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 circumstances

Forensic Geology Important Forensic properties Mineral content Rock content Plant matter Animal matter Artificial material

Forensic Geology Uses Vehicle Accidents Rape/Assault Burglary Vehicles frequently strike natural objects Rape/Assault Can be useful if crime occurs outdoors Burglary 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 Fossils Man-made components Color of material

Geologic Terminology Geology Mineralogy Petrology Paleontology The study of the Earth and its processes Mineralogy Study of minerals Petrology Study of rocks Paleontology 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) Solid 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 Rock 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 Igneous Sedimentary The direct result of volcanic processes Sedimentary The result of weathering and erosion of other rocks Metamorphic The result of intense heating or pressure of other existing rocks

Mineral and Rock Identification Minerals are largely identified by specific physical and chemical properties 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 setting) Quartz is the most common mineral on Earth 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 properties Mohs Scale Used as a standard 1-10 Field Hardness Scale Uses approximations of common items Fingernail = 2.5 Penny = 3 Glass = 5.5 Steel = 7

Forensic Mineralogy Essentials Density Determined 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 Luster Appearance of reflected light from the surface of the mineral Main classifications are metallic and non-metallic Non-metallic subcategories Glassy Resinous Fibrous Waxy Earthy/dull Brilliant Pearly

Forensic Mineralogy Essentials Color 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 Streak 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 Quartz A glassy, hard crystal Often looks like broken glass Can appear in many colors Will easily scratch glass

Forensic Mineralogy Essentials Feldspar Fleshy appearance Slightly softer than quartz Will often have up to 4 flat sides

Forensic Mineralogy Essentials Mica Very soft – usually found in flakes Flat and shiny Has two varieties 1 – Coppery color (more common) 2 – Gloss black

Forensic Mineralogy Essentials Calcite 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 Granite 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 grey Will fizz with HCl or vinegar Metamorphic version is marble (uncommon) 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 “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 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 investigation 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 gravels/sands Most rocks will be sedimentary or their metamorphic versions Sandstone, limestone shale are common Many artificially introduced minerals/rocks