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