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Trace Evidence 1
Forensic Geology
“Life is hard. Then you die.
Then they throw dirt in your face. Then the
worms eat you.
Be grateful it happens in that order.”
—David Gerrold
Forensic Geology
 The legal application of
earth and soil science
 Almost always an issue
of “transfer”
 Can be individualized
under the right
Forensic Geology
Important Forensic properties
Mineral content
Rock content
Plant matter
Animal matter
Artificial material
Forensic Geology Uses
Vehicle Accidents
Vehicles frequently strike natural objects
Can be useful if crime occurs outdoors
Properties often have flowerbeds, etc.
beneath common entry points
History of Forensic Geology
 1887–1893
 Sir Arthur Conan Doyle
 Several Sherlock Holmes cases
suggested the possible use of soil
in criminal investigations
 1893
 Hans Gross considered to be the
first criminalist
 First manual included the study of
“dust, dirt on shoes and spots on
“Dirt on shoes can
often tell us more about
where the wearer of
those shoes had last
been than toilsome
-Hans Gross
History of Forensic Geology
 1910
 Edmond Locard
 Was interested in the fact
that dust was transferred
from the crime scene to
the criminal
 Helped to establish his
Exchange Principle
Edmond Locard
What is Forensically Valuable?
Color of material
Geologic Terminology
The study of the Earth and its processes
Study of minerals
Study of rocks
Study of the Earth’s past
Minerals and Rocks
To be considered a mineral, 5
requirements must be met
Naturally occurring
Inorganic (Exception: material formed by
the activity of animals...pearls)
Definite chemical structure which provides
for specific physical properties
Recurring atomic structure (crystal)
~4000 exist but only a few dozen are
found in large quantities
Minerals and Rocks
An group of minerals combined together
Each mineral found in the rock keeps its
original properties
A few rocks contain only one primary
mineral (calcite – limestone)
Minerals and Rocks
Rocks come in three major types
The direct result of volcanic processes
The result of weathering and erosion of
other rocks
The result of intense heating or pressure of
other existing rocks
Mineral and Rock Identification
Minerals are largely identified by
specific physical and chemical
Rocks are largely identified by
physical appearance
Properties can vary since there’s no
specific “formula” for a rock
Example: Granite
Mineral and Rock Identification
In general (not considering geologic
Quartz is the most common mineral on
Most earth samples will contain only 3-5
different minerals and rocks
75% of anything picked up will be a
sedimentary rock
Forensic Mineralogy Essentials
Among the most useful and reliable of
Mohs Scale
Used as a standard
Field Hardness Scale
Uses approximations of common items
Fingernail = 2.5
Penny = 3
Glass = 5.5
Steel = 7
Forensic Mineralogy Essentials
Determined by relative chemical
composition and closeness of atoms in
Most rock forming minerals: 2.0 - 3.0
Most metallics: >5.0 g/cm3
Forensic Mineralogy Essentials
 Luster
 Appearance of reflected light from the surface of
the mineral
 Main classifications are metallic and nonmetallic
 Non-metallic subcategories
 Glassy
 Resinous
 Fibrous
 Waxy
 Earthy/dull
 Brilliant
 Pearly
Forensic Mineralogy Essentials
Not a reliable property since small
impurities can change color (Corundum)
Only a few minerals occur in one color
General guesses about composition
can be made based on colors
Dark (black, greys, greens, etc.) - contain
metals, Fe
Light (tans, clears, reds) - contain Si or Al
Forensic Mineralogy Essentials
Color of the mineral in a powdered form
Found by rubbing the mineral across a
streak plate
Streak the same regardless of mineral
color differences
Reliable for hardnesses of ~7 or less
Forensic Mineralogy Essentials
A glassy, hard crystal
Often looks like broken glass
Can appear in many colors
Will easily scratch glass
Forensic Mineralogy Essentials
Fleshy appearance
Slightly softer than quartz
Will often have up to 4 flat sides
Forensic Mineralogy Essentials
Very soft – usually found in flakes
Flat and shiny
Has two varieties
1 – Coppery color (more common)
2 – Gloss black
Forensic Mineralogy Essentials
Chief mineral in limestone
Glassy to white blocky crystals
Slightly harder than a fingernail
Will fizz in HCl
Forensic Mineralogy Essentials
Hornblende and Pyroxene
Hard, dark (dk. Green to black) minerals
Almost always blocky
Unusual in most settings
Forensic Petrology Essentials
Most common igneous rock
Combination of light and dark minerals
discussed earlier
Forensic Petrology Essentials
Sandstone (sed.) and quartzite (meta.)
Typically quartz and feldspar combo.
Looks like sand
Quartzite will have similar color but sand
grains will be smashed/fused together
Forensic Petrology Essentials
Limestone (sed.)
Limestone is almost always a shade of
Will fizz with HCl or vinegar
Metamorphic version is marble
Also look for calcite veins – limestone is
composed of calcite
Forensic Petrology Essentials
Shale (sed.) and slate (meta.)
Very smooth appearance
Obvious layering
Usually dark grey but also brick red or
olive green
Shale is very brittle
Slate will be same color but not brittle
Forensic Petrology Essentials
Caution should be taken when using this
“Sand” is a general term that describes
grain size, not specific mineral content
Gravel > > Sand > > Silt > > Clay
Forensic Petrology Essentials
Siltstone (sed.)
Similar to sandstone but smaller particles
Often mica-rich (look for the “shine”)
Very soft and brittle
Usually a stream deposit
Mineral and Rock Identification
Geologic Setting
The sum total of geologic conditions (past
and present) for a particular area
Absolutely essential in any geologic
Greatly assists in including or eliminating
possible geologic “species”
UD Geologic Setting
Light colored minerals are common
Quartz, feldspar and micas
Micas especially common in stream
Most rocks will be sedimentary or their
metamorphic versions
Sandstone, limestone shale are common
Many artificially introduced