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Thin-Section Petrography of Ceramics

Clear epoxy

Blue epoxy

Plane Polarized Light

• Use transmitted light (light comes from below through the specimen

• Add a polarizer to filter out all light except that vibrating in ONE direction

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MINERAL FEATURES IN PLANE POLARIZED LIGHT

• Transparency or Opacity• Color• Refractive Index• Relief (how visible it is)• Morphology (size and shape)• Cleavage (breakage pattern)

TRANSPARENT OPAQUE

• Some light is transmitted through the grain

• The grain is either clear or colored in plane polarized light

• Does not transmit any visible light

• Black in plane polarized light

• Need reflected light for further identification

Transparent Chlorite and Quartz with Opaque Magnetite

COLOR

• May be a distinctive feature

• Minerals may be deeply colored OR have pale, subtle tints

Glaucophane

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Pleochroism in BiotiteREFRACTIVE INDEX (n)

• Measure of speed with which light passes through the crystal

• Usually simply measure relative to the mounting medium (1.54 for thin sections) or to quartz (also 1.54) using Becke line test

garnet: n = 1.72-1.89quartz: n = 1.54-1.55epoxy: n = 1.54

Garnet has high reliefQuartz has low relief

RELIEF

• A measure of the relative difference in n between a mineral grain and its surroundings

• Determined visually in ppl, used to estimate n

Grains with low relief

High relief

REFRACTIVE INDEX (n)

• Isotropic crystals have only 1 n

• Anisotropic minerals have 2 or 3 in different directions within the crystal (often almost exactly the same, occasionally very different as with calcite)

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Calcite varies in relief depending on degree of microscope stage rotation

MORPHOLOGY

• Grain Size

• Grain Shape

6-sided grain of olivine Large Blobby Quartz and Small Radiating Fans of Chlorite

Platy tablets and elongated grains of biotite micaCLEAVAGE

• Ability of a mineral to separate into smaller and smaller particles along a smooth plane surface

• Example: Micas have 1 good cleavage, peels off in perfect layers

• In thin section, mica cleavage planes appear as parallel lines if grain is oriented side up; if looking down onto the top surface of the grain it will just be very flat

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Mica: side orientation showing single good cleavage Mica from top, smooth surface

Cleavage Variables

• Presence/absence of cleavage

• Number of cleavage directions

• Angle between two cleavage directions

• Cleavage orientation relative to morphology (parallel to short or long direction)

• Cleavage quality (good or poor)

Two Cleavages at 90o, One Higher Quality

Two Cleavages, 60/120o

CROSSED POLARIZED LIGHT (cpl)

• Uses two polarizing plates with their allowed planes of vibration at right angles to each other

• Creates a different set of optical properties thatcan be observed in thin sections

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ISOTROPIC

• Random or uniform arrangement of atoms in all directions

• Light moves with equal velocity in all directions, so there is only 1 refractive index (n)

• Dark in crossed polars

Garnet, ppl

Garnet, cpl

ANISOTROPIC

• Visible in crossed polarized light except at 4 positions of extinction as you rotate the stage

• Colors called interference colors or birefringence colors

• Color seen depends on retardation (slowness) () of 1 set of light waves compared to the other

• Also depends on crystal thickness (d)

= d (n2 – n1)

Michel-Levy Color Chart To Determine Birefringence (Δ, n1-n2)

Go to 0.03 thickness for standard thin sections, move right across to color you seeunder microscope, then follow the angled line to determine birefringence

Quartz in cpl, Low Interference Colors

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Quartz in ppl is colorless

Polycrystalline Quartz, cpl Microcrystalline Quartz, cpl

Olivine in cpl, Strong Interference Colors Anhydrite, cpl

Anhydrite, ppl Calcite, ppl

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Calcite, cpl Multiple (polysynthetic) twinning in plagioclase feldspar, cpl

Simple and multiple twinning in feldspars, cplMajor Rock-Forming Minerals

• <40 of the more than 2,400 minerals known

• Most are silicates (contain SiO4)

• Another 30 occur as accessory minerals

Quartz, ppl Quartz, cpl

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PLAGIOCLASE FELDSPARS

Albite Na (AlSi3O8) Anorthite Ca (Al2Si2O8)

Solid solution

Plagioclase feldspar, ppl

Plagioclase feldspar, cpl Simple + multiple twinning

Quartz PlagioclaseFeldspar

Orthoclase feldspar, ppl

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Orthoclase feldspar, cpl

Microcline Feldspar

ppl

cpl

MuscoviteMica

ppl

cpl

Biotite mica, ppl

Biotite mica, cpl

Chlorite

ppl

cpl

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Pyroxene

ppl

cpl

Amphibole (Hornblende)

Calcite, ppl Calcite, cpl

IGNEOUS ROCKS

Volcanic

Magma erupts onto surface of the earth, cools rapidly, forms fine-grained rocks (or even glass if cools too quickly to form mineral crystals)

Example: Basalt

Basalt

ppl

cpl

Plagioclase

Pyroxene

Iron oxide

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IGNEOUS ROCKS

Plutonic

Formed deep within the earth from magma which has a chance to cool slowly and somore coarse-grained rocks are formed

Example: Granite

Granite

ppl

cpl

Microcline feldspar

Biotite mica

Quartz

Detrital Sedimentary Rock Examples

• Shale

•Siltstone

•Sandstone

•Conglomerate and Breccia

Fine

Coarse

Shale: Mainly clay-sized grains (<0.005 mm)

Shale (ppl)

Siltstone(0.005 – 0.0625 mm)

ppl

cpl

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SandstoneSandstone (lithic grain), cpl

Sedimentary Carbonates:

Limestone with microfossils, ppl

Chert, cpl (microcrystalline quartz, 0.01 – 0.1 mm) Chalcedony, cpl (cryptocrystalline quartz, < 0.01 mm)

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METAMORPHIC ROCKS

Slate

Phyllite

Schist

Gneiss

Fine

Coarse

Quartz-muscovite schist, cpl

Metamorphic Rocks: Marble

Marble, ppl

Quartz,ppl

Quartz,cpl

Muscovite mica, cpl

Igneous rock, cplMinerals and rock inclusions in ceramics

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