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Trace Evidence 1

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”

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Trace Evidence 1

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  1. 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

  2. Forensic Geology • The legal application of earth and soil science • Almost always an issue of “transfer” • Can be individualized under the right circumstances

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

  4. Forensic Geology Uses • Vehicle Accidents • Vehicles frequently strike natural objects • Rape/Assault • Can be useful if crime occurs outdoors • Burglary • Properties often have flowerbeds, etc. beneath common entry points

  5. 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

  6. 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

  7. What is Forensically Valuable? • Unusual mineral/rock components • Fossils • Man-made components • Color of material

  8. Geologic Terminology • Geology • The study of the Earth and its processes • Mineralogy • Study of minerals • Petrology • Study of rocks • Paleontology • Study of the Earth’s past

  9. 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

  10. 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)

  11. Minerals and Rocks • Rocks come in three major types • Igneous • 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

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

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

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

  22. Forensic Mineralogy Essentials • Calcite • Chief mineral in limestone • Glassy to white blocky crystals • Slightly harder than a fingernail • Will fizz in HCl

  23. Forensic Mineralogy Essentials • Hornblende and Pyroxene • Hard, dark (dk. Green to black) minerals • Almost always blocky • Unusual in most settings

  24. Forensic Petrology Essentials • Granite • Most common igneous rock • Combination of light and dark minerals discussed earlier

  25. 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

  26. 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

  27. 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

  28. 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

  29. 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

  30. 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”

  31. 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

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