IGNEOUS ROCKS

The main types of igneaous rocks

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SILICIC - GRANITE

CRYSTAL GRAIN SIZE: Coarse

TEXTURE: Porphyritic

COLOUR: Dark Grey to Pink

SILICA PERCENTAGE: >66%

ENVIRONMENT OF FORMATION: Plutonic

MINERAL COMPOSITION: Orthoclase Feldspar, some biotite and Muscovite Mica and Hornblende

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SILICIC - GRANODIORITE

CRYSTAL GRAIN SIZE: Coarse

TEXTURE: Porphyritic

COLOUR: Grey or White

SILICA PERCENTAGE: >66%

ENVIRONMENT OF FORMATION: Plutonic

MINERAL COMPOSITION: Mainly plagioclase, minor orthoclase feldspar, hornblende and biotite.

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SILICIC - RHYOLITE

CRYSTAL GRAIN SIZE: Medium

TEXTURE: Porphyritic

COLOUR: Pale - Shades of Brown (may show flow banding)

SILICA PERCENTAGE: >66%

ENVIRONMENT OF FORMATION: Hypobyssal

MINERAL COMPOSITION: Quartz, Plagioclase, Mica and Orthoclase

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SILICIC - OBSIDIAN

CRYSTAL GRAIN SIZE: Glassy

TEXTURE: Equicrystaline

COLOUR: Grey or Black 

SILICA PERCENTAGE: >66%

ENVIRONMENT OF FORMATION: Extrusive

MINERAL COMPOSITION: Quartz, and Feldspar

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INTERMEDIATE - ANDESITE

CRYSTAL GRAIN SIZE: Fine

TEXTURE: Porphyritic

COLOUR: Speckled Grey or Brown

SILICA PERCENTAGE: 52% - 66%

ENVIRONMENT OF FORMATION: Extrusive

MINERAL COMPOSITION: Mainly composed of Plagioclase Feldspar

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INTERMECIATE - DIORITE

CRYSTAL GRAIN SIZE: Medium

TEXTURE: Porphyritic

COLOUR: Dark - Black + White or Pink + Green

SILICA PERCENTAGE: 52% - 66%

ENVIRONMENT OF FORMATION: Hypabyssal

MINERAL COMPOSITION: Plagioclase feldspar and hornblende

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MAFIC - BASALT

CRYSTAL GRAIN SIZE: Fine

TEXTURE: Equicrstaline

COLOUR: Dark - may have vesicles

SILICA PERCENTAGE: 45% - 52%

ENVIRONMENT OF FORMATION: Extrusive

MINERAL COMPOSITION: Quartz, Plagioclase, Coliate and Pyroxene

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MAFIC - DOLERITE

CRYSTAL GRAIN SIZE: Medium

TEXTURE: Porphyritic

COLOUR: Dark - Grey, Black or Green

SILICA PERCENTAGE: 45% - 52%

ENVIRONMENT OF FORMATION: Hypabyssal

MINERAL COMPOSITION: Quartz, Feldspar and Plagioclase

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MAFIC - GABBRO

CRYSTAL GRAIN SIZE: Coarse

TEXTURE: Porphyritic

COLOUR: Dark - Black or Muddy Green

SILICA PERCENTAGE: 45% - 52%

ENVIRONMENT OF FORMATION: Plutonic

MINERAL COMPOSITION: Plagioclase Feldspar and pyroxene

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ULTRA-MAFIC - PERIDOTITE

CRYSTAL GRAIN SIZE: Coarse

TEXTURE: Equicrystaline

COLOUR: Dark - Green

SILICA PERCENTAGE: <45%

ENVIRONMENT OF FORMATION: Plutonic

MINERAL COMPOSITION: Olivine and pyroxene

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