Freeman-Lynde GEOL1122 Minerals & Rocks

Minerals & Rocks

THE COMPOSITION OF MATTER

ATOMS AND MOLECULES
- the smallest particles that retain the chemical characteristics of an element (atoms), or a compound (molecules)
  - MORE THAN 100 ELEMENTS, although ONLY ABOUT 90 are NATURALLY OCCURRING
- molecules are 2 or more atoms bonded together
  - CAN BE the SAME ELEMENT
  - MOSTLY DIFFERENT ELEMENTS to form compounds
ATOMIC STRUCTURE
- Nucleus - dense positively-charged protons & electrically-neutral neutrons
- Electrons - negatively-charged particles with little mass orbiting the nucleus at various energy levels (shells)
  - EACH SHELL HOLDS ONLY A CERTAIN NUMBER OF ELECTRONS
- Nearly all of the mass is in the nucleus & the electrical charge is balanced by the protons & electrons
- Atomic number - the number of protons in the nucleus; determines the element
- Atomic mass - the number of protons plus neutrons in the nucleus
  - PROTONS & NEUTRONS EACH HAVE A MASS OF 1
  - ISOTOPES - DIFFERENT ATOMS OF A GIVEN ELEMENT HAVING DIFFERENT ATOMIC MASSES (same # of protons, different # of neutrons)
BONDING
- Chemical bonds are electrical in nature & involve electrons in the outermost shell
- Octet rule - one atom bonds with another atom so that each bonded atom contains 8 (or 2 for the 1st shell) electrons in its outermost shell, thereby filling the outermost shell
  - ELECTRONS CAN BE GAINED, LOST OR SHARED
  - ELEMENTS that ALREADY HAVE a FILLED OUTERMOST SHELL (noble gases) are CHEMICALLY INERT
- Ionic bonds
  - ELECTRONS GAINED AND LOST
  - ATOMS are ELECTRICALLY CHARGED - they have become IONS
  - BONDING occurs BY ELECTROSTATIC ATTRACTION of negatively- and positively-charged ions
  - RESULTING MOLECULE is ELECTRICALLY-NEUTRAL
- Covalent bonds
  - ELECTRONS ARE SHARED
  - SHARING is NOT ALWAYS EQUAL

MINERALS

DEFINITIONS
- naturally occurring, inorganic solids with specific chemical compositions & definite internal structures
- Crystals - orderly (regular, repeating) arrangement of atoms chemically bonded together
  - EACH MINERAL has a PARTICULAR CRYSTAL STRUCTURE
    - polymophs are different minerals with the same chemical composition (for example calcite & aragonite [CaCO₃])
PHYSICAL PROPERTIES OF MINERALS
- crystal form, color, hardness, density (specific gravity), cleavage, fracture, luster, streak
MINERAL GROUPS
- Only 20-25 minerals are important as rock-forming minerals in the crust (out of >4000)
- Only 8 elements make up most of these minerals (represent >98% of the continental crust)
  - Oxygen (O) - 46.6%; Silicon (Si) - 27.7%; Aluminum (Al) - 8.1%; Iron (Fe) - 5.0%; Calcium (Ca) - 3.6%; Sodium (Na) - 2.8%; Potassium (K) - 2.6%; and Magnesium (Mg) - 2.1%
- Most rock-forming minerals are silicates
SILICATE STRUCTURE
- Basic building block - Silica Tetrahedron - SiO₄^-4 (a complex ion)
- Silica tetrahedra can be linked into single & double chains, sheets, 3-D networks by sharing oxygen atoms
  - the RATIO OF O TO SI VARIES
  - ISOLATED TETRAHEDRA - 4:1, CHAINS - 3:1, 3-D - 2:1
- Positive ions are plugged into the silicate structure to make electrically-neutral compounds
- These ions have different sizes - determine mineral structure
  - IONS OF about the SAME SIZE can SUBSTITUTE for each other WITHOUT ALTERING MINERAL STRUCTURE
- Mineral series - array of different compositions of a particular mineral
  - OLIVINE - (Mg, Fe)₂SiO₄
  - PLAGIOCLASE FELDSPAR - CaAl₂Si₂O₈ to NaAlSi₃O₈
IMPORTANT SILICATE MINERALS
- Olivine - (Mg, Fe)₂SiO₄ - single tetrahedra
- Pyroxene - (Mg, Fe)SiO₃ - single chain
- Amphibole - (Ca₂Mg₅)₂Si₈O₂₂(OH)₂ - double chain
- Mica - KAl₃Si₃O₁₀(OH)₂ (muscovite) & K(Mg, Fe)₃Si₃O₁₀(OH)₂ (biotite) - sheet
- Feldspar -KAlSi₃O₈ (orthoclase) & NaAlSi₃O₈/CaAl₂Si₂O₈ (plagioclase) - 3-D network
- Quartz - SiO₂ - 3-D network
- Ferromagnesian minerals - contain iron & magnesium
  - OLIVINE, PYROXENE, AMPHIBOLE, BIOTITE, & GARNET
- Non-ferromagnesian minerals - don't contain iron and magnesium
  - MUSCOVITE, FELDSPAR, QUARTZ, & CLAY
NON-SILICATE MINERALS
- Carbonates - calcite, dolomite
- Sulphates - gypsum
- Sulfides - pyrite, galena
- Oxides - magnetite, hematite
- Native elements - gold, silver, copper
- Halides - halite (table salt)
- Hydroxides
- Phosphates

ROCKS

INTRO
- a rock is an aggregate of one or more minerals
- 3 types: igneous, sedimentary & metamorphic
IGNEOUS ROCKS
- Form by the solidification & crystallization of magma
- Magma - naturally occurring molten rock originating up to 200 km deep (in the aesthenosphere)
  - LAVA - MAGMA ERUPTED AT the SURFACE
- Plutonic igneous rocks form when magma remains & cools in the subsurface
- Volcanic igneous rocks form when magma erupts & cools at the surface
- Igneous Textures (overall appearance based on the size & arrangement of interlocking crystals)
  - GREATLY AFFECTED BY RATE OF COOLING
    - slow cooling = large crystals; fast cooling = small crystals
  - APHANITIC = FINE GRAINED (fast cooling - small crystals cannot be seen with the naked eye)
    - characteristic of volcanic rocks at surface or small shallow intrusives
  - PHANERITIC = COARSE GRAINED (slow cooling - individual crystals can be seen with the naked eye), most crystals about the same size
    - characteristic of plutonic rocks cooled far below the surface (& subsequently exposed by erosion)
  - PHORPHYRITIC = LARGE CRYSTALS (phenocrysts) IN a matrix of SMALLER CRYSTALS (groundmass)
    - results from differential crystallization followed by eruption of magma + large crystals to surface and subsequent rapid cooling of the lava
  - GLASSY = NON-CRYSTALLINE SOLID (extremely rapid cooling)
- Igneous Rock Names
  - GRANITE - plutonic (volcanic equivalent is rhyolite)
    - K feldspar + quartz (minor biotite, amphibole & Na plagioclase) - light gray to pink
  - BASALT - volcanic (plutonic equivalent is gabbro)
    - Ca rich plagioclase + pyroxene (minor olivine & amphibole) - dark colored
  - MELTING TEMPERATURE: BASALTIC MAGMA - ~1200^oC, GRANITIC MAGMA - ~800^oC

SEDIMENTARY ROCKS

Form by lithification of sediments
Lithification = Compaction and Cementation
- COMPACTION - IMPORTANT ONLY IN FORMATION OF SHALE (MUDS COMPRESS UP TO 40%)
  - Cementation - material precipitated from water into pores

Clastic Sedimentary Rocks

MECHANICAL BREAKUP OF OTHER ROCKS
TRANSPORTED TO SITE OF DEPOSITION BY WATER, ICE OR WIND UNDER THE INFLUENCE OF GRAVITY

SEDIMENTARY PARTICLE SIZES

Name	Grain Size	Comment
Gravel	>2 mm
Sand	0.064 to 2 mm
Silt	0.004 to 0.064 mm [4-64 microns]	also called mud
Clay	<0.004 mm [<4 microns]	also called mud

SHALE = CLAY- + SILT-SIZED SEDIMENTS DEPOSITED IN QUIET WATER
SANDSTONE = SAND-SIZED SEDIMENTS
CONGLOMERATE = GRAVEL-SIZED SEDIMENTS

Chemical/Biogenic Sedimentary Rocks
- ORGANIC OR INORGANIC PRECIPITATION OF SEDIMENTS
- LIMESTONE - CaCO₃
  - reef limestone - built by corals (and other critters in other eras)
  - coquina - poorly cemented large shells and shell fragments
  - chalk - poorly cemented microscopic shells
- DOLOSTONE - altered limestone composed of dolomite [MgCa(CO₃)₂]
- CHERT - hard, dense rock consisting of microcrystalline silica (SiO₂)
- EVAPORITES - form during evaporation of sea water (rock salt - mostly halite [NaCl] & rock gypsum - mostly gypsum[CaSO₄^.2H₂0])
- COAL - carbon rich

METAMORPHIC ROCKS
- Change in Form

SEDIMENTARY, IGNEOUS, (METAMORPHIC ROCKS) -> HEAT, PRESSURE, CHEMICALLY-ACTIVE FLUIDS -> METAMORPHIC ROCKS

Changes are Textural ± Mineralogical
- INCREASE IN GRAIN SIZE IN FINE-GRAINED ROCKS (10 microns - 1 cm)
- DEVELOPMENT OF FOLIATION IN FINE-GRAINED ROCKS
  - alignment of platey minerals to direction of least pressure
  - flattening of non-platey minerals
- GROWTH OF MICAS
- GROWTH OF PORPHYROBLASTS - Garnet, Staurolite, Sillimanite
Setting - Deep in Earth with an Increase in Temperature and Pressure
- TEMPERATURE INCREASES WITH DEPTH (20^o-30^oC/km = geothermal gradient)
- at 20 km, temperatures are 400^o-600^oC
- PRESSURE ALSO INCREASES WITH DEPTH (250 bars/km [1 bar = 1 atmosphere])
Types of Metamorphism
- REGIONAL - LARGE AREA affected by MOUNTAIN BUILDING
- CONTACT - ASSOCIATED WITH an individual PLUTON OR FLOW
Common Metamorphic Rocks
- SLATE - MINUTE MICA flakes & slately CLEAVAGE
- PHYLLITE - larger but still UNIDENTIFIABLE MICA & GLOSSY SHEEN
- SCHIST - VISIBLE MICA (or other flattish minerals) & strong SCHISTOSITY
  - Amphibolites result from intermediate to high grade metamorphism of basalt
- GNEISS - banded metamorphic rocks bearing ELONGATED, BUT GRANULAR, MINERALS like quartz & feldspar
  - granitic in composition
- MARBLE - RECRYSTALLIZED LIMESTONE
- QUARTZITE - RECRYSTALLIZED SANDSTONE
- MIGMATITES - PARTIALLY-MELTED METAMORPHIC rocks, get granite intrusion
Index Minerals
- DIFFERENT MINERALS INDICATING DIFFERENT METAMORPHIC CONDITIONS, OR GRADE
- Chlorite, Muscovite, Biotite, Garnet, Staurolite, Sillimanite
  
  Low Grade
  ->
  High Grade

THE ROCK CYCLE

TRANSFORMATION OF 3 BASIC ROCK TYPES FROM ONE TO ANOTHER
- Igneous rocks - initially molten, solidifies, then uplifted & eroded
- Sedimentary rocks - weathering products of igneous (+ sedimentary + metamorphic) rocks
- Metamorphic rocks - deep burial of sedimentary (+ igneous + metamorphic) rocks
- Very deep burial = high temperatures - metamorphic -> igneous rocks

Back to GEOL1122 Course Notes Home page

Chlorite,	Muscovite,	Biotite,	Garnet,	Staurolite,	Sillimanite
Low Grade		->			High Grade