Igneous Rock Classificiation
Mineralogical Classification
Igneous Rock Occurence
Outline
Igneous rocks are classied on the basis of three factors:
Grain Size and Texture
Intrusive or Extrusive
Silica Content and Mineral Composition
Grain Size and Texture
- Vitreous - glassy, no crystals
- Aphanitic - fine grained < 2mm crystals
- Phaneritic - coase grained > 2mm crystals
- Pegmatitic - very coase grained > 10mm crystals
- Vesicular - voids created by gases
- Amigdaloidal - filled voids created by gases
- Porphyritic - some large crystals (phenocrysts)in fine matrix
Intrusive or Extrusive
- Intrusive Igneous Rocks - usually medium to coarse grained texture due
to slow cooling. Typically light coloured.
- Extusive Igneous Rocks - glassy or fine-grained due to rapid cooling.
May have voids due to degassing of the magma. Typically dark coloured.
Silica Content and Mineral Composition
- Acid Igneous Rocks - silica rich ( > 65% by weight) composed mainly
of felsic or light coloured minerals (eg. feldspars, quartz)
- Intermediate Igneous Rocks - silica 50-65% by weight, composed mainly
of plagiocalse feldspars, some mafic minerals, with some quartz.
- Basic Igneous Rocks - silica poor ( < 50% by weight) contain significant
amounts of mafic or dark coloured ferromagnesian minerals (eg. olivines,
pyroxenes, amphiboles, micas). No free quartz.
Mineralogical Classification
Igneous Rock Occurence
Outline
(c) Department of Civil and Geological Engineering, University of Saskatchewan,
57 Campus Drive, Saskatoon, SK, Canada, S7N 5A9