JE

EESB15 - Earth History Course: 4.5(6) Billion Years - The Big Riddle

Earth History and Geologic Time

  • Formation of Earth: 4.6\ \text{billion years ago}
  • Oldest fossil: around 3\ \text{billion years ago}
  • Evolution of cells with nucleus (eukaryotes): around 2\ \text{billion years ago}
  • Dinosaur extinction: around 65\ \text{million years ago}
  • Major time units: Precambrian, Paleozoic, Mesozoic, Cenozoic; current epoch: Holocene
  • Key geologic time scale elements: Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylavian, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, Quaternary
  • Focus: interpret geologic record and relate past processes/events to the present environment

Earth Materials: Minerals, Rocks, and Rock Formation

  • Minerals: naturally occurring, inorganic, solid crystalline substances with definite chemical composition and orderly internal arrangement
  • Rocks: aggregate of minerals
  • Three major rock types and formation processes:
    • Igneous: cooling of magma/lava
    • Sedimentary: weathering/erosion -> transport -> lithification of sediments
    • Metamorphic: alteration by heat, pressure, and fluids
  • Building blocks: elements -> minerals -> rocks
  • Common building materials: silica (SiO₂), oxygen (O), silicon (Si), etc., used in daily life (glass, etc.)

Mineral Basics and Identification

  • Minerals grow by precipitation from oversaturated fluids or cooling
  • Properties used for identification and use: physical properties, formation environment, associated rocks
  • Common mineral uses: Quartz (SiO₂) for glass; Halite (NaCl) for salt; Mica for cosmetics; Garnet for jewelry

Mineral Properties: Diagnostic Tools

  • Color and Clarity/Diaphaneity (transparent, translucent, opaque)
  • Streak (color of powder when scratched on porcelain)
  • Luster (metallic vs non-metallic; vitreous/glassy)
  • Hardness (Mohs scale) — resistance to scratching
  • Cleavage (breaks along planes) vs Fracture (breaks irregularly)
  • Magnetism, reaction to acid (HCl), taste, tenacity, feel, gravity/density
  • Crystal habit (euhedral vs anhedral)
  • Internal atomic arrangement drives all properties

Mineral Internal Structure and Why It Matters

  • Layered sheets in some minerals (e.g., biotite) lead to:
    • High heat resistance (e.g., oven windows)
    • Cleavage along planes
    • Flexibility vs brittleness depending on structure
  • Exsolution color patterns and other internal features affect appearance and properties

Mineral Color, Clarity, and Streak Details

  • Color is often not diagnostic alone; can indicate chemistry
  • Clarity/diaphaneity indicates how light passes through
  • Streak: color of powdered mineral on porcelain plate

Mineral Luster, Cleavage, and Fracture

  • Luster: metallic vs non-metallic (dull, glassy)
  • Cleavage: how a mineral splits along planes of weakness
  • Fracture: breaks without cleavage planes (e.g., conchoidal)

Mineral Habit and Growth

  • Euhedral: grew with ample space -> well-formed crystal faces
  • Anhedral: restricted growth -> imperfect faces

Mohs Scale: Hardness and Testing

  • Hardness range: 1-10
  • Harder minerals can scratch softer ones
  • Common reference values:
    • Talc ~1, Gypsum ~2, Calcite ~3, Fluorite ~4, Apatite ~5, Orthoclase ~6, Quartz ~7, Topaz ~8, Corundum ~9, Diamond ~10
  • Testing caveats: weathering surface, cleavage/fractures can bias results

Common Rock-forming Silicate Minerals (Key Examples)

  • Quartz: ext{SiO}_2; Hardness 7; no cleavage; conchoidal fracture
  • Orthoclase (K-feldspar): ext{KAlSi}3 ext{O}{8}; Hardness 6; two-direction cleavage; pink/white
  • Plagioclase feldspar: Na-Ca plagioclase series; Hardness 6-6.5; two-direction cleavage; striations on cleavage planes
  • Muscovite (mica): ext{KAl}2( ext{AlSi}3 ext{O}{10})( ext{OH})2; Hardness 2-3; perfect basal cleavage; light-colored
  • Biotite (mica): ext{K(Mg,Fe)}3 ext{(AlSi}3 ext{O}{10})( ext{OH})2; Hardness 2-3; dark color; perfect cleavage
  • Pyroxene group: Hardness 5-6; cleavage in two directions at ~90°
  • Amphibole group: Hardness 5-6; cleavage at ~60° and 120°
  • Olivine: Hardness 6.5-7; no cleavage; green
  • Garnet: Hardness 6.5-7; no cleavage; usually glassy crystals

Silicate Mineral Table (Overview)

  • Silicate minerals are the main constituents of many rocks and vary by composition and structure. Common groups include quartz, feldspars (orthoclase and plagioclase), micas (biotite, muscovite), pyroxenes, amphiboles, olivine, and garnet. Composition ranges from pure SiO₂ to complex aluminosilicates with Mg, Fe, Ca, Na, K, and H₂O; physical properties (hardness, cleavage, gravity) reflect internal structure.

Everyday Earth Materials: From Elements to Minerals

  • Elements form minerals in defined geometric order
  • Minerals are inorganic, solid, crystalline with definite composition
  • The crust is enriched in light elements (e.g., Si, O); the bulk Earth contains heavier elements (e.g., Fe, Mg)

Quick Practice Cues (Study Prompts)

  • What defines a mineral? What properties are most diagnostic for ID?
  • How do rock types form, and how do they relate to the geologic record?
  • Why do minerals have specific physical properties?
  • How does the internal structure influence mineral hardness and cleavage?

Notes on Study Flow and Course Tasks

  • Lecture 1 emphasizes: origins of Earth materials, mineral groups, rock-forming minerals, and mineral identification
  • Practice includes: online modules, lab guides, and quizzes to reinforce mineral properties and rock formation concepts
  • Focus for quick recall: core definitions, major mineral groups, basic Mohs scale values, and the rock cycle