trace evidence 1 forensic geology “life is hard. then you die. then they throw dirt in your face....
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Trace Evidence 1Forensic 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 GeologyThe legal application
of earth and soil science
Almost always an issue of “transfer”
Can be individualized under the right circumstances
Forensic GeologyImportant Forensic properties
Mineral contentRock contentPlant matterAnimal matterArtificial material
Forensic Geology UsesVehicle Accidents
Vehicles frequently strike natural objects
Rape/AssaultCan be useful if crime occurs
outdoorsBurglary
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 cloth”
“Dirt on shoes can often tell us more about where the wearer of those shoes had last been than toilsome inquiries.”
-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?
Unusual mineral/rock components
FossilsMan-made
componentsColor of
material
Geologic TerminologyGeology
The study of the Earth and its processes
MineralogyStudy of minerals
PetrologyStudy of rocks
PaleontologyStudy of the Earth’s past
Minerals and RocksTo be considered a mineral, 5
requirements must be metNaturally occurringInorganic (Exception: material formed
by the activity of animals...pearls)SolidDefinite 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 RocksRock
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 RocksRocks come in three major typesIgneous
The direct result of volcanic processes
SedimentaryThe result of weathering and
erosion of other rocksMetamorphic
The result of intense heating or pressure of other existing rocks
Mineral and Rock IdentificationMinerals are largely identified by
specific physical and chemical properties
Rocks are largely identified by physical appearanceProperties can vary since there’s no
specific “formula” for a rockExample: Granite
Mineral and Rock IdentificationIn general (not considering
geologic setting)Quartz is the most common mineral
on EarthMost 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 properties
Mohs ScaleUsed as a standard1-10
Field Hardness ScaleUses approximations of common
itemsFingernail = 2.5Penny = 3Glass = 5.5Steel = 7
Forensic Mineralogy Essentials
DensityDetermined by relative chemical
composition and closeness of atoms in crystal
Most rock forming minerals: 2.0 - 3.0 g/cm3
Most metallics: >5.0 g/cm3
Forensic Mineralogy Essentials
LusterAppearance of reflected light from the
surface of the mineralMain classifications are metallic and non-
metallicNon-metallic subcategories
GlassyResinousFibrousWaxyEarthy/dullBrilliantPearly
Forensic Mineralogy Essentials
ColorNot a reliable property since small
impurities can change color (Corundum)
Only a few minerals occur in one colorGeneral guesses about
composition can be made based on colorsDark (black, greys, greens, etc.) -
contain metals, FeLight (tans, clears, reds) - contain Si
or Al
Forensic Mineralogy Essentials
StreakColor of the mineral in a powdered
formFound by rubbing the mineral
across a streak plateStreak the same regardless of
mineral color differencesReliable for hardnesses of ~7 or
less
Forensic Mineralogy Essentials
QuartzA glassy, hard crystalOften looks like broken glassCan appear in many colorsWill easily scratch glass
Forensic Mineralogy Essentials
FeldsparFleshy appearanceSlightly softer than quartzWill often have up to 4 flat sides
Forensic Mineralogy Essentials
MicaVery soft – usually found in flakesFlat and shiny Has two varieties1 – Coppery color (more common)2 – Gloss black
Forensic Mineralogy Essentials
CalciteChief mineral in limestoneGlassy to white blocky crystalsSlightly harder than a fingernailWill fizz in HCl
Forensic Mineralogy Essentials
Hornblende and PyroxeneHard, dark (dk. Green to black)
mineralsAlmost always blockyUnusual in most settings
Forensic Petrology Essentials
GraniteMost common igneous rockCombination of light and dark
minerals discussed earlier
Forensic Petrology Essentials
Sandstone (sed.) and quartzite (meta.)Typically quartz and feldspar
combo.Looks like sandQuartzite will have similar color but
sand grains will be smashed/fused together
Forensic Petrology Essentials
Limestone (sed.)Limestone is almost always a shade
of greyWill fizz with HCl or vinegarMetamorphic version is marble
(uncommon)Also look for calcite veins –
limestone is composed of calcite
Forensic Petrology Essentials
Shale (sed.) and slate (meta.)Very smooth appearanceObvious layeringUsually dark grey but also brick red
or olive greenShale is very brittleSlate will be same color but not
brittle
Forensic Petrology Essentials
“Sand”Caution should be taken when using
this term“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
particlesOften mica-rich (look for the
“shine”)Very soft and brittleUsually a stream deposit
Mineral and Rock IdentificationGeologic Setting
The sum total of geologic conditions (past and present) for a particular area
Absolutely essential in any geologic investigation
Greatly assists in including or eliminating possible geologic “species”
UD Geologic SettingLight colored minerals are common
Quartz, feldspar and micasMicas especially common in stream
gravels/sandsMost rocks will be sedimentary or
their metamorphic versionsSandstone, limestone shale are
commonMany artificially introduced
minerals/rocks
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