Forensic Examination of Soils

Question 1

constitute excellent trace evidence in criminal cases because there are an
almost unlimited number of identifiable soil types based on the content of rocks, minerals, glasses, human-made particles, and chemicals.

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Question 2

commonly identifies the original geographic location of soils associated
with a crime, thus assisting an investigation.

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Question 3

EXAMINATION METHODS

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Question 4

The value of soil evidence rests on the fact that there are an almost unlimited
number of rock types, mineral types, fossils, and artificial rock material, including
glass and brick and concrete.

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Question 5

In addition, it is not uncommon for soil material to have incorporated human-made
particles such as plastics, metals, and chemicals such as fertilizer and pesticides.

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Question 6

Various instruments, methods, and procedures are used to study minerals, rocks,
soils, and related materials for forensic purposes. These methods are used to
collect data from the various questioned and known samples, which are then used
to make a judgment as to comparison or lack of comparison.

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Question 7

The evidential value of some of these methods is greater than others. However,
they are all standard methods long accepted for use in the identification of material.

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Question 8

Because of the extreme diversity of soils and related material, the examiner may
decide that in a particular case some other method will provide information that will
assist in reaching a conclusion.

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Question 9

Soils often have added particles, such as fibers, hair, and/or paint chips, these can
be collected for examination by experts in those forensic science fields

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Question 10

One of the reasons that microscopic examination is so important is that it is only
way that particles of other forms of evidence can be found, and unusual mineral
particles discovered.

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Question 11

most important identifying characteristics of minerals and soils.

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Question 12

form a mosaic of grays, yellows, browns,
reds, blacks, and even greens and brilliant purples.

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Question 13

the native minerals contribute directly to the soil color.

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Question 14

Soil particles become stained, coated, and impregnated with mineral and organic substances, giving the soil an appearance
different from its original one.

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Question 15

sands along a river channel are examined, the color of each sand grain can generally be recognized individually; however, after a
deposit has weathered for a long period of time, there is a degree of leaching, accumulation, and/or movement of substances within
the soil.

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Question 16

The mineral grains, especially the larger ones, are generally coated. I

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Question 17

most situations the coatings on the soil particles consist of
iron, aluminum, organic matter, clay, and other substances

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Question 18

The coloring of the coatings alone can give some indication as to the
history of the sample.

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Question 19

The “redness” of a soil depends not only on the

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Question 20

amount of iron present but also on its state of oxidation, with a highly oxidized condition tending to have a
more reddish color.

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Question 21

The iron on the coatings of the particles probably is in the form of hematite, limonite, goethite, lepidocrocite, and other iron-rich mineral forms.

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Question 22

Black mineral colors in the soil are generally related to manganese or various iron and manganese combinations.

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Question 23

Green colors are generally due to
concentrations of specific minerals rather than of the mineral coatings

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Question 24

some copper minerals, chlorite, and glauconite are usually green.

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Question 25

Deep blue to purple coloration in the soil is generally due to the mineral vivianite, an iron phosphate.

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Question 26

Apart from the mineral colors in the soil are those that result from organic matter.

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Question 27

The organic litter on the soil surface is generally black.

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Question 28

Humus percolates
through the mineral horizons, giving various dark colors.

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Question 29

some instances, the iron and humic acids combine to form a dark reddish brown to nearly black
color.

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Question 30

To have some uniformity in descriptions of the color of geologic materials and soils, certain standards have been established. The color standards most
frequently used in the United States are those of the Munsell Color Company.

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Question 31

The color standards are established on three factors: hue, value, and chroma.

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Question 32

This standardization of colors offers some degree of
uniformity, but moisture content will also affect the color of the soil, as will light intensity and wavelength.

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Question 33

Soil color is different in natural light than in fluorescent and different still in incandescent light. If a soil is air dry, it may be recorded as yellow, but if moist the recording may be yellowish brown. • Moisture added to a dry soil will usually result in a more brilliant appearance. It is therefore important to record not only the color of the soil but also an estimate of the “wetness factor” at the
time of the recording.

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Question 34

Soil, being a mixture of materials of various sizes and compositions, contains individual minerals of different colors. If soil is fractionated into various sizes—coarse sand, medium sand, fine
sand, silt, and clay—there is a tendency for the finer-sized particles to exhibit more red or reddish-brown colors as opposed to grays and yellows in the coarser fractions. In considering coarser
sand particles, the matrix will commonly have a speckled appearance, with the quartz and feldspar particles being gray or yellowish; but the heavy minerals such as ilmenite and magnetite will
generally be black.

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Question 35

Sand particles from soils of recent origin, such as recent glacial or stream deposits, usually retain their original mineral appearance and we can usually detect a mosaic of colors. But sand
fractions from the old landscapes commonly have coatings of clay, and the sand grains may be iron stained, which results in a more uniform color of the entire matrix. Soil grains veneered with
organic matter give the particles a dark gray appearance. It is important first to record the color of the untreated soil sample and then to remove the organics so that the true color and
appearance of the sand grains can be studied.

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Question 36

In studying soil samples for forensic purposes, the sample is normally dried at
approximately 100◦C and viewed with natural light, preferably coming from a
northerly direction. A north-facing window is a good location for such observations.

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Question 37

Such studies should be made on samples that have the same general size
distribution of particles. The color of samples prepared from the individual sieved-out
particle size ranges gives important additional data. Two or more samples, collected
for study, can be compared directly by the observer.

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Question 38

It is then possible to use a color chart with the samples to determine the Munsell
color numbers for precise description of the color.

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Question 39

Instrumental determination of color provides more quantitative information and can
now be done with a precision which exceeds that the human eye.

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Question 40

Pye Associates of Great Britain have found that rapid, reliable, and highly
reproducible results are achieved using the Minolta CM-2002 photo spectrometer.
This instrument covers the visible wavelength range 400 to 700 nm, providing
several light source types and observer angles, and producing a range of color
indices, including Munsell hue, value, and chroma; Lab* indices; and reflectance
data and curves.

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Question 41

Calibration of the instrument is carried out for every analysis to black, white, and
CERAM II color standard materials.

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Question 42

Particle-Size Distribution

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Question 43

The determination of the distribution of particle sizes in a
sample can often provide significant evidence. This
determination is often produced for a variety of reasons:

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Question 44

to produce samples for comparison studies that are similar, in which case
the control sample may contain some larger or smaller particles that are
not present in the sample being questioned or an associated sample, and
they must be removed;

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Question 45

the samples may be broken down into subsamples in which all the particles
are in the same size range for mineral or color studies;

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Question 46

a determination of the distribution of particle sizes may be produced as a
method of comparison.

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Question 47

when abrasive particles have been introduced into
machinery for the purpose of sabotage, the size distribution of
the particles may be diagnostic of the material, if changes in
particle size have not taken place in the machinery.

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Question 48

Before making a mechanical analysis to determine the size
distribution of particles, it is necessary to disperse the soil.
Individual soil particles tend to stick together in the form of
aggregates.

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Question 49

Cementing agents of the aggregates must be removed;
otherwise, a cluster of silt and clay particles would have the
physical dimensions of sand or gravel. Cementing agents
consist of organic matter, accumulated carbonates, and iron
oxide coatings, and in some situations, there is a mutual
attraction of particles by physicochemical forces.

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Question 50

• If carbonates have cemented the particles together, it is
desirable to pretreat the sample with dilute hydrochloric acid to
remove the carbonates. The sample is then treated with
hydrogen peroxide to remove the organic cementing agents.

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Question 51

All samples must be treated in the same way, and it must be
determined before treatment that important information will not
be lost, such as dissolving carbonate cement from grains that
should be treated as single grains. It is almost always desirable
to determine the size distribution of soil by sieving in a liquid,
usually water.

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Question 52

Dry sieving of the entire sample is generally unsatisfactory
because the small particles tend to cluster together, and clay
tends to adhere to larger particles. Sometimes a dispersing
agent is added to the water.

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Question 53

Several methods can then be used to determine the size distribution of the finer particles in a dispersed
suspension.

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Question 54

rapid method for determining the percentage of sand, silt, and clay in a sample

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Question 55

hydrometer

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Question 56

based on the principle of a decreasing density of the suspension as the solid particles settle out

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Question 57

This
method, although rapid and accurate, is unsatisfactory if we want to make a subsequent examination of the
various size ranges, because there is no physical separation of the various-sized particles.

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Question 58

pipette method

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Question 59

One of the most accurate and satisfactory procedures for fractionating soil samples is

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Question 60

This consists of pretreating the sample as is done in the hydrometer method, dispersing the soil in water,
and calculating the time required for various-sized particles to settle out from the suspension.

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Question 61

based on the rate of settling depends on the size of the mineral matter, with larger particles
settling at a more rapid rate.

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Question 62

procedure is based on Stokes’ law: V = (2/9)gr2(d − d )/n,

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Question 63

Although this method is generally considered the most satisfactory regarding accuracy, it is not infallible.

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Question 64

Several assumptions are made: that all particles have the same shape and that all the soil particles have
the same density, neither of which is usually the case. Nevertheless, the pipette method is generally
considered to be the best available mechanical method.

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Question 65

In making a mineral analysis of a sample, one fact becomes
apparent: the sample contains different groups of minerals
within various size ranges.

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Question 66

Sands are made up of a set of
minerals that are usually completely different from those within
the clay size range. Therefore, in comparative analysis it is
important to make comparisons within the same size ranges.

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Question 67

can be quite deceptive to compare the minerals found in one
size range in one sample with a different size range in another
sample.

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Question 68

When a transfer is made from a soil it is unlikely that the
transferred sample is truly representative of the distribution of
sizes that existed at that place.

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Question 69

two samples
that are to be compared using grain size distribution may not
show the same distribution of grain sizes.

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Question 70

The method can be
useful in concluding comparison or lack of comparison but is
seldom definitive.

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Question 71

Stereo Binocular Microscope

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Question 72

can be a most useful tool to the forensic geologist.

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Question 73

After an analysis of color has been
made, the information that can be obtained by examination of soil samples with this instrument makes it the logical first
step in the direct examination of particles.

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Question 74

Light microscopes are generally of two types, transmitted light and reflected light.

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Question 75

light source is placed beneath the specimen, which must be transparent.

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Question 76

that are used in
studying tissue are of this type.

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Question 77

have a light source above the object, and the surface features of the particle are viewed.

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Question 78

Such a microscope is essentially a stationary, higher-power magnifying glass.

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Question 79

Most of these microscopes have two sets
of lenses, and thus the object is viewed in three dimensions, that is, in stereo.

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Question 80

The magnification of a microscope is determined by multiplying the magnification of the ocular lens, commonly 10×, by
the magnification of the objective lens, which differs from microscope to microscope but seldom exceeds 10×. • This gives a maximum magnification of approximately 100×.

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Question 81

The objectives may be individual lenses of fixed
magnification, or in some microscopes a zoom objective is used.

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Question 82

Such lenses can change magnification continuously from less than 1× to about 5×. Most viewing with stereo binocular
microscopes is done at magnifications between 10× and 40×

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Question 83

Some of these microscopes are seated on a base that
contains a second light source so that objects can be viewed in both transmitted and reflected light.

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Question 84

When the
transmitted light is polarized, the microscope may be used for both stereo reflected light viewing and low-power
transmitted polarizing light studies.

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Question 85

Objects as small as approximately 10 μm in diameter can be viewed with a

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Question 86

The upper limit
is determined by how large a sample can fit under the instrument so that the surface of pebbles and cobbles can easily
be viewed.

Question 87

normally placed on a tray having a dull black finish for ease of viewing light-colored minerals or a white
finish for viewing dark colored minerals.

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Question 88

Various inserts are available for these microscopes that permit measurement of the size of objects or provide grids that
aid in counting the various particles.

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Question 89

The sample tray may have an etched grid that serves a similar purpose.

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Question 90

Trays are
available with various gummed surfaces that hold the grains in place for ease of counting.

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Question 91

examining a soil sample or similar material, the scientist will commonly first examine the entire sample as it is
received and observe the types of grains and particles.

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Question 92

Recording a general impression of the material is normally done
currently. It is not uncommon to observe in soil samples nonmineral materials such as fibers, metals, paint, glass, and
plastics.

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Question 93

In some situations, they can be the most important parts of the
sample. Materials such as metals, hair, fibers, paint, and plastic are removed for further examination by specialists.
Plant particles can be of great value.

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Question 94

The total amount of plant material is in general relatively useless for forensic purposes. However, identification of the
individual grasses, seeds, leaves, and the like can be most useful.

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Question 95

Preliminary examination of the entire sample with the binocular microscope is normally very difficult.

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Question 96

The mixture of particles of all sizes commonly
obscures the grains and makes identification difficult.

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Question 97

The presence of organic material contributes to this problem. The sample must be cleaned for the study of minerals and rock.

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Question 98

Sieving of the samples removes the larger particles and most of the larger organic fragments. If the sample is washed carefully in water, the lighter
organic particles will generally float and can be removed and saved for study.

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Question 99

Treatment with hydrogen peroxide will remove the fine organic matter and clean the sample.

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Question 100

The use of ultrasonic cleaners is dangerous in many cases. Many times the scientist has taken a sample that contained chips of red shale before
ultrasonic cleaning and found that the sample contained thousands of silt-sized quartz grains and clay minerals after cleaning and that the shale chips
had been broken up into its components.

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Question 101

• If the rocks and minerals are of a type that would not be disaggregated by ultrasonic cleaning, the method can be most useful.

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Question 102

Properties of grains such as shape, rounding, weathering, inclusions, color, and polish can be observed and
recorded.

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Question 103

The counting of different types of grains is especially important. When we record our information in numbers, it is normally more useful than qualitative
impressions.

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Question 104

In counting grains of different types, it is important that the sample counted be representative of the whole sample, that the identification be consistent
and accurate, and that the same grain not be counted twice because the sample moved.

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Question 105

latter problem can usually be avoided by placing the
sample on a gummed surface or by removing the grains as they are counted and placing each grain, as it is removed, in a container or gummed
individual tray.

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Question 106

Most important is the judgment and caution used by the scientist, whatever method is used.

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Question 107

Petrographic Microscope

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Question 108

differs in detail from an ordinary compound microscope.

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Question 109

However, its primary
function is the same: to produce an enlarged image of an object placed on the stage.

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Question 110

The magnification is produced by a combination of two sets of lenses, the objective and the ocular.

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Question 111

The
function of the objective lens, at the lower end of the microscope tube, is to produce an image that is
sharp and clear.

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Question 112

The ocular lens merely enlarges this image. For mineralogical work, three objectives—low, medium, and
high power—are normally used.

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Question 113

The magnification produced by objectives is usually 2× (low), 10× (
medium), and 50× (high).

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Question 114

Oculars have different magnifications, usually 5×, 7×, 10×, 15×, and 20×.

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Question 115

The total magnification of the image is determined by multiplying the magnification of the objective by that
of the ocular as follows: 50× times 10× = 500×

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Question 116

contain a crosshair that is useful for locating grains under high power when changing
objectives.

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Question 117

condensing lens system is normally provided under the stage for use with high magnification
and for assisting the viewing of the various optical effects produced by minerals.

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Question 118

have a rotating stage and a polarizing filter under the stage that transmits light
usually vibrating in a N-S (front-to-back) direction.

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Question 119

Above the stage a second, removable polarizing filter is placed in the tube of the microscope.

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Question 120

transmits light usually in an E-W
(left-to-right) direction.

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Question 121

When the upper filter is inserted, light is blocked out from passing through the microscope.

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Question 122

the filters, which are called polars, are said to be crossed. Only when an anisotropic material, that is, a material that is not
isotropic (meaning that it forms in the isometric crystal system or is amorphous), is placed on the microscope stage under
crossed polars can it be seen.

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Question 123

The effect of the anisotropic mineral is to rotate the N-S vibrating light from the lower polarizing filter, thus permitting some of it to
pass the upper E-W polarizing filter.

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Question 124

When the stage is rotated, there will be four positions when the vibration directions in an
anisotropic crystal will line up with the N-S and E-W direction.

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Question 125

• If the mineral has a higher refractive index, the Becke line will move into the grain.

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Question 126

the liquid of known refractive index is higher, the Becke line will move away from the grain into the liquid.

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Question 127

At this point the grain will be almost
invisible in the liquid. In most cases the refractive index of the grain is found to fall between the
refractive indexes of two liquids, and the value can be estimated by an experienced observer.

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Question 128

important tool in many aspects of forensic work and is the
best method for a study of the optical properties of rocks and minerals.

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Question 129

study of individual
mineral grains or thin sections of rocks and related material is easily accomplished by anyone
trained in the use of the instrument.

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Question 130

thin section is a thin slice of rock mounted on a glass slide.

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Question 131

The slice is normally 30 μm in
thickness and may be prepared from a solid rock or loose material impregnated with plastic.

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Question 132

The
rock is cut with a diamond saw and the surface polished.

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Question 133

This polished surface is cemented to a glass microscope slide with an adhesive of known
refractive index such as epoxy or Canada balsam.

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Question 134

saw cut is then made parallel to the glass,
leaving a wafer of rock cemented on the slide.

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Question 135

Grinding of the wafer proceeds until it is thinned to approximately 30 μm.

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Question 136

thin class cover is
then glued onto the polished rock surface to protect the rock and improve viewing with the
microscope.

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Question 137

Most rocks are transparent at this thickness and can be viewed in transmitted light.

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Question 138

Loose mineral grains of the same general size, also commonly mounted in epoxy or Canada
balsam on a microscope slide, are covered with a thin platelet of glass (cover glass) and studied.

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Question 139

This is the method used when the heavy minerals, that is, those minerals with high specific
gravity (such as rutile, garnet, zircon, and tourmaline), are separated from the more common
lighter minerals (such as quartz and feldspar) by settling in a heavy liquid such as sodium
polytungstate or one of the other tungsten-based heavy liquids and studied.

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Question 140

Scanning Electron Microscope

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Question 141

wide range of magnifications, generally from
25× to over 650,000× and can record something as small as 1.5 nm.

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Question 142

has the advantage that the surface of a sample may be viewed directly.

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Question 143

However, an
ultrathin coating of carbon or gold plated on the specimen improves the quality of the picture.

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Question 144

The depth of field is very large, and most SEM pictures have an excellent three-dimensional
appearance.

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Question 145

In using the instrument it is possible to change magnification easily and thus study the
appearance of the surface from very low to very high magnification. K

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Question 146

Differences in very small fossils that were not previously known can now be seen during
routine examination. Surface features of individual grains of minerals such as quartz can be
seen and shown to have many different types of scratches, pitting, and mineral growth.

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Question 147

Some of these features may be useful in telling us the past history of the individual grain. It is
not uncommon to observe other minerals, such as clay flakes, filling the scratches and thus
adding another characteristic that can be useful in comparing the minerals.

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Question 148

X-Ray Diffraction

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Question 149

CHEMICAL METHODS

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Question 150

one of the most important and reliable methods of
identifying the composition of geologic, soil, and other crystalline
substances.

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Question 151

based on the arrangement of atoms, ions, and molecules
within the specimen.

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Question 152

sample is analyzed by passing x-rays through a
crystal and measuring the angle of the diffracted x-rays. Each crystalline
material has its own distinctive x-ray pattern.

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Question 153

pattern of a sample is controlled by the internal
structure of the specimen.

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Question 154

The diffraction pattern can be collected on film,
on an image plate, or by using an electronic detector.

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Question 155

The interpretation
of x-ray patterns under normal situations is a comparatively simple matte

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Question 156

There are at least two avenues for interpreting x-ray diffraction data:

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Question 157

The first involves measuring the d values and intensities and
comparing this information with published lists of data on minerals.

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Question 158

The second involves comparing the x-ray pattern directly with the
pattern produced by a known mineral.

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Question 159

If a comparison between samples is to be made, there are situations in
which the x-ray diffractograms themselves may be compared without
actual identification of the substance.

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Question 160

were analyzed chemically,
they would be identical because both are composed of pure carbon, but x
-ray diffractograms of the two minerals would be quite different.

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Question 161

determining the amounts and types of elements in a sample may rely on the fact that as one of its properties, an
element selectively emits or adsorbs light.

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Question 162

the basis for emission spectroscopy and atomic adsorption spectrophotometry. • Neutron activation analyses is a nondestructive method that produces detection sensitivity of one-billionth of a gram.

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Question 163

requires a nuclear reactor to bombard the sample with neutrons. T

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Question 164

resulting gamma-ray radioactivity is measured to
identify the elements present and their amounts.

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Question 165

This method is expensive to operate and maintain and leaves the samples
radioactive.

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Question 166

Organic compounds contain carbon, and their identification requires different methods.

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Question 167

There are several techniques for
separating out the various compounds in a mixture.

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Question 168

these methods depend on the relative amounts of the gas phase of a compound and the liquid phase under fixed
conditions.

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Question 169

That amount is a characteristic property of each compound. Because those compounds that have a higher tendency
to go to the gas phase will move faster away in each time, the distance they move in that time identifies and separates the
compounds.

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Question 170

Identification is also done using spectrophotometers, which measure the
light-adsorbing properties of a compound.

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Question 171

could uniquely identify compounds if analyzed under proper laboratory procedures. T

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Question 172

instrument
bombards the sample with high-energy electrons, causing the molecules to lose electrons and become positively charged.

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Question 173

The instrument then passes the fragments through an electric or magnetic field where they are separated according to their
masses.

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Question 174

permits the specific identification of the compounds because the distribution of masses is a unique property.

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