Forensic Examination of Soils Flashcards
(174 cards)
1
Q
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.
A
;
2
Q
commonly identifies the original geographic location of soils associated
with a crime, thus assisting an investigation.
A
L
3
Q
EXAMINATION METHODS
A
L
4
Q
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.
A
L
5
Q
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.
A
L
6
Q
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.
A
L
7
Q
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.
A
L
8
Q
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.
A
L
9
Q
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
A
L
10
Q
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.
A
L
11
Q
most important identifying characteristics of minerals and soils.
A
L
12
Q
form a mosaic of grays, yellows, browns,
reds, blacks, and even greens and brilliant purples.
A
L
13
Q
the native minerals contribute directly to the soil color.
A
O
14
Q
Soil particles become stained, coated, and impregnated with mineral and organic substances, giving the soil an appearance
different from its original one.
A
K
15
Q
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.
A
K
16
Q
The mineral grains, especially the larger ones, are generally coated. I
A
K
17
Q
most situations the coatings on the soil particles consist of
iron, aluminum, organic matter, clay, and other substances
A
O
18
Q
The coloring of the coatings alone can give some indication as to the
history of the sample.
A
K
19
Q
The “redness” of a soil depends not only on the
A
K
20
Q
amount of iron present but also on its state of oxidation, with a highly oxidized condition tending to have a
more reddish color.
A
K
21
Q
The iron on the coatings of the particles probably is in the form of hematite, limonite, goethite, lepidocrocite, and other iron-rich mineral forms.
A
I
22
Q
Black mineral colors in the soil are generally related to manganese or various iron and manganese combinations.
A
I
23
Q
Green colors are generally due to
concentrations of specific minerals rather than of the mineral coatings
A
I
24
Q
some copper minerals, chlorite, and glauconite are usually green.
A
I
25
Deep blue to purple coloration in the soil is generally due to the mineral vivianite, an iron phosphate.
O
26
Apart from the mineral colors in the soil are those that result from organic matter.
I
27
The organic litter on the soil surface is generally black.
K
28
Humus percolates
through the mineral horizons, giving various dark colors.
I
29
some instances, the iron and humic acids combine to form a dark reddish brown to nearly black
color.
I
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.
K
31
The color standards are established on three factors: hue, value, and chroma.
O
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.
I
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.
L
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.
I
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.
O
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.
I
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.
I
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.
I
39
Instrumental determination of color provides more quantitative information and can
now be done with a precision which exceeds that the human eye.
I
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; L*a*b* indices; and reflectance
data and curves.
I
41
Calibration of the instrument is carried out for every analysis to black, white, and
CERAM II color standard materials.
I
42
Particle-Size Distribution
I
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:
I
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;
I
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;
I
46
a determination of the distribution of particle sizes may be produced as a
method of comparison.
I
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.
K
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.
O
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.
O
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.
I
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.
I
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.
I
53
Several methods can then be used to determine the size distribution of the finer particles in a dispersed
suspension.
I
54
rapid method for determining the percentage of sand, silt, and clay in a sample
I
55
hydrometer
I
56
based on the principle of a decreasing density of the suspension as the solid particles settle out
I
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.
J
58
pipette method
I
59
One of the most accurate and satisfactory procedures for fractionating soil samples is
O
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.
K
61
based on the rate of settling depends on the size of the mineral matter, with larger particles
settling at a more rapid rate.
I
62
procedure is based on Stokes’ law: V = (2/9)gr2(d − d )/n,
K
63
Although this method is generally considered the most satisfactory regarding accuracy, it is not infallible.
I
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.
I
65
In making a mineral analysis of a sample, one fact becomes
apparent: the sample contains different groups of minerals
within various size ranges.
K
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.
K
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.
I
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.
I
69
two samples
that are to be compared using grain size distribution may not
show the same distribution of grain sizes.
I
70
The method can be
useful in concluding comparison or lack of comparison but is
seldom definitive.
0
71
Stereo Binocular Microscope
L
72
can be a most useful tool to the forensic geologist.
J
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.
U
74
Light microscopes are generally of two types, transmitted light and reflected light.
J
75
light source is placed beneath the specimen, which must be transparent.
I
76
that are used in
studying tissue are of this type.
M
77
have a light source above the object, and the surface features of the particle are viewed.
I
78
Such a microscope is essentially a stationary, higher-power magnifying glass.
I
79
Most of these microscopes have two sets
of lenses, and thus the object is viewed in three dimensions, that is, in stereo.
I
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×.
I
81
The objectives may be individual lenses of fixed
magnification, or in some microscopes a zoom objective is used.
O
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×
I
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.
I
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.
K
85
Objects as small as approximately 10 μm in diameter can be viewed with a
J
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.
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.
N
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.
K
89
The sample tray may have an etched grid that serves a similar purpose.
I
90
Trays are
available with various gummed surfaces that hold the grains in place for ease of counting.
I
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.
I
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.
J
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.
K
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.
K
95
Preliminary examination of the entire sample with the binocular microscope is normally very difficult.
U
96
The mixture of particles of all sizes commonly
obscures the grains and makes identification difficult.
I
97
The presence of organic material contributes to this problem. The sample must be cleaned for the study of minerals and rock.
K
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.
K
99
Treatment with hydrogen peroxide will remove the fine organic matter and clean the sample.
I
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.
I
101
• If the rocks and minerals are of a type that would not be disaggregated by ultrasonic cleaning, the method can be most useful.
I
102
Properties of grains such as shape, rounding, weathering, inclusions, color, and polish can be observed and
recorded.
I
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.
I
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.
J
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.
I
106
Most important is the judgment and caution used by the scientist, whatever method is used.
K
107
Petrographic Microscope
K
108
differs in detail from an ordinary compound microscope.
J
109
However, its primary
function is the same: to produce an enlarged image of an object placed on the stage.
J
110
The magnification is produced by a combination of two sets of lenses, the objective and the ocular.
J
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.
I
112
The ocular lens merely enlarges this image. For mineralogical work, three objectives—low, medium, and
high power—are normally used.
K
113
The magnification produced by objectives is usually 2× (low), 10× (
medium), and 50× (high).
I
114
Oculars have different magnifications, usually 5×, 7×, 10×, 15×, and 20×.
I
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×
K
116
contain a crosshair that is useful for locating grains under high power when changing
objectives.
I
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.
I
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.
I
119
Above the stage a second, removable polarizing filter is placed in the tube of the microscope.
J
120
transmits light usually in an E-W
(left-to-right) direction.
I
121
When the upper filter is inserted, light is blocked out from passing through the microscope.
K
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.
O
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.
K
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.
I
125
• If the mineral has a higher refractive index, the Becke line will move into the grain.
K
126
the liquid of known refractive index is higher, the Becke line will move away from the grain into the liquid.
O
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.
J
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.
K
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.
K
130
thin section is a thin slice of rock mounted on a glass slide.
J
131
The slice is normally 30 μm in
thickness and may be prepared from a solid rock or loose material impregnated with plastic.
I
132
The
rock is cut with a diamond saw and the surface polished.
I
133
This polished surface is cemented to a glass microscope slide with an adhesive of known
refractive index such as epoxy or Canada balsam.
I
134
saw cut is then made parallel to the glass,
leaving a wafer of rock cemented on the slide.
I
135
Grinding of the wafer proceeds until it is thinned to approximately 30 μm.
I
136
thin class cover is
then glued onto the polished rock surface to protect the rock and improve viewing with the
microscope.
I
137
Most rocks are transparent at this thickness and can be viewed in transmitted light.
J
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.
K
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.
K
140
Scanning Electron Microscope
‘
141
wide range of magnifications, generally from
25× to over 650,000× and can record something as small as 1.5 nm.
L
142
has the advantage that the surface of a sample may be viewed directly.
L
143
However, an
ultrathin coating of carbon or gold plated on the specimen improves the quality of the picture.
O
144
The depth of field is very large, and most SEM pictures have an excellent three-dimensional
appearance.
O
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
J
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.
I
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.
K
148
X-Ray Diffraction
I
149
CHEMICAL METHODS
K
150
one of the most important and reliable methods of
identifying the composition of geologic, soil, and other crystalline
substances.
K
151
based on the arrangement of atoms, ions, and molecules
within the specimen.
K
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.
I
153
pattern of a sample is controlled by the internal
structure of the specimen.
I
154
The diffraction pattern can be collected on film,
on an image plate, or by using an electronic detector.
I
155
The interpretation
of x-ray patterns under normal situations is a comparatively simple matte
I
156
There are at least two avenues for interpreting x-ray diffraction data:
I
157
The first involves measuring the d values and intensities and
comparing this information with published lists of data on minerals.
I
158
The second involves comparing the x-ray pattern directly with the
pattern produced by a known mineral.
I
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.
K
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.
K
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.
I
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.
I
163
requires a nuclear reactor to bombard the sample with neutrons. T
I
164
resulting gamma-ray radioactivity is measured to
identify the elements present and their amounts.
I
165
This method is expensive to operate and maintain and leaves the samples
radioactive.
I
166
Organic compounds contain carbon, and their identification requires different methods.
I
167
There are several techniques for
separating out the various compounds in a mixture.
I
168
these methods depend on the relative amounts of the gas phase of a compound and the liquid phase under fixed
conditions.
K
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.
K
170
Identification is also done using spectrophotometers, which measure the
light-adsorbing properties of a compound.
I
171
could uniquely identify compounds if analyzed under proper laboratory procedures. T
K
172
instrument
bombards the sample with high-energy electrons, causing the molecules to lose electrons and become positively charged.
J
173
The instrument then passes the fragments through an electric or magnetic field where they are separated according to their
masses.
I
174
permits the specific identification of the compounds because the distribution of masses is a unique property.
K
