineral resources of Federal lands, and serves as a repository for geochemical
data generated by numerous Federal programs. These data are being applied to new
economic and environmental concerns and provide a cost effective method to solve
geochemical problems, often with no impact on wilderness or fragile refuges.
Questions about geology -the science of the Earth- can
be difficult to answer because many times we can`t safely get close enough to
the event. Even if we can, our senses are not sharp enough to detect everything
that is happening. The Earth is relentless in its course of change, but the
transformation occurs over a vast amount of time. Some geologic processes can
take a million years or more to complete. We know that today`s events have also
occurred repeatedly throughout geologic time. To understand our planet Earth, we
need to read and interpret the permanent records in the Earth`s crust and
interior. These records are the key to the future, and many of these clues are
preserved in the chemistry of geologic samples.
Everything we touch in our daily lives is made up of
elements. There are 92 elements that occur naturally, and in most cases, the
human senses cannot recognize these elements when they are present in a
compound. If, for example, we could always recognize what something is made of,
there would be no such thing as "fool`s gold" (a natural combination or iron and
sulfur called pyrite). Because we have difficulty identifying these relatively
pure compounds, it`s not surprising that when rock or soil contains only a very
small amount of an element we are incapable of recognizing the element`s
Using only our vision, pyrite is easily confused with
gold, so much so that the common name for pyrite is "fools gold."
Using analytical chemistry, we can even determine trace
elements (elements present at very low levels) at the parts per million (ppm) or
parts per billion (ppb) level. It`s difficult to comprehend the concentration of
a substance at this low a level. To get a mental picture, imagine an average
3-bedroom home. It would take about 1 million marbles to cover the floors of the
home. One part per million would be represented by just one marble among all the
other marbles. For that same marble to represent one part per billion, however,
it would take 20 football fields covered with marbles.
Different elements have different physical properties.
These properties determine what methods can be used to analyze each element (or
group of elements). The methods described in the WWW document can be applied to
many different geological problems, but no one method can solve every problem.
The analytical methods described here are only a few that were selected to show
the role of chemistry in geology.
II. Understanding the Earth
IIa. History recorded in chemistry
How old is the Earth?
The question of when the Earth was formed and when
various events on it occurred has long fascinated humanity. In the past, various
estimates of the age of the Earth have been made using the available technology.
All estimates of this type changed drastically with the modern application of
radioactivity.Return to this point in index.
Elements, isotopes, and radioactivity
Matter is made up of atoms, and atoms are made up of a
complex array of subatomic particles. Let`s consider only three of these
particles: protons (positively charged), neutrons (no charge), and electrons
(negatively charged). Every element has a fixed number of protons that cannot be
changed without creating a different element. If, for example, we add a proton
to an atom of sulfur, it becomes heavier and is now an atom of chlorine. If we
change the number of neutrons in an atom, however, it has almost no effect on
the chemical properties and outward appearance but does have an effect on the
atomic mass. It can also have an extreme effect on the atomic stability of the
element. If we take an atom of potassium that has 39 neutrons in it and add one
more, the atom now becomes unstable and can radioactively decay.
Each combination of an element with a different number
of neutrons is called an isotope. Isotopes that are radioactive disintegrate or
decay in a predictable way and at a specific rate to make other isotopes. The
radioactive isotope is called the parent, and the isotope formed by the decay is
called the daughter. A radioactive isotope decays at a constant rate
proportional to the number of radioactive atoms remaining. A simple way of
describing the speed of decay is to see the time it takes for half of the atoms
of a radioactive parent to decay and form the daughter element(s). This is
called the half life. Various events (especially melting of the rock) will cause
the isotopes in a rock to redistribute. When the rock solidifies it can be
thought of as starting a stopwatch. By determining the amount of the parent and
daughter isotopes present scientists can determine when the stopwatch started.
Naturally occuring radioactive isotopes (called the
parent isotope) disintegrate at specific rates to make other isotopes (called
daughter isotopes). The amount of time it takes for half the quantity of the
original isotope to decay is constant, no matter how much as present at the
beginning. Based on this principle, the age of geologic events can be measured.
As an example, the parent-daughter system used to
determine the age of the Earth is the uranium-lead system. The decay of the
parent uranium isotopes to daughter lead isotopes in samples of the Earth, Moon,
and meteorites indicates that all the planets in our solar system formed 4.5
billion years ago.
While determining the age of the Earth is intriguing,
radiometric dating has recently been useful in more practical issues like the
following: With what age of granite formation are ore deposits in a particular
region associated? How recently has a fault been active, and is it likely to be
safe to build near it now? How often does a volcano erupt and how often do
On May 18, 1980 the Cascade volcano, Mt. St. Helens,
erupted exposively causing a great deal of destruction and a number of deaths.
In addition to telling us the Earth is 4.5 billion years
old, geologic dating can answer important questions such as: When was the last
time a fault moved? What areas are a safe place to construct a nuclear reactor?
How frequently does a particular volcano erupt? Is a volcano nearing an eruptive
part of its cycle?
Because different isotopes of an element have different
masses, they can be viewed as an arrangement of masses in a spectrum. An
instrument that separates and electronically measures a spectra of atomic masses
is called a mass spectrometer. There are many types of mass spectrometers, but
the most frequently used in earth-science age determinations are magnetic sector
mass spectrometers. These magnetic spectrometers operate on the principle that
if you put an electric charge on an object and throw it into a magnetic field,
the object s path will form a circle. The radius of the circle will depend on
the strength of the magnetic field and the mass of the charged atom divided by
its electric charge. Thus, if you have a purified portion of an element from a
sample with several isotopes, each can be made, in sequence, to travel the same
circular path to the detector by varying the strength of the magnetic field.
Magnetic sector mass spectrometers consist of at least three components as
illustrated in this figure. (1) A source of sample ions, (2) a magnetic field,
and (3) a detector.
The atoms on the filament are ionized and accelerated at
a specific velocity through a magnetic field, causing them to take a specific
curved path depending on the ion`s mass. This type of mass spectrometer scheme
most commonly used in geologic dating shows how ions with a specific mass are
directed into the collector for counting, while others, like a race car taking
the curves at the wrong speed, are lost.
A difficult chemical procedure is used to concentrate
the element of interest so that isotopes can be measured on a mass spectrometer.
In many cases the recovered amount is no larger than a spot on the sample
filament and could pass through the eye of a needle. Return to this point in
But how do you take a rock and purify a portion of it
for mass spectrometry, and how do you analyze a rock sample on an instrument
that only analyzes liquids? In most cases, before a rock s chemical composition
can be determined, it must pass through both a physical and a chemical
preparation to free the element(s) of interest from the rock and present them in
a dissolved or liquid form.
Initially, fist-size pieces of rock are broken down to
pea-size fragments using a crusher with steel jaws. A pulverizer grinds this
coarse material into a powder as fine as flour.
Next, the powder is further broken down, or decomposed,
by using either an acid treatment or fusion. During this chemical decomposition,
the weighed sample of powdered rock releases its elements into solution.
Because most rocks are composed of a combination of many
types of minerals, each having different chemical and physical properties,
digestion is accomplished by using a combination of acids. Most commonly used is
a mixture of hydrofluoric, nitric, hydrochloric, and perchloric acids, which
will decompose all but the most resistant minerals. The acids are heated with
the sample powder in Teflon containers, on a hot plate, or in a specially
designed microwave oven.
In the fusion technique, a powdered inorganic reagent
(known as a flux) is mixed with the rock powder and heated above the melting
point of the flux; the molten flux then attacks the sample and decomposes it
into a uniform melt. The melt may then be poured into a mold and cooled for
methods that require a uniform solid such as X-ray fluorescence spectrometry
(scroll down to picture of arm pouring red hot samples for a discussion of XRF)
or dissolved in a diluted acid to create a liquid solution. The higher
temperatures (500 to 1,200 C) and caustic nature of the molten chemicals used
for fusions increases the efficiency of the decomposition as compared to acid
techniques and renders most minerals soluble. Each form of sample decomposition,
acid or flux, has its advantages and disadvantages that must be considered. In
addition, the importance of safety and simplicity must not be ignored.Return to
this point in index.
Disaster from space
One of the mysteries of the history of the earth is the
layer of clay that was deposited around the entire globe 65 million years ago.
The layer marks the K-T boundary the end of the Cretaceous and beginning of the
Tertiary periods. It is best known as the time when not only the dinosaurs but
nearly half of all life forms became extinct.
Chemical evidence in this layer of clay preserved from
65 million years ago in Caravaca, Spain, indicates an asteroid or comet struck
the Earth at up to 170 times the speed of sound, possibly causing a disaster
resulting in the extinction of half of all life forms, including the dinosaurs.
At the beginning of the last decade, Nobel Laureate Luis
Alvarez and his team members discovered a 9 ppb abundance of the element iridium
while using neutron activation analysis to study 1-cm-thick samples at the K-T
boundary layer. The fact that the high level of iridium coincided exactly with
the classic end of the Cretaceous mass extinction event led them to propose a
theory linking these two observations. They theorized that an asteroid between 6
and 14 km in diameter struck the Earth, and the impact lofted enormous amounts
of pulverized target material high into the Earth s atmosphere. They speculated
that this dust- size, impact ejecta caused an environmental catastrophe.
Under a microscope, these quartz grains show lines that
are characteristic of high shock and are found only with meteorite impacts or
atomic explosions. This 1/3 millimeter grain is from the K-T boundary clay at
Teapot Dome, Wyoming.
Additional research by other scientists suggests that if
the extraterrestrial object was an asteroid, it most likely impacted the Earth
at a velocity of 50 times the speed of sound and measured 15 km in diameter.
Because asteroids of this size are very few in number in our solar system, the
object could also have been a comet, most likely moving even faster, possibly
170 times the speed of sound but measuring only 10 km in diameter.
To test the impact theories, we have applied a new
analytical technique called laser ablation, inductively coupled plasma,
quadrupole mass spectrometry (LA-ICP-QMS). To allow efficient, rapid, spatial
sampling, a laser is used. The technique is highly sensitive for almost all
As depicted below, the energy of the laser is focused
onto a spot about 80 micrometers in diameter (slightly more than the diameter of
a human hair) to vaporize and sputter material from small zones of the sample.
The operating conditions of the laser range from 1 million to 1 trillion watts
per square centimeter. This incredibly high energy density is created when the
energy is packed into small bursts of 160 microseconds, which are then focused
with a lens onto a very small spot.
A laser ablation, induction coupled plasma, quadrupole
mass spectrometer vaporizes a small spot on the sample. The vapor is then
ionized in the plasma. The four charged rods (the quadrupole) then cause only
the appropriate ions to arrive at the detector for counting; all others are
lost. By changing the electric charge on the rods, different elements can be
The vapor from the sample is then carried by a stream of
argon gas into a 7,000 C argon plasma, where the vapor is ionized. These ions
are then drawn into a quadrupole mass spectrometer (QMS). The QMS consists of
two sets of electrically charged, machined rods. A radio-frequency signal is
applied to both sets of rods. Under specific operating conditions, one unique,
mass-to-charge ratio of ions will be directed down the opening between the four
rods and exit to the detector. All other ions will be lost.
Based on these 250- micron-wide, black laser trails
across the brown layer of clay from the K-T boundary in Caravaca, Spain, the
quadrupole mass spectrometer found abnormally high abundances of platinum-group
elements (up to 1,000 ppb), most likely coming from an extraterrestrial source.
The LA-ICP-MS is sensitive for all the platinum group
elements (PGEs) that would appear from an asteroid impact. The laser, which has
fine sampling resolution, was used to sample the 1-cm layer analyzed by Alvarez
and coworkers but in bands only 0.25-mm thick. In this way, we were able to
sample just the layer of PGE-enriched material and found the concentration in
this zone to be nearly 1 ppm, a factor of 100 times higher than that previously
reported. This greater concentration of the PGEs gives additional support to the
theory that an extraterrestrial body collided with the Earth 65 million years
ago. Return to this point in index.
IIb. Geologic processes
Volcanoes erupt when molten rock (magma) deep in the
Earth s interior makes its way to the surface. On average, for every cubic
kilometer of magma erupted from a volcano, 3 to 10 cubic kilometers are stored
beneath the surface in shallow reservoirs called magma chambers.
We can see what these magma chambers look like by
studying ancient reservoirs that have solidified and been exposed by erosion.
One of these is Half Dome in Yosemite National Park.
Half Dome, in Yosemite Park, is the remains of a magma
chamber that cooled slowly and crystallized beneath the Earth`s surface. The
solidified magma chamber was then exposed and cut in half by erosion. Similar,
still molten magma chambers are thought to underlie many active volcanoes.
The degree of violence of an eruption depends
principally on the chemical composition of the magma. Of major importance is the
interplay between the proportion of silicon dioxide (SiO2 or silica ), which
controls the viscosity of the magma, and volatile components, such as water,
carbon dioxide, and sulfur dioxide. Magmas that are poor in silica usually
release their gases non- explosively and produce slow-moving lava flows, like
those commonly seen in Hawaii. Although such eruptions can be destructive,
humans can usually avoid the lava flow and are rarely threatened by such
Low silica magma, typical of Hawaiiain volcanoes,
produces lava flows that move slowly and can rarely overtake a human who wants
Because buildings and structures can not easily be moved
out of harms way, even slow moving lava flows can cause significant property
Under certain conditions, however, the magma and
surrounding rocks are blown apart by the release of volatiles, resulting in a
dangerous explosive eruption, as happened on May 18, 1980 at Mount St. Helens,
near Portland, Oregon. With only about 0.5 cubic km of erupted magma, however,
this was by no means considered a large volcanic eruption. The 1991 eruption of
the Pinatubo Volcano, near Manila in the Philippines, was approximately 14 times
larger, involving about 7 cubic km of magma. But even the Pinatubo eruption is
relatively small compared to infrequent giant eruptions of volatile- and
silica-rich magma that have occurred throughout the history of the Earth.
In the early 1900`s a chemist could analyze about 200
samples per year for the major rock-forming elements. Today, using X-ray
fluorescence spectrometry, two chemists can perform the same type of analyses on
7,000 samples per year.
Major-element chemical analysis is a front-line tool in
the study of volcanoes and volcanic hazards. The analysis of a volcanic rock
provides a fundamental common ground for comparing the styles and violence of
previous eruptions of similar composition. During the first half of the 20th
century, these analyses were performed exclusively by classical wet chemical
analyses chemically separating each element of interest from the other elements
in the sample. This procedure was extremely laborious. A good analytical chemist
could analyze only a couple of hundred rocks per year for their complete major
element chemistry. U.S. Geological Survey scientists now use technology called
X-ray Fluorescence Spectrometry (XRF) to perform the same type of analyses.
XRF Spectrometry starts at the atomic level. Atoms
consist of protons and neutrons in a central nucleus with electrons in different
orbitals around that nucleus. If an electron from an inner orbital is knocked
out, the vacancy created is filled by an electron previously residing in a
higher orbit. The excess energy resulting from this transition is dissipated as
an X-ray photon with a characteristic wavelength. In X-ray fluorescence
analyses, the electron vacancies are created by bombarding the sample with a
source of X-rays or gamma rays most frequently from an X-ray tube or a
radioactive isotope. By detecting the characteristic X-rays that are fluoresced,
the element of interest is shown to be present in the sample. The more abundant
the X-rays are, the more of that element is present in the sample.
Bombarding the sample with X-radiation does not require
a liquid sample. In fact, because solid samples are more stable than liquids,
virtually all samples presented to X-ray spectrometers are solids. Furthermore,
there is almost no permanent change that takes place in a solid sample analyzed
by XRF, allowing it to be saved and reanalyzed. This is especially important for
the repeated analysis of the same calibration standards over periods of years,
permitting the use of the same analysis protocol. Homogeneity requirements are
frequently solved by dissolving a portion of the pulverized sample in molten
flux that is then poured into a mold and cooled to form a solid glass disc with
a precise, flat, analytical surface.
To analyze samples by X-ray fluorescence spectrometry,
samples are fused at 1120xC with a flux; the chemist then pours the molten
mixture into special molds to produce solid glass discs with a precise
A team of two analysts, using this method, can analyze
over 7,000 samples a year. Because so many more analyses are now available,
geologists can answer more difficult types of questions such as what changes are
happening in the magma chamber during an eruptive cycle.
At a number of frequently active volcanoes, such as
Mount St. Helens (which has erupted about every 100 years), a thick and complex
sequence of volcanic rocks has been deposited. Geochemists and geologists can
reconstruct the eruptive history of the volcano through field studies and
analyses of these rocks. They conclude that the eruptive activity at Mount St.
Helens is separated by longer periods of repose. Like many other volcanoes,
there are systematic changes in major- and trace-element composition through
time. The 1980 eruption appears to be at the end of a chemical cycle that began
about 500 years ago.
With this information we can predict the style,
frequency, and warning signs of future eruptions. Newly erupted lava, pumice, or
ash may then be evaluated in a historical context. In some instances, XRF
analyses can be rapidly completed in less than 24 hours by express delivery of
the samples to the lab and electronic transmission of data back to the volcano
being examined. This is something that would have been impossible for the
While systematic changes in overall chemistry contribute
a great deal of information about a volcano, there is still a desire to
understand more about what happens deep within the Earth s crust how the magma
forms and what triggers the volcano into eruption. Return to this point in
Application of instrumental neutron activation analysis
Some of our understanding of the source of molten magma
has been obtained by analyzing rocks for a group of 15 elements called the
rare-earth elements (REE). In a type of rock called basalt, the total amount of
all the REE s is often less than 100 parts per million (ppm).
One well proven analytical technique used to determine
the concentrations of REE in rocks and minerals is instrumental neutron
activation analysis (INAA). In this technique, a rock or a single mineral that
the rock contains is irradiated using a nuclear reactor. This causes the
elements to become radioactive and to emit gamma rays with distinct energies.
The sample is then placed on a detector that measures how many gamma-rays of
these energies are emitted. The number of distinct gamma rays emitted is
proportional to the abundance of that particular element.
To get better sensitivity necessary to measure
rare-earth elements in specific rocks, samples can be irradiated in a low-power
reactor. It turns some of the element into an unstable isotope whose decay can
then be detected and counted to determine the quantity of the element in the
To understand what the REE can tell us about how magmas
are formed, scientists have developed mathematical formulas. These formulas
suggest that when certain minerals interact with molten rock, there can be
appreciable effects on the rock s REE contents. In a process called partial
melting, for example, if a source rock contains minerals (such as garnet) that
can hold high concentrations of certain REE, then these elements tend to be
prevented from entering the molten rock. Because Hawaiian basalts have low
concentrations of the heavier REE, and garnet has high concentrations of heavy
REE, some Earth scientists conclude that the magmas have formed by partial
melting of a source rock that contains garnet, and the garnet held back the
heavy REE. Return to this point in index.
The smallest clues
To understand more about the causes of eruptions,
geologists have to look more closely into the fine details of the solidified
magma samples to find a record of the conditions before and during eruption.
Mineral crystals within magmas vary in composition depending on the surrounding
magma and the temperature at which they are formed.
Why do some volcanoes explode catatrophically with
rapid, life-threatening devastation? Recent research indicates that magma does
not necessarily move directly from its source to an eruption. A magma chamber
may contain stable reservoirs or layers of one composition with a lower
temperature. Subsequent influx and mixing of a second higher temperature lava
overheats the mixture, triggering an explosion. The 1991 Pinatubo eruption
appears to have been triggered because a hot, low-silica basalt magma penetrated
a stable resevoir of cooler, high-silica type, forming an explosive mixture. The
explosion forced the closing of Clark Naval Air Station and interrupted numerous
air flights because of ash clouds that damaged engines.
Mineral compositions from the 1991 eruption of Mt.
Pinatubo indicate that low-silica magma at a temperature of about 1,250 C mixed
with high-silica magma (780 C) just before the eruption. Based on this
information, volcanic rocks produced in previous eruptions were analyzed. The
results suggest that the 1991 eruption is the latest in a series of eruptions
that were triggered by the mixing of magmas. Magma mixing has also triggered
eruptions at a number of other volcanoes.
Shortly after World War II, physicists in the United
States, England, Germany, and Japan began to perfect a new analytical instrument
called the electron microscope. Instead of producing a visually magnified image,
this new instrument accelerated and focused electrons through a column of
magnetic lenses onto a small spot on the sample. The ability to magnify objects
is limited by the energy or wavelength of the radiation that is used to observe
the object. Because the accelerated electrons from the column have a much
shorter wavelength than light, it is possible to produce images at much higher
magnifications than can be obtained using an optical microscope. Today, the most
powerful electron microscopes can produce images at magnifications as high as 1
When electrons are accelerated into an object, they
interact with the atoms in that object and produce three important types of
radiation: (1) X-rays (you may scroll back to the picture of the Early 1900`s
Laboratory where a description of how X-rays are formed was presented for the
related technique of X-ray Fluorescence), (2) the secondary electrons that are
used to see the sample, and (3) back-scattered electrons, which are bounced back
as a function of the mass of the sample.
In the 1950 s, the French physicists, Castaing and
Guinier, developed an instrument based on the characteristic X-rays produced by
the electron bombardment of the sample. This instrument can measure the number
of X-rays emitted from the small spot irradiated on the sample. By counting the
X-rays produced, Castaing determined the chemical composition of a portion of a
sample no larger than the size of a human blood cell. This new instrument was
called the electron microprobe (EMP).
During the same period of time, another instrument was
brought into production the Scanning Electron Microscope (SEM). Like the
electron microscope, it uses the secondary electrons created from the sample s
surface to record an enlarged image of the object. Its principal advantage is
that it deflects the electron beam and scans it back and forth over the sample
surface (called rastering) in a pattern similar to that in which wallpaper
covers a wall.
In order to see objects smaller than what normal light
allows, scientists have developed an instrument that accelerates electrons. The
Scanning Electron Microscope uses electromagnetic lenses to focus the electrons,
since glass lenses cannot.
The secondary electrons are continuously detected, and
the signal is directed to a television monitor where the image is displayed.
Zooming in or backing out by changing the size of the raster area (hence
changing the magnification), the scientist can use the enlarged image to aim the
scanning electron microscope. At the same time, X-rays characteristic of the
composition are generated. These X- rays can be detected by an X-ray analyzer
and used to create a map of the element`s abundance.
In this example, calcium X-rays produced from a
pinhead-size sample from the 1991 eruption of Mt. Pinatubo are mapped and color
coded by a scanning electron microscope to show the range of calcium content
from high (white) to low (green).
Analyzing a single particle of smoke
Because of their similarities, EMPs and SEMs overlap in
their capabilities. The modern EMP has become a true hybrid that combines the
viewing capability of the SEM with the analytical power of the electron
microprobe. Both EMPs and SEMs are capable of obtaining images at magnifications
over 100,000 times. These instruments can see and then analyze something that
wouldn`t show up with a light microscope, such as the following single particle
of volcano smoke in this picture.
After seeing the invisible, the next question is "wonder
what that`s made of?" "Is it bad for our health?" Small samples like this
particle of volcanic smoke, the size of a single human red blood cell, can be
analyzed by a scanning electron microscope in 4 minutes with errors of less than
Analytical chemistry in the search for ore deposits
Analytical chemistry plays a key role in our continuing
quest to understand how ore deposits form and in the practical exploration for
ore deposits. If you pick up an ordinary rock that builds the crust of the Earth
and determine its chemical composition, for every billion atoms, 1 to 10,000
atoms will be metallic elements such as gold, silver, platinum, mercury, copper,
cobalt, nickel, chromium, lead, zinc, molybdenum, tin, and tungsten. Natural
processes in the Earth s crust have the remarkable ability to concentrate and
purify certain rare metallic elements to form unusual deposits of minerals that
contain 1,000 to 10,000 times the amounts found in ordinary rocks.
With today s modern mining and extraction technology, it
has become possible to mine very low-grade deposits. For example, gold can be
economically recovered from rocks that contain less than one tenth of an ounce
of gold per ton of rock. But gold continues to be expensive because of the cost
in locating the deposit, mining the rock, and extracting the small amount of
gold in each ton of rock. All of the inorganic raw materials used to manufacture
the products of today s technological society have to be either mined or
Almost every process that takes place in the Earth s
crust, whether from the action of molten rock, heat and pressure at depth, hot
springs or steam, running water, weather, or biological activity can contribute
to the formation of an ore deposit. Geologists use the principles of chemistry
to try to understand how these processes scavenge elements from ordinary rock,
transport them, and concentrate them to form an ore deposit. Geologists have
developed models that describe the physical characteristics and chemical
composition of each ore deposit type and how they relate to the geologic
environment in which they form similar to the way biologists describe how an
organism fits into a particular environmental niche.
In North America and many other parts of the world,
almost all of the rich ore deposits exposed at the surface have already been
discovered. Most of the ore yet to be found is not visible to the human eye.
Therefore, geologists have had to improve their understanding and develop more
sophisticated ways to detect where ore deposits can occur.
Two main approaches are used to detect deposits hidden
below the surface. One uses the ore-deposit model, and the other is based on the
detection of a dispersion halo that extends for some distance from the deposit
(for more discussion of dispersion halos, scroll down to the section on "Mapping
the Chemistry of the Earth`s Surface") .
The following analogy shows how geologists use
ore-deposit models. If all but the tip of the tail of an elephant was buried by
a landslide, a biologist could recognize from the skin, hair, and shape of the
appendage that the tail belonged to a mammal. With advanced testing of tissue
samples, a biologist could prove that the tail belongs to an elephant and could
easily predict that the body should be buried about 1 meter below the tip of the
Most ore-deposit models are not as advanced as
biologists models for elephants, but a few are nearly so. Several copper and
molybdenum porphyry deposits, located as deep as 2,000 to 4,000 feet below the
surface, have been discovered based on small surface exposures measuring several
feet across. These exposures were of breccia pipes (vertical pipe-shaped bodies
of pulverized rock), which are known to extend thousands of feet above the main
body of porphyry deposits. Because not all porphyries contain deposits of
economic metals, geologists can collect and analyze field samples to determine
what metals the porphry will contain, and if it is worth drilling.
Schematic cross section of a copper-molybdenum porphyry
model. Explosive release of steam and gases during the cooling of the intrusion
result in the formation of pipes filled with broken rock fragments that extend
for thousands of feet towards the surface and often contain fragments of the ore
body present at depth.
Analysis of fossil fluids and gases from tiny time
A great many ore-deposit models are tied to the cause of
formation of the deposit. Questions about the environmental conditions related
to formation of the deposit are temperature, pressure, source of the metals, and
composition of any fluids and gases that transported and formed the ore or
Many crystals in the Earth s crust have formed in some
kind of fluid. Small quantities of the fluid that surrounded the crystals during
growth are commonly trapped as tiny fluid inclusions within these crystals. In
many cases, these fluid inclusions are less than 0.1 mm but record important
information about the conditions when the ore was being formed.
Trapped in a time capsule the same size as the diameter
of a human hair, the ore-forming liquid in this inclusion was so hot and
contained so much dissolved solids that when it cooled, crystals of halite,
sylvite, gypsum, and hematite formed. As the samples cooled, the fluid shrank
more than the surrounding mineral, and created a vapor bubble. Heating the
inclusion to the temperature at which the bubble is reabsorbed and daughter
crystals dissolve gives an estimate of the minimum temperature at the moment of
Current understanding of movements within continents
reveals that throughout the Earth s history periods of large-scale fluid
movements occurred in the Earth s crust. Some of these fluid migrations resulted
in the deposition of metallic ore deposits and accumulations of oil and gas.
Characteristics of fluid inclusions are extremely
variable. In the simplest case, when fluid inclusions cool from the elevated
temperature at which they formed, the liquid shrinks and separates into a liquid
and a vapor bubble. Detailed microthermometric studies give a reasonable
estimate of the temperature at which the mineral was formed. Studies of this
type reveal that the inclusions were trapped at temperatures from less than 50 C
to over 600 C and at pressures equivalent to what is experienced at the Earth s
surface and ranging to what would be found several kilometers deep.
Because of the extremely small size of so many fluid
inclusions, determining the composition of the trapped fluids is difficult.
First, the total amount of dissolved solids is determined by observing with a
microscope the freezing/melting points of the inclusions. The sample is then
crushed and rinsed with water. This water is recovered and analyzed by using a
sensitive analytical technique to determine the ratios of the elements
contributed by the trapped fluid. These ratios are used to calculate the
composition of the fluid. The compositions range from aqueous solutions with
salt content similar to rainwater to fluids with dissolved solid concentrations
of over 60 percent nearly 20 times the amount found in seawater.
Analytical data on fluid inclusions are needed to
understand the chemical and physical processes involved in the formation of
economic mineral deposits. These data are also critical in understanding modern
mineral-deposit models, which promote cost-effective mineral exploration vital
to our healthy industrial economy.
Most fluid inclusions contain dissolved gases, and in
some environments the inclusions consist entirely of gases. Recently, the USGS
has designed a gas quadrupole mass spectrometer (QMS) that will analyze the
amounts and chemical identity of gas ions in small gas samples (for more details
on QMS instruments, scroll back to the QMS illustration in the "Disaster from
Space" section). This instrument is extremely sensitive (8 parts per billion
detection) and capable of millisecond speeds of analysis important for gas
bubbles as small as 1/100 of a millimeter in diameter.
The QMS is used extensively to study ore- deposit models
as well as environmental and geologic hazards. Examples include: identifying
carbon dioxide as the responsible gas at the Lake Nyos, Cameroon disaster where
2,000 people suffocated in 1986; tracking atmospheric gases from bubbles in
climate- study ice cores of Greenland and Antarctica; tracing dispersal of
smokestack emissions and gases of geothermal energy wells and springs.
Scientists sample air trapped in the snowpack at the
Greenland Ice Sheet Project 2 site in Central Greenland. These samples will be
analyzed by mass spectrometry to determine the composition of ancient air. These
studies help us to predict climate changes.
Global change in the geologic past
An exciting new application of the QMS instrument uses a
high-energy laser fired through a modified microscope to open individual gas
inclusions in ice. Ice from Greenland and Antarctica contain atmospheric gases
that were captured in snow as it formed. The gases were retained as the snow
turned into ice and formed bubbles. Analysis of these bubbles provides detailed
information on the past composition of the atmosphere.
Sea-level changes, changes in solar activity, and,
according to some astrophysicists, even the signals from distant supernovas, are
also recorded in the ice. Compiling and studying this record helps us to
evaluate current changes in the atmosphere and to predict future trends.
Ice-core studies provide valuable information about the levels of human
pollution, past climate patterns, sources of moisture, the altitude of the ice
when it formed, frequency and magnitude of natural events, and biological
activity at the ocean surface. Return to this point in index.
Air bubbles, amber, and dinosaurs
Ages of ice samples found on the Earth cover a span
approaching 200,000 years. But how can we tell what the Earth s atmosphere was
like before that? Recently, USGS scientists have used a gas QMS to determine the
oxygen level of ancient samples of Earth s atmosphere from a most unlikely place
amber. The fossilized resin of conifer trees, amber is interesting to scientists
as a medium that traps insects, small animals, and plants, preserving them
through geologic time for future study.
Amber --the fossilized resin of conifer trees--provides
a unique means of protecting intricate samples of the past. This mosquito, lying
trapped for 45 million years in a piece of amber, is almost perfectly preserved.
The recent extraction by scientists, of ancient DNA from
organisms entombed in amber much like in the science-fiction novel and movie,
Jurassic Park is an example of why scientists are intensely interested in amber.
Minute bubbles of ancient air trapped by successive flows of tree resin during
the life of the tree are preserved in the amber. Analyses of the gases in these
bubbles show that the earth s atmosphere, 67 million years ago, contained nearly
35 percent oxygen compared to present levels of 21 percent. Results are based
upon more than 300 analyses by USGS scientists of Cretaceous, Tertiary, and
recent-age amber from 16 world sites. The oldest amber in this study is about
130 million years old.
This 84-million-year-old air bubble lies trapped in
amber (fossilized tree sap). Using a quadrupole mass spectrometer, scientists
can learn what the atmosphere was like when the dinosaurs roamed the earth.
The consequences of an elevated oxygen level during
Cretaceous time are speculative. Did the higher oxygen support the now extinct
dinosaurs? Their demise was gradual in the transition from late Cretaceous to
early Tertiary times, as was the decrease in oxygen content of the atmosphere.
This chart shows a major decrease in oxygen content in
the atmosphere from 35 percent to the present day level of 21 percent. This
decrease occured about the same time that the dinosaurs disappeared--65 million
Recent methane emissions from Gulf Coast marshes
The Earth s atmosphere is still changing. Natural
environmental processes (geological, biological, and geochemical) produce carbon
dioxide (CO2) and methane (CH4). These gases, along with water vapor, are
responsible for trapping heat at the Earth s surface.
Because biological processes are responsible for the
production of methane in environments where organic matter ferments, wetlands
(swamps, bogs, etc.) were previously the principal source of methane. Now,
however, the combination of rice cultivation and cattle raising have taken over
as the principal contributor. Studies of methane sources help us to understand
their relative contributions and the factors that control the methane production
and release to the atmosphere.
The studies show that when coastal wetlands are flooded
by sea-level rise, salt marshes are inundated, up-slope brackish marshes become
saltier, and some fresh marshes near the coast become brackish. Consequently,
total methane emissions decrease because salt marshes do not produce as much
methane as fresh marshes.
Fifteen miles inland from the Gulf of Mexico in a
brackish marsh in Terrebonne Parish, Louisiana, methane emissions are collected
in inverted buckets and measured with a portable gas analyzer. Using these
measurements, scientists can determine one effect of global sea-level rise.
USGS studies of methane in Gulf Coast Louisiana indicate
that brackish marshes emit between one-fourth and one-half the methane of the
fresh marshes they replace during sea-level rise. The results of these local
measurements in Louisiana can be used to project the world-wide effects of
sea-level rise on methane emissions. By the year 2050, projected world-wide,
sea-level rise will replace 50 percent of coastal fresh-water marshes with
brackish water marshes. This will reduce the world s methane emissions by 2
percent. Return to this point in index.
Acid rain steals our heritage
In addition to affecting people, plants, and wildlife,
air pollution also affects rocks and soils. One of the problems it causes is the
degradation of buildings and monuments, especially those built out of limestone
or marble. These rock types, both almost pure calcite (calcium carbonate), are
commonly used throughout the world as a building stone.
These balusters, on the Pan American Union Building,
Washington, D.C., were made from Georgia marble, and were installed in 1910.
They demonstrate the effects of dry deposition of sulfur dioxide, which causes
the formation of gypsum. Gypsum traps particulate matter to form heavy, black
incrustation. In some areas, the gypsum crust has flaked off the balusters
exposing a fresh but very rough surface.
Studies to determine damage caused by air pollution have
pointed to changes in the acidity of the air and rain. In fact, the term acid
rain is now commonly used in the media as well as scientific studies. Acid rain
affects carbonate stone buildings and monuments in two ways. The first is by dry
deposition of sulfur dioxide gas, increasingly contributed to the atmosphere by
the combustion of fossil fuels. The gas reacts with calcium-carbonate building
stone to form calcium sulfate (gypsum). As gypsum forms on the surfaces of the
stone, it traps particulate matter, forming a blackened crust.
The second effect of acid rain is wet deposition.
Natural rain water is a weak carbonic acid solution and all carbonate-stone
surfaces that are washed by rainwater are subject to gradual erosion. This
erosion is accelerated, however, by the increased acidity of rain in the eastern
United States, which is often 10 times greater than in areas where acidic
pollutants are absent.
Current research on acid rain is directed at defining
the degree of stone damage due to both dry and wet deposition. Scientists are
measuring the effects of acid rain on historic stone buildings and monuments
across the country. They are exposing samples of marble and limestone to
weathering at specific field sites and simulating depositional processes under
highly controlled laboratory conditions.
The effects of both dry and wet deposition are evaluated
by the chemical analyses of the stone surfaces before and after exposure and of
rain run-off solutions collected from test slabs.
Recent research by the USGS and other agencies conducted
under the National Acid Precipitation Assessment Program has shown that test
samples of marble erode 15 to 30 micrometers per year, while limestone (which is
less compact than marble) erodes from 25 to 45 micrometers per year. (These
measurements are slightly less than those of the diameter of a human hair).
Approximately 20 percent of this erosion is caused by acid rain. The remaining
80 percent is the result of the natural solubility of the stone in rain water.
Because the effects of acid rain only develop over an extended period of time,
high-precision analytical chemistry plays a central role in measuring these
effects. Return to this point in index.
The chemistry of mine drainage
Mine drainage is water that drains from mines. The water
can be of the same quality as drinking water, or it can be very acidic and laden
with high concentrations of toxic, heavy metals. In general, the more acidic the
water is, the poorer the water quality.
Because the chemistry of water samples can rapidly
change if they are removed from the natural site, many measurements are made in
the field. One of the first of these field measurements is for acidity, which is
read by a meter and reported as the pH of the sample. Water with a pH of 2 has a
high concentration of hydrogen ions and is acidic, whereas water with a pH of 7
is neutral. A study of mine drainage in Colorado, for example, shows that the pH
of mine waters ranges from a low of 1.8 to a high of 8.
A companion field measurement made on mine water is for
specific conductance. This property of water measures the electrical
conductivity associated with a water sample and is useful as a quick estimate of
total dissolved solids. A low number from 10 to about 200
microsiemens/centimeter (the unit of specific conductance measurements) could be
considered to be drinking-water quality. Specific conductance measurement of
mine waters in the Colorado study range from 100 to 38,000
The full characterization of mine water requires a
number of other instrumental and analytical measurements that are carried out
using both mobile and laboratory facilities. Three main, instrumental,
analytical techniques are used to complete the characterization of mine-water
samples. These techniques are: ion chromatography (IC), which is used to
determine the concentration of fluoride, chloride, nitrate, and sulfate in
aqueous samples; ICP-AES, which determines the concentration of major and trace
elements(for additional discussion on ICP-AES and an illustration of the
instrument, scroll down to "10,000 element determinations a day"); and liquid
ICP-QMS , which is used to determine elements below the ppm level (for
additional discussion and an illustration of a laser ablation ICP-QMS
instrument, scroll back to the "Disaster from space" section).
Why is it so important to characterize mine drainage?
Because mine- drainage water almost always flows into a stream where it can
dramatically affect the aquatic organisms and the quality of the water received
by downstream communities. To successfully reduce the effect of the toxic
elements, their abundances must be known.
Mineral-laden water from the Argo drainage tunnel in
Colorado, entering into Clear Creek, illustrates the possible environmental
impact of untreated mine drainage.
From the analytical chemistry of mine drainage,
scientists have concluded that the major cause of high acidity of the water is
the bacterially catalyzed oxidation of the mineral pyrite. This acidity
stimulates the dissolution of many other sulfide minerals, resulting in the high
concentration of metals such as copper and zinc.
While it is difficult or impossible to stop mine
drainage, it might be possible to cut back the rate of the introduction of toxic
elements into the environment. This can be done by hindering the bacteria that
speed up the oxidation of the pyrite or by neutralizing the drainage and
extracting toxic elements. Recent studies have shown that wetlands can
concentrate heavy metals from mine drainage. Constructed wetlands could,
therefore, be used to accumulate the pollution from mine drainage. By analytical
monitoring of the toxic, metal build-up in these wetlands we can avoid any
impact on the wildlife that might try to live there. Return to this point in
IIe. Pollution Prevention
Cleaning up coal burning
While hundreds of abandoned mines across the country are
releasing pollutants, active mines can also produce pollutants. Among the best
examples of air and water pollution control are advances in coal technology. For
years coal has been a major source of both energy and pollution in the United
States. Supplies of natural gas and petroleum are dwindling. Alternative energy
sources are not expected to contribute significantly to the energy needs of the
United States in the near future. Coal will continue to play an important role
for energy production through the first half of the 21st century.
Significant improvements in coal processing and burning
in modern power plants have dramatically reduced pollution. The process has been
improved in three ways. First, sophisticated equipment has significantly reduced
fly ash and soot compared to the equipment used many years ago; other
specialized equipment greatly reduces sulfur-dioxide emissions.
Coal, a major source of energy in the United States,
does not have to cause pollution. This coal-burning power facility at Brilliant,
Ohio, uses a process wherein sulfur-dioxide emissions are cut by 90 percent,
nitrogen oxides by 50 percent, and carbon dioxide by 15 percent. (Photo provided
by American Electric Power Service Corporation).
A second way of reducing coal pollution is by selective
mining of low-ash and low-sulfur coals that pollute less. Detailed chemical
analyses of coal prior to mining is required to determine the concentrations of
ash, sulfur, and other toxic elements. A new multielement analytical technique
that introduces the sample in liquid form to an inductively coupled plasma
quadrupole mass spectrometer (ICP-QMS) is proving very useful for this purpose.
This technique can determine over 70 elements at the ppm to ppb levels. To
analyze coal by this method, it must first be converted to ash, fused with a
flux, and dissolved. The solution is then sprayed into the 7,000 C thermal
environment of an argon plasma where it is ionized. The resulting charged atomic
particles are drawn into a high vacuum portion of the instrument where a
quadrupole mass spectrometer (shown in Disaster from space section) separates
and counts the number of atoms for each different mass.
Detailed mapping of trace elements in a coal seam may be
required to locate low-polluting coal resources. The major drawback to selective
mining is that only small quantities of clean coals exist, and those that can be
found may be too far from power plants or too deep to be economically recovered.
The principal source of sulfur emissions from burning
coal is pyrite, whose presence is shown in this photomicrograph of a coal
Coal cleaning is the third method for reducing
pollution. Sulfur minerals such as pyrite can be removed by using various
techniques. Chemical analysis of the coal and identification of mineral
inclusions determines what cleaning procedure will be most effective. This
requires looking at the coal under high- power microscopes or performing tests
that separate mineral and coal species by using complex physical and chemical
techniques. Understanding the chemistry and mineralogy of coals has contributed
significantly to the progress that has been made in recent years toward the
prevention of coal-burning pollution.
For additional information on environmental geochemistry
order a paper copy of Understanding Our Fragile Environment USGS Circular 1105 a
publication in the Public Issues in Earth Science Series. Return to this point
III. Mapping the Chemistry of the Earth`s Surface
IIa. Assessment of public lands
Mapping stream sediments for resource exploration
The successes of the old-time and latter-day prospectors
have diminished the likelihood for the discovery of additional mineral resources
on the surface of our planet. Yet our national and global dependence on mineral
resources continues to grow unabatedly, and recycling can only provide a
fraction of our needs. By necessity, today s search for the many minerals vital
to society is focused on ore deposits that lie beneath the Earth s surface.
Earlier in this Session (back by the illustration of the
copper-molybenum porphyry cross section) we discussed the use of models to
locate ore deposits . Another way of locating mineral resources is by
identifying element-dispersion halos. Dispersion halos are abnormal levels of
the metals that develop around deposits. This halo can extend for long distances
from the deposit and, once recognized, can be used to trace down the source. The
most familiar example of a halo is the dispersion of gold nuggets in drainages
downstream from gold mother lodes.
Using today s technology, collected stream-sediment
samples may be processed and analyzed for as many as 40 elements, giving an
indication of very faint halos at some distance from a variety of deposit types.
If elements of economic interest, such as gold, silver, copper, lead, or zinc,
are present, they will be revealed in these analyses. This process is repeated
for many samples until the entire study area is covered.
By evaluating our nation`s mineral resources, we can
determine tha appropriate use of Federal lands. Helicopters have little impact
on the land and can be used in remote areas, such as Alaska, to efficiently
gather samples for geochemical analysis.
Keeping track of the resources of our country
Congress has mandated the USGS to assess the
mineral-resource potential of public lands, especially those lands set aside as
wilderness or proposed wilderness. These assessments provide an inventory of
mineral resources for future generations. In 1964, the Wilderness Act was
passed, and a 20-year program to assess the mineral resources of U.S. Forest
Service wilderness areas began. A large amount of this work involved the
analysis of stream sediments to determine the presence or absence of halos.
Subsequent laws have required mineral-resource assessments on additional public
lands. The USGS also works with the Bureau of Indian Affairs and individual
tribes to assess the mineral resources on Indian lands.
By relating ore-deposit models and geochemical data to
geologic observations and plate tectonic theory, geologists can predict what
types of ore deposits may be found in a given geographic area. The USGS supplies
this information to the public and to other government agencies. Assessments are
published by the USGS for use by land-use planners, Federal, state, and local
government agencies, environmentalists, and private individuals. Many maps, such
as the map of lead in stream sediments of Colorado, are useful for both resource
evaluations and environmental assessments.
This map of lead in Colorado stream sediments was
generated with existing data from the National Geochemical Data Base. It shows
the presence of a geochemical halo from the Colorado Mineral Belt and also the
lead caused by industry in some cities.
It is important to weigh the mineral-resource potential
of a tract of land against other potential uses such as water resources,
grazing, forestry, recreation, tourism, and scenic value. Chemistry plays a
vital role in this assessment process. Return to this point in index.
Looking for halos of mineralized areas or testing for
pollution is like playing the game of hide-and-seek. The target can be found
more easily if you are given hotter or colder clues. To provide these clues,
analysts in mobile laboratories perform chemical analyses for geologists in the
field. As a result, samples can be evaluated quickly. The use of mobile
laboratories by the USGS dates back to the turn of the century. These pictures
show a mobile laboratory used in Montana. It was a horse-drawn wagon that
carried the necessary reagents, glassware, etc. that were set up in a tent.
You can find the halo of a deposit faster, if you are
constantly aware whether you are getting closer or farther away. To rapidly
determine the distribution of elements in the field in 1907, portable analytical
equipment was used.
When the field area was reached, a tent was set up and
wet chemical analyses were performed in primative conditions from a kneeling
Over the years, the mobile laboratories have become more
refined. Since the 1960 s, these laboratories have provided USGS geologists and
geochemists with over 1 million analyses, providing timely information for
evaluating the mineral-resource potential of public lands.
Today, the principle of mobile laboratories is the same
as the wagon of earlier times, but the equipment is significantly more advanced
in technology. In addition to clean, relatively comfortable surroundings, which
are protected from the weather, sophisticated electronic equipment shown here
can be used to run a large number of sensitive analyses.
Exploration for covered ore deposits
Ore deposits covered by transported overburden, such as
gravels, are more difficult to locate than ore deposits that are buried in the
host rock in which they formed. New research using super-sensitive analytical
techniques provide scientists with a way to see through that covering.
This research is based on the idea that buried ore
deposits may release trace amounts of ore-related elements that are transported
through the overburden. These trace elements that are found at the surface,
however, may have been originally introduced with the transportation of the
overburden and don t necessarily indicate the presence of a covered ore deposit.
The ability to distinguish between the trace elements already in the overburden
and those migrating from an ore deposit would provide a powerful tool for
subsurface exploration. Two of the methods that are currently being researched
by the USGS are ground-water analysis and selective chemical extractions of
overburden samples for the loosely bonded migrating elements on the surface of
the gravel fragments.
Ground water collected from wells, springs, and drill
holes may provide clues to the presence of covered deposits. This water moves
very slowly through the overburden until it discharges at the surface as a
spring or seeps into a body of water. Subsurface flow rates vary from almost
zero to over 100 feet per year. The slower rates cause water to have a longer
contact time with the subsurface gravels, rocks, and, if present, ore deposits,
permitting minute amounts of metals to be leached from the rocks.
Geochemists can sample water from previously drilled
holes to detect the "halo" of an ore deposit.
Detecting gold in a ground-water dispersion pattern
requires an extremely sensitive analytical technique. The USGS has developed a
method for detecting gold in water at the one-part-per-trillion (ppt)
determination level. One ppt could be represented by one marble on 20,000
football fields (almost 39 square miles) covered with marbles.
In this technique, gold ions are removed from relatively
large-volume water samples by the use of anion-exchange resin, in a manner
similar to the exchange of ions that takes place inside a commercial water
softener. Later, the gold ions are stripped from the resin and analyzed using
graphite-furnace, atomic- absorption spectroscopy. (AAS is discussed in the Maps
of natural contamination section)
The USGS is working on a new method to gather
information from nonproductive drill holes.
Using a simple device, a ground-water sample is
recovered from the drill hole in hopes that it will show proximity to an ore
The relatively large dilute water sample is filtered and
stabilized prior to being transported to a mobile laboratory for analysis.
Mineral scavengers provide a clue
There is another way of detecting the trace elements
carried from a deposit by ground water. Ground water is drawn upward by
evaporation at the surface. During this upward migration, trace elements in the
water are affixed to minerals in the overburden. The affixation, or bonding, may
range from weak to very strong. The strength of this bonding depends on the
chemical nature of both the trace element and the host mineral. The differences
in bond strength is comparable to the difference between the weak electrostatic
attraction that holds an inflated balloon to a wall and a nail driven into a
Minerals that are capable of scavenging trace elements
from ground water with increasing bond strength include hydrated aluminum
silicates (clays), secondary carbonates, amorphous (noncrystalline) oxides of
manganese, and the amorphous and crystalline oxides of iron. Trace elements
scavenged by these minerals are removed by treating samples of overburden with
chemicals that react selectively with each mineral phase. Sequential selective
extractions are used to release trace elements from the host minerals in the
order of increasing bond strength such as clays first and crystalline iron
The principal advantage of selective extractions is that
they facilitate the distinction of elements that have migrated from other
sources from those normally present in the overburden. Thus the presence of a
gold deposit in Nevada may well be indicated by the occurrence of gold, or its
associated elements, arsenic and antimony, in a specific mineral phase in the
overburden. Return to this point in index.
IIIb. Geographic chemistry: National Geochemical Data
Chemistry of a nation on file
For many decades, samples of geologic materials (rocks,
soils, sediments, waters, and others) have been chemically analyzed. The
geochemical data collected from these and other scientific programs and projects
provide the basis of a growing national geochemical data base. A part of the
data base contains chemical analyses of stream sediments from hundreds of
thousands of drainage basins throughout the United States. These analyses
represent the chemistry of surface materials in these basins and may be used in
many applications concerning health, the environment, and natural resources.
(Scroll up a few screens to see the map of lead in stream sediments of
This map shows the coverage of analytical data generated
during theNational Uranium Resource Evaluationprogram that is stored in the
National Geochemical Data Base.
10,000 element determinations a day
One of the principal methods of analyzing samples that
shows up frequently in the National Geochemistry Data Base is inductively
coupled plasma-atomic emission spectrometry (ICP-AES). This method provides a
rapid and precise means of monitoring up to 50 elements simultaneously for
minor- and trace- levels. The ICP-AES technique is widely regarded as the most
versatile analytical technique in the chemistry laboratory.
When the sample solution is introduced into the
spectrometer, it becomes atomized into a mist-like cloud. This mist is carried
into the argon plasma with a stream of argon gas. The plasma (ionized argon)
produces temperatures close to 7,000 C, which thermally excites the outer-shell
electrons of the elements in the sample.
In an inductively coupled plasma-atomic emission
spectrometer the (1) aqueous sample is pumped and (2) atomized with argon gas
into the (3) hot plasma. The sample is excited, emitting light wavelengths
characteristic of its elements. (4) A mirror reflects the light through the (5)
entrance slit of the spectrometer onto a (6) grating that separates the element
wavelengths onto (7) photomultiplier detectors.
The relaxation of the excited electrons as they return
to the ground state is accompanied by the emission of photons of light with an
energy characteristic of the element. Because the sample contains a mixture of
elements, a spectrum of light wavelengths are emitted simultaneously. Just as
rain breaks sunlight into a rainbow, the spectrometer uses a grating to disperse
the light, separating the particular element emissions and directing each to a
dedicated photomultiplier tube detector. The more intense this light is, the
more concentrated the element. A computer converts the electronic signal from
the photomultiplier tubes into concentrations. The determination portion of the
process takes approximately 2 minutes to complete. In 1 day a chemist using the
ICP-AES can analyze 200 samples for a total of 10,000 elemental determinations.
Return to this point in index.
IIIc. Public health and safety: Element maps of soils
Maps of natural contamination
Recently, an environmental problem was solved by mapping
the soil chemistry in the San Joaquin Valley of California. In the 1980 s,
wildlife managers noticed increasing reproductive failure among nesting water
birds at the Kesterson Wildlife Refuge in the northern end of the San Joaquin
Valley. Chemical analyses of tissues from both birds and fish indicated toxic
levels of selenium. The question was not only why, but why at that particular
time? What had suddenly changed?
Beginning in the 1870`s, irrigation was used to turn the
nonproductive desert land of the San Joaquin Valley into the patchwork quilt of
fields shown in this 1985 satellite image.
To avoid the loss of crops caused by the buildup of
salinity in the soil, a drain was built to transport the used irrigation water
away. Instead of completing the projected 290-mile drain to the sea, it was
halted 205 miles short of its goal, forming a wetland and leaving the water to
Wildlife moved into the wetlands and prospered until
selenium leached from marine shales in the Coast Ranges built up in the food
cahin and resulted in terrible mutations in higher life forms such as this baby
Irrigation of arid soils in the San Joaquin Valley began
in the 1870 s, accumulating salts in shallow ground water perched on impermeable
clay layers. Within a decade, farmers recognized the need for drainage
facilities to lower the level of salts in the ground water or risk permanent
loss of agricultural capacity, but the problem persisted. Finally in 1960,
California voters approved financing for the State Water Project that included
an extensive drainage system. Between 1968 and 1975, 85miles (of the projected
290 miles) of the San Luis drain facility had been completed with a temporary
termination at Kesterson Wildlife Refuge, still many miles short of the
Sacramento-San Joaquin Delta, its projected destination.
By 1978, drainage into Kesterson had increased
significantly. Unseen, the selenium levels were also increasing and by 1982 had
built to toxic levels in the food chain of the wildlife refuge. Fish were
affected first followed by waterfowl. Ultimately, the ponds were closed and
filled in as the quickest solution to an environmental disaster. One question
remained where did the selenium come from? The eastern side of the San Joaquin
Valley has a deficiency in selenium, and, in fact, livestock grazing in the area
needed to have selenium added to their food as a supplement. So why did
Kesterson have too much selenium?
Further chemical studies focused on the Panoche Fan, the
source of most of the drain water. Through chemical analyses using hydride
generation atomic absorption spectrometry (AAS), high-selenium soils were found
and mapped near the mountain front on mud-flow debris derived from
selenium-enriched marine shales in the Coast Ranges.
AAS uses a bright source of the element s characteristic
light, usually from a lamp whose cathode contains a large amount of the element.
This light is then passed through a cloud of non-excited, ground-state atoms
from the sample where it is absorbed proportional to the amount of the element
present in the cloud. Next, the light goes to a monochrometer, that separates
the energy wavelength of interest. The light is then converted into an
electrical current, amplified, and rectified. A computer calculates the quantity
of the element in the samples.
With the selenium data generated by AAS, the source of
the selenium in the wildlife refuge was studied. Is the source of the selenium
natural or caused by humans? The answer is both. The occurrence of selenium in
the soils and ground water of the Panoche Fan is perfectly natural. Humans,
however, are interacting with one part of the natural hydrologic cycle in which
elements are transported from minerals to the ultimate sink the ocean. Here the
elements would have been naturally recycled by reprecipitation as minerals in
marine shales. By increasing the amount of rainfall (via irrigation), human
activity has sped up the leaching of selenium out of the Panoche fan sediments.
The temporary halt of the San Luis drain had left the
project 205 miles short of the sea, and the drain water was instead contained in
holding ponds. The extra water turned the holding ponds into wetlands where
birds that used the flyway made nesting sites. The ultimate solution to the San
Joaquin drainage may lie in finishing the drain and discharging the water
directly to the ocean so that nature can recycle it into marine sediments again.
To some people, the Kesterson Wildlife Refuge has been
considered an environmental disaster. Nevertheless, it has served as an
environmental lesson. The holding ponds demonstrate the feasibility of creating
wetlands to clean up some forms of metal pollution. They also prove, however,
that if we create wetlands for bioremediation, they cannot be built and left
untended without risk to wildlife. The levels of toxic elements being
concentrated in the wetland will have to be monitored so that they do not build
up to levels that are toxic to wildlife. Return to this point in index.
Industrial sources of contamination
A similar cause for environmental concern is the
presence of mercury in the agricultural soils of the Panoche Fan. To the west of
the San Joaquin Valley, a major, mercury mineralization district is located near
the town of Idria. The New Idria Mine, operated between 1858 and 1972, was the
second largest mercury producer in North America.
Streams that drain the north, east, and south sides of
the mining district all contribute sediment to the Panoche Fan. Chemical
analyses of soil samples from the Fan clearly show that the soil contains
elevated mercury levels. These high levels of mercury could be caused by a
combination of natural geochemical dispersion and mining activity, considering
the time period of major mercury production at the New Idria Mine (scroll back
to "Mapping the Chemistry of the Earth` Surface" for a discussion of dispersion
Like the selenium, the mercury data were generated using
AAS. The rocks were digested and the solution was then reduced to form elemental
mercury, which was separated as mercury vapor and measured with AAS. This method
is called the cold vapor-AAS method. Return to this point in index.
Robots in the laboratory
Geochemical studies generate large quantities of samples
to be analyzed in the laboratory. Although technological advances have produced
vast improvements in analytical measurements and data reduction, the manual
preparation of samples has remained a time-consuming problem. As a result, one
of the most rapidly growing areas in laboratory automation is the use of
robotics for sample preparation.
What does a robot have in common with a technician= An
arm, hand, and fingers. The robots arm moves up/down, in/out, and rotates 360x.
The fingers grip tubes and flasks, and the hand rotates
for pouring liquids.
A laboratory robot generally consists of an arm, a hand,
and a pair of fingers. These components are programmed to duplicate the sample
preparation usually performed by a laboratory technician. The centrally
positioned robot moves samples in and out of laboratory work stations. Each work
station performs a specific function such as dispensing acids, mixing, heating,
centrifuging, filtering, and weighing.
There are several advantages to the use of robotics.
Robots have improved productivity by a factor of 2 or 3. Because sample
preparation requires the use of hazardous chemicals, the robot minimizes human
exposure to these chemicals. By delegating the repetitive applications to the
robot, the technician is available to assume greater responsibilities. Finally,
robots provide consistency in sample preparation and improve the precision of
In USGS laboratories, robotics have been applied to a
range of techniques including sample disaggregation, the decomposition of tens
of thousands of samples per year for the ICP-AES methods, the weighing of 7,000
charges of flux per year for the XRF major element analyses method, and other
similar sample preparation methods. The use of laboratory robotics continues to
increase as the benefits from each application are realized. Return to this
point in index.
IV. Can we depend on chemical analyses?
IVa. Measuring quality
The importance of measurements
One of the tasks facing scientists is to measure and
define unknown quantities. These measurements are important because they can
warn us of potential hazards from volcanoes and environmental contamination or
help us develop our mineral resources to stay competitive in the worldwide
economy. In Earth sciences, the measurements of geological samples are used in
making policy decisions. These decisions can affect all Americans in topics
ranging from pollution prevention and control to evaluation of mineral resources
and wilderness areas.
Decisions are made every day based upon measurements of
various substances (or areas containing them). Without quality measurements,
misleading or dangerous conclusions could be drawn.
The uncertainty of measurements
There are many difficulties associated with making
measurements. Quality assurance involves minimizing mistakes and correcting
problems before the information is used.
When an archer releases an arrow at a target, both the
distance from which the archer shoots and the size of the target define what is
considered acceptable accuracy and precision. Shots from 5 yards would be
expected to hit closer to the bull s-eye than shots from 50 yards. If the arrows
miss the target completely, the archer is considered inaccurate. The closer
together repeat shots hit the target to each other, the more precise the archer.
The strength and dexterity of the archer, the acuity of the archer s eye, the
adjustment of bow sights, wind conditions, and the number of shots taken also
contribute to the accuracy and precision of the archer. A laboratory procedure
is similar in the need to understand the variables involved and the
possibilities for error.
Searching for the best
A mistake in measurements can impact decisions made on
endangered animal habitat, mineral exploration, or remediation of an
environmental problem. When a quality assurance program claims 99.9 percent
accuracy, consider what that could mean in terms of error: 1 hour of unsafe
drinking water per month, 16,000 lost pieces of mail per hour, or 176,000 checks
deducted from the wrong bank accounts every day. The quest is for 100 percent
accuracy and precision, even if it is not attainable. Return to this point in
The USGS Reference Materials Project
In the field of analytical chemistry, reference
materials serve an important role in the development of new techniques and the
periodic testing of established methods. Used correctly, reference materials
provide investigators with a mechanism to objectively compare their results with
established values and determine if any bias exists.
It was this drive to produce quality data that led the
USGS and the Massachusetts Institute of Technology in the early 1950 s to
jointly develop the first geochemical reference materials. This early work
started a USGS tradition of preparing high-quality reference materials that are
used for both domestic and international geochemical programs. To date, 29
different geochemical standards have been produced with an estimated worldwide
distribution of over 20,000 units.
Samples are dried, crushed, powdered, mixed thoroughly,
then bottled and analyzed. Some reference materials are distributed by the USGS
in limited quantities directly to researchers and analytical laboratories. Other
standards can be prepared on a contract basis for individual government
agencies. Geologic analytical laboratories can compare their results to these
Initially, the need for quality control led to the
development of several silicate rock standards that were important in such
diverse activities as the lunar program, ore-genesis studies, and volcano
monitoring. When the mining and exploration industries clamored for reference
materials, the USGS responded by generating six exploration standards designed
to contain elevated concentrations of key elements. The USGS involvement with
the mining industry continues today with the recent development of coal and
gold-ore standards, which will be useful in resource appraisals.
Environmental concerns are becoming a major part of the
national agenda, and the USGS Reference Material Project provides quality
reference material to aid in this field of study. A major emphasis of this
effort will be to conduct cooperative studies with other Federal agencies, thus
helping them respond to national needs. Return to this point in index.