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MICROSCOPES and LABORATORIES |
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HOW
BIG IS SMALL?
Our microscopes look at tiny pieces of rocks. The best way
of measuring small distances is with the metric scale. The metric
scale starts with a unit called a meter which is a little bit
bigger than a yard.
 Divide
a meter into a hundred pieces and you have a centimeter (cm).
There are about two and a half centimeters in an inch.
Divide a meter into a thousand pieces and you have a millimeter
(mm). You can see millimeters easily on a ruler.
Divide a millimeter into a thousand pieces and you have a micrometer
(mm). There are a billion micrometers in a meter.
Divide a micrometer into a thousand pieces and you have a nanometer
(nm). There are a trillion nanometers in a meter.
The metric system also measures big distances. A kilometer
(km) is a thousand meters. A kilometer is a little more than
half a mile.
| [pictured
above] A millimeter is the smallest measure we can
see on a ruler. One thousand millimeters make a meter. A
meter is a little bit bigger than a yard. |
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|  ATOMS,
ELEMENTS, MINERALS AND ROCKS
Atoms are made up of a nucleus that contains protons and neutrons,
and electrons that circle around the nucleus.
A chemical element is material made of only one type of atom.
All the atoms of any one chemical element have the same number
of protons in their nucleus. A helium balloon is filled with helium
atoms that all have three protons in the nucleus. A sheet of aluminum
foil is made of aluminum atoms that all have thirteen protons
in the nucleus.
A mineral is a crystalline substance. A mineral can be made of
only one element. An example is diamond which is made of carbon.
Or, a mineral can be made of more than one element. An example
is salt which is made of sodium and chlorine. Minerals that make
up rocks usually contain silicon, oxygen and other elements like
magnesium, aluminum, iron and calcium. Each mineral has its own
chemical composition, and its own arrangement of atoms.
Rocks are made up of many small mineral grains that are put together
in different ways depending on how the rock formed.
| [pictured
above] This model shows you how the atoms in a mineral
are organized. It is a model of salt, the mineral we eat every
day. Salt contains the elements sodium and chlorine. In the
model, the blue circles are sodium atoms and the green circles
are chlorine atoms. The atoms are arranged in a very organized
pattern that is repeated over and over, throughout a single
mineral grain. |
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 MAKING
A THIN SECTION
Scientists look at thin slices of rocks under a microscope
to learn more about what the rocks are made of.
A thin section is a highly polished slice of a rock that is thin
enough for light to pass through. A thin section is 30 micrometers
thick (that's three hundredths of a millimeter). To make a thin
section, we first cut a slice of the rock that is about a millimeter
thick, using a rock saw. Then we glue the slice to a glass slide,
grind it down until it is the right thickness, and finally polish
it until it is as smooth and shiny as a mirror.
| [pictured
above] Thin section of the Milton pallasite. Crystals
of olivine are white, and the black material is iron-nickel
metal. A thin section is a highly polished slice of a rock
that is 30 micrometers thick. The slice is glued to a circular
glass slide that is 2.5 cm in diameter. |
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|  LIGHT
MICROSCOPE
A light microscope, or optical microscope, is used to study
a thin section of a rock. The section is so thin that light passes
through the rock.
Light in an optical microscope passes through two polarizing
plates that are like the lenses in polarized sunglasses. Different
minerals can be identified because polarized light passes through
them in different ways. We can easily look at grains that are
a few millimeters down to about 10 micrometers across.
Thin sections viewed in plane polarized light (one polarizing
plate) are usually not very brightly colored. Different minerals
are slightly different colors and often have different shaped
grains. Minerals like metals, sulfides and oxides are opaque,
which means that light does not pass through them. We see these
as black grains in plane polarized light.
With crossed polars (two polarizing plates at right angles to
one another), many minerals show bright colors and the thin section
looks like a kaleidoscope. The minerals olivine and pyroxene are
usually green, pink, yellow and blue, and feldspars and quartz
are usually grey. In cross-polarized light, we can see twinning
in minerals, differences in chemical composition and many other
characteristics that help us understand how the rock formed.
| [top
picture above] Light microscope (optical microscope)
at the University of New Mexico. This microscope uses transmitted
light that passes through the sample, as well as reflected
light that is reflected off the surface of the sample. The
sample, usually a thin section of a rock, sits on the round
black platform.
[center
picture above] Thin section photograph of the SAH99555
angrite (achondrite), taken in plane polarized light. The
long, thin, white crystals are plagioclase feldspar and
the pink crystals are pyroxene. The image is 2 mm across.
[bottom
picture above] Thin section photograph of the SAH99555
angrite (achondrite), taken in cross-polarized light. The
long, thin, white and grey crystals are plagioclase feldspar
and the brightly colored crystals are pyroxene. The image
is 2 mm across. |
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| SCANNING
ELECTRON MICROSCOPE
In a scanning electron microscope (SEM), pictures are made
with electrons instead of light.
An SEM can be used to look at a 3-dimensional piece of rock a
few centimeters across, or a polished thin section. The sample
is placed under a beam of electrons. Images are always black-and-white.
With an SEM we can take pictures of objects that are about a micrometer
across.
An SEM can make several different kinds of images. A secondary
electron image (SE image) looks at electrons that are reflected
off the sample. The image looks just like a picture you would
take with your camera. A back-scattered electron image (BSE image)
makes a picture in which the grey level is related to the atomic
number (number of protons) of each grain. A grain of iron appears
very white because iron has a high atomic number, and a grain
of quartz (silicon and oxygen) appears dark grey because it has
a low atomic number.
| [above
left] Scanning electron microscope (SEM) laboratory
at the University of New Mexico. This instrument is used
to take photographs of rocks using electrons that scatter
off the surface of the sample. The white cylinder is the
column for the electron beam.
[above center]
SEM image of a chondrule from the Mokoia (CV3) carbonaceous
chondrite. This is a secondary electron image. The chondrule
is like a ball, made mostly of pyroxene crystals that poke
out of the surface. The image is 1.3 mm across.
[above right]
SEM image of the Bishunpur (LL3.1) ordinary chondrite. Round
objects are chondrules. White grains are iron-nickel metal
and iron sulfide. Grey grains are silicate minerals, mostly
olivine and pyroxene. The different shades of grey show
different chemical compositions. The white line at the top
of the image is a 1 mm scale bar. |
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|  TRANSMISSION
ELECTRON MICROSCOPE
In a transmission electron microscope (TEM), pictures are
made with electrons that pass straight through the sample.
For electrons to pass through a mineral grain, the grain must
be about a tenth of a micrometer thick. One way to prepare a sample
for a TEM is to take a thin section and make a hole in it with
high-energy ions. Around the edge of the hole the grain is thin
enough for TEM imaging. The sample is placed under a beam of electrons
with very high energies. We use the TEM to look at objects that
are a few micrometers down to a few nanometers in size.
[above left]
Transmission electron microscope laboratory at the University
of New Mexico. This instrument is used to take photographs using
electrons that pass right through the sample. The tall white cylinder
is the column for the electron beam.
[left]
TEM image of tiny grains of magnetite in the martian meteorite,
ALH84001. Each grain is about 10 nanometers across.
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|  MINERAL
ANALYSIS: ELECTRON MICROPROBE
An analysis of a mineral tells us what chemical elements it
is made of. This helps us understand how the mineral grain formed.
We can analyze a mineral using an electron microprobe. This machine
is like a scanning electron microscope (see above) but it has
extra attachments called spectrometers that are used for analyses.
An electron microprobe can be used to determine which elements
are present in a mineral, and how much of each element there is.
An electron microprobe is like a scanning electron microscope:
a sample is put underneath a beam of electrons. Where the electrons
hit the sample, X-rays are given off. Each element gives off X-rays
of different energies. We collect the X-rays and use them to determine
the composition of the mineral. Each analysis tells us the composition
in a spot that is about 1 micrometer across.
| [top
picture above] Electron microprobe at the University
of New Mexico. This instrument is used to take pictures
like a scanning electron microscope. It is also used to
measure the chemical composition of mineral grains. The
tall white cylinder is the column for the electron beam.
It is surrounded by spectrometers that are used to measure
the X-rays that come off the sample when it is hit by the
electron beam.
[above left]
X-ray spectrum for pyroxene. X-rays are produced when atoms
are hit by an electron beam. Each element in the pyroxene
grain emits X-rays with a distinct energy. The pyroxene
grain that was analyzed here contains oxygen (O), magnesium
(Mg), silicon (Si), calcium (Ca) and iron (Fe).
[above right]
X-ray image of a metal grain in the PAT 91546 (CH3) carbonaceous
chondrite. The image is made with X-rays that have been
emitted from nickel atoms. The green circle is the grain
of metal, which is a mixture of iron and nickel. Red patches
show where there is a lot of nickel. The white line in the
bottom left corner of the image is 10 micrometers long. |
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|  DATING
ROCKS: MASS SPECTROMETERS
We can find out the age of a rock using natural radioactivity
that can be used like a clock.
All rocks contain very small amounts of elements that are naturally
radioactive. Some of the atoms of the radioactive elements are
changing into atoms of other elements. For example, atoms of the
element rubidium change into atoms of strontium. This happens
very slowly and can take billions of years. If we measure how
much strontium is in the rock, we can tell how old the rock is.
We can make these measurements using a mass spectrometer.
Atoms of the same element that have different numbers of neutrons
in their nucleus are called isotopes. For example, a hydrogen
atom can have one, two or three neutrons in its nucleus: we call
these isotopes hydrogen, deuterium and tritium. Not all isotopes
of an element are radioactive. The rubidium-strontium clock used
to date rocks looks only at the decay of the rubidium-87 isotope
(87Rb) to the strontium-87 isotope (87Sr). The mass spectrometer
measures the concentrations of elements that are only about one
billionth of the total mass of the rock. The rock must be dissolved
in acid in order to make these measurements.
| [pictured
above] Mass spectrometer at the University of New Mexico.
This instrument is used to date rocks. The rock is dissolved
in acid and each chemical element is separated. The amounts
of each isotope are measured in the mass spectrometer, and
the isotopes are used to determine the age of the rock. |
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| CLASSROOM
ACTIVITIES |
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