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Petrographic Examination on

Aggregates

Basic Steps for Identification of Minerals and

Rock Fragments under the Polarizing

Microscope

By

Dr. Kyi Khin

2008

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Aggregates

Fine Aggregates- Sand (Fine to Coarse: 0.125-4mm) - Fine gravel (>4mm)

Coarse Aggregates- 20mm gravels - Rock Fragments (mostly granitic rocks)

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Minerals

Crystal Forms

Specific Chemical Composition

Composed with minerals

Igneous, Sedimentary, Metamorphic Origins

ROCKSROCKS

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Polarizing Microscope

Transmitted Light, Fluorescent Light Polarizer, Analyser (Polaroid Lens)

-Under Plane-Polarized Light (PPL):

Not insert analyser

-Under Cross-Polarized Light (XN):

Analyser cross 90° to Polarizer

Analyser

Polarizer

Transmitted light

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I. Minerals under the Polarizing

Microscope

Isotropic Minerals (Opaque)

Anisotropic Minerals (Optically colored)

1. Color

1st Order 2nd Order 3rd Order

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2. Crystal Shape

Euhedral (completely bounded by its crystal faces)

Subhedral (partially bounded by its crystal faces)

Anhedral (lacks any of its crystal faces)

Euhedral Olivine Subhedral Olivine Anhedral Olivine

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3. Extinction

-Check Under the Cross-Polarized Light

-Dark because direction of 2 different rays

coincide with light source when rotate the

stage

-Straight, Wavy, Symmetrical (Crystal boundary)

-Parallel, Incline (Refer with cleavage)

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Extinction

Wavy Extinction Straight Extinction

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4. Zoning & Twining

Zoned Feldspar Polysynthetic Twins

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5. Cleavages

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5. Polycrystallinity

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6. Crystallinity

Holocrystalline

(100% Crystals)

Hypocrystalline

(50% Crystals)

Cryptocrystalline

(100% fine crystals)

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I. Minerals under the Polarizing I. Minerals under the Polarizing

MicroscopeMicroscope

Common Minerals (RockCommon Minerals (Rock--forming Minerals)forming Minerals)

1. Quartz (Sio2) - Mono-crystalline Quartz (Single Grain)

- Poly-crystalline Quartz (Many Grains)

- Strained Quartz (>20º extinction angle)** Including metamorphic Quartz (elongated)

- Micro-crystalline Quatrz (<1µm)

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2. Feldspar

Orthoclase Feldspar (Potassium: KAlSi3O8)

- Simple Twining or untwined- Grey, Pale yellow color

Plagioclase Feldspar (Calcium, Sodium: Na, Ca AlSi3O8)

- Multiple Twining or untwined- Grey, Dark Grey color

- Overgrowth with Quartz

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3. Mica

Biotite Mica - Brown Color (PPL), Dark Brown (XN)

- Altered to Chlorite (Green)

- Altered to Iron-oxide (Dark brown)

Muscovite Mica - White Color (PPL), Bright colors (XN)

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3. Heavy Minerals

Hornblende/Augite - Pale Green (PPL), Orange/pale Brown/Dark Green (XN)

- 1 or 2 sets of Cleavage

- Extinction: Augite (Parallel): Hornblende (Inclined)

Epidote - Pale Green (PPL), Bright green/Orange/Red (XN)

- Partings

- Extinction: Straight

Iron-oxide - Reddish Brown (PPL), Dark brown (XN)

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II. Rockfragments under the

Polarizing Microscope

Igneous Origin

- Crystalline contact

- Euhedral/Subhedral Shape

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II. Rockfragments under the

Polarizing Microscope

Igneous Origin

- Crystalline contact

- Euhedral/Subhedral

Shape

- Composed with 2 or more

minerals

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II. II. RockfragmentsRockfragments under the under the

Polarizing MicroscopePolarizing Microscope

Hornblende GraniteHornblende Granite

MicrograniteMicrogranitePorphyriticPorphyritic GraniteGranite

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II. Rockfragments under the

Polarizing Microscope

Sedimentary Origin

CHERT (Ch)

SILTSTONE (Sst)

SHALE (Sh)

Ch

Sst

Sst

Sst Sh

Ch

PPL

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II. II. RockfragmentsRockfragments under the under the

Polarizing MicroscopePolarizing Microscope

Sedimentary OriginSedimentary Origin

CHERT (Ch)CHERT (Ch)

SHALE (SHALE (ShSh))

SILTSTONE (SILTSTONE (SstSst))

Ch

Ch

Ch Sh

Sst

Sst

Sst

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II. II. RockfragmentsRockfragments under the under the

Polarizing MicroscopePolarizing Microscope

Varieties of CHERTVarieties of CHERT

Banded Chert Granular Chert

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II. Rockfragments under the

Polarizing Microscope

Sedimentary Origin

MUDSTONE (Md)

SHALE (Sh)

Md

Md

Sh

Sh

Sh

PPL

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II. II. RockfragmentsRockfragments under the under the

Polarizing MicroscopePolarizing Microscope

Sedimentary OriginSedimentary Origin

SILTSTONESILTSTONE

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II. Rockfragments under the

Polarizing Microscope

Metamorphic Origin

- Grains orientation

- Anhedral Shape

- Tabular

- Colored under PPL

PPL

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II. Rockfragments under the

Polarizing Microscope

Metamorphic Origin

- Grains orientation

- Anhedral Shape

- Tabular

- Colored under PPL

XN

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II. Rockfragments under the

Polarizing Microscope

Metamorphic Origin

- Grains orientation

- Anhedral Shape

- Tabular

- Colored under PPL

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II. II. RockfragmentsRockfragments under the under the

Polarizing MicroscopePolarizing MicroscopeVolcanic OriginVolcanic Origin

-- Needle shape crystals in fineNeedle shape crystals in fine--grained grained

ground massground mass

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III. ALTERATIONIII. ALTERATION

1. 1. SericitizationSericitization (altered from Feldspars)(altered from Feldspars)

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III. ALTERATIONIII. ALTERATION

2. 2. ChloritizationChloritization (altered from Micas)(altered from Micas)

Dolomite in Carbonate RocksDolomite in Carbonate Rocks

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EndEnd

Thank You!