Igneous Rocks - Suitabke to print back to back

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  • Grain Size: Over 5mm, phenocrysts can be several centimeters in length
  • Colour: colourless grains, also mottled in white, pink or red, some grey or dark grains.
  • Percentage Silica: over 65%
  • Minerals: essential quartz, alkali feldspar and plagioclase in variable amounts, usually with hornblende and/or biotite; muscovite may occur.
  • Texture: coarse to very coarse-grained; usually granular; may be porphyritic with well-shaped phenocrysts (large crystals) of feldspar
  • Geological Description: An intrusive, plutonic, felsic igneous rock.
  • Formation: Slow cooling deep underground, common in batholiths in the continental crust
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  • Grain Size: Fine grained - under 1mm unless phenocrysts or xenoliths are present
  • Colour: White/Grey sometimes slightly pink
  • Percentage Silica: Over 65%
  • Minerals: Quartz, Feldspars, Hornblende, some mica
  • Texture: At times some crystals can grow large enough to see and then the texture is call porphyritic. Porphyritic texture means that there are larger crystals surrounded by a fine grained or glassy matrix. Sometimes there are rounded sphericules of quartz or feldspar in the matrix.
  • Geological Description: A fine grained, extrusive acidic igneous rock
  • Formation: Rhyolite is found in volcanic arcs where crustal rocks have been subducted under continental crust and melted into a lighter magma rich in silica
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  • Grain Size: Under 1mm unless phenocrysts are present
  • Colour: black, very dark grey; paler and greenish or reddish if altered
  • Percentage Silica: Between 45 and 52%
  • Minerals: essential plagioclase and pyroxene in variable amounts, with olivine, spinel, ilmenite, magnetite or apatite
  • Texture: usually large crystals (phenocrysts) set in finely crystalline matrix, may have vesicles (air pockets) which can be filled with minerals (amygdaloidal texture).
  • Geological Description: Extrusive, basic igneous rock
  • Formation: Extruded from basic volcanoes in rift valleys constructive margins and from hotspot volcanism.
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  • Grain Size: Intermediate - between 1 and 5mm
  • Colour: Dark grey/green to black
  • Percentage Silica: Between 45 and 52%
  • Minerals: Mafic minerals common, Plagioclase, Pyroxene, Augite and olive
  • Texture: Dolerites may be porphyritic, containing phenocrysts of plagioclase, olivine and even quartz
  • Geological Description: A medium grained, hypabyssal basic igneous rock (mafic)
  • Formation: Usually in Hypabyssal intrusions such as dykes, sills and laccoliths
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  • Grain Size: Normally fine grained, though included crystals are common
  • Colour: Grey
  • Percentage Silica: Can be intermediate to acidic, ie over 52% silica
  • Minerals: quartz, feldspars and a very wide range of others
  • Texture: Highly vesicular, often over 90% air spaces hence it floats on water, it can form rafts of debris after major eruptions, reputed to reach over 30km in size, forming an important part of the marine ecosystem.
  • Geological Description: A rapidly cooled igneous rock, which is effectively the froth on the top of the magma chamber
  • Formation: Rapid cooling due to gases coming out of solution in a highly pressurized magma chamber. When pressure is released, the rock expands and the bubbles are fossilised in the melt, often distorted by flow
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  • Grain Size: Fine (under 1mm) usually under 1mm but porphyritic textures are common
  • Colour: Dark to light grey, sometimes stained red with weathering due to high iron content
  • Percentage Silica: usually 57% but ranges from 52 to 66%
  • Minerals: The primary ingredient of most andesites is andesine, a feldspar of the plagioclase series. Smaller amounts of quartz or minerals rich in iron and magnesium such as olivine, pyroxene, biotite, or hornblende are also present
  • Texture: Commonly porphyritic
  • Geological Description: A fine grained, intermediate extrusive rock
  • Formation: This is the most common extrusive rock after basalt, found usually at convergent margins and intermediate volcanoes
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  • Grain Size: Microscopic
  • Colour: black, dark colours
  • Percentage Silica: normally high - over 60%
  • Minerals: essential glass, other minerals can include alkali feldspar, quartz
  • Texture: none; massive glass with conchodial fracture
  • Geological Description: A very fine grained extrusive igneous rock, commonly called volcanic glass.
  • Formation: Normally found where acidic lava experiences a very significant temperature change on extrusion, therefore expecially common in underwater eruptions, or as the crust of an acidic lava flow, where grain size coarsens towards the centre.
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  • Grain Size: over 5mm
  • Colour: dark grey, greenish, brownish
  • Percentage Silica: 45 - 52%
  • Minerals: essential plagioclase and pyroxene, other minerals include olivine, hornblende, spinel, ilmenite, magnetite, apatite.
  • Texture: course grained, granitic texture, crystals intergrown
  • Geological Description: A basic, intrusive, plutonic igneous rock
  • Formation: Deeper continental crust areas where slow cooling can occur.
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