Igneous Textures

1
Igneous Textures
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Crystallinity

• Holocrystalline 100 crystalline
• Hypocrystalline 50-100 crystalline
• Hypohyaline 0-50 crystalline
• Holohyaline 0 crystalline

Holocrystalline Diorite Porphyry
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Grain Size

• Phaneritic
• Grains are visible to the
• unaided eye
• Aphanitic
• Grains not easily seen w/unaided eye
• Glassy
• Few to no mineral grains
• visible in a glass-rich matrix
• Obsidian, tachylyte

Phaneritic Granite
Glassy Obsidian
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Phaneritic

• Pegmatite (very large crystals)
• Coarse (gt5 mm)
• Medium (1-5 mm)
• Fine (lt1 mm)

Diorite Porphyry
Phaneritic Granite
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Aphanitic

• Microcrystalline
• Grains are visible with a hand lens or microscope
• Cryptocrystalline
• Crystalline but individual grains are not easily
  visible with a hand lens or microscope
• Chert, Flint

Aphanitic Basalt
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Grain Shape

• Euhedral
• Well formed bounded by crystal faces
• Subhedral
• Shape has some relation to crystal habit
• Anhedral
• Irregular no crystal faces

Euhedral OPX
Anhedral CPX
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Fabric
Textural relationship between/among grains

• Granular
• Porphyritic
• Some phenocrysts are considerably larger than the
  matrix
• Poikilitic
• Small grains of one mineral are irregularly
  scattered within a larger crystal of another
  mineral

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Granular

• Equigranular
• All grains are about the same size
• Inequigranular
• Grains vary in size
• Serate
• Grain sizes range from large
• to small

Equigranular
Serate
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Granular

• Allotriomorphic
• All or nearly all grains are anhedral
• Hypidomorphic
• Mixture of euhedral, subhedral and anhedral
• Panidiomorphic
• All or nearly all of the grains are euhedral or
  subhedral

Allotriomorphic Pyroxenite
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Porphyritic

• Porphyry
• gt 50 phenocrysts
• Used as a name after a rock
• Granite porphyry
• Porphyritic
• lt 50 phenocrysts
• Used as a prefix
• Porphyritic Biotite Granite

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Porphyritic

• Glomeroporphyritic
• Phenocrysts occur in clusters
• Vitrophyric
• Phenocrysts in a glassy groundmass

Vitrophyre
Glomeroporphyritic
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Color Index

• Leucocratic
• 0-35 dark minerals
• Mesocratic
• 35-65 dark minerals
• Mafic
• gt 50 dark minerals
• Melanocratic
• 65-95 dark minerals
• Ultramafic
• gt90 dark minerals
• Often pyroxene and olivine

Ultramafic Pyroxenite
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Cooling History

• Phaneritic
• Cools slowly
• Crystals have time to form and grow
• Intrusive
• Aphanitic
• Cools quickly
• Extrusive
• Glassy
• Cools very quickly
• Extrusive