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Igneous rocks contain crystals, glass, and/or combination with rock fragments. Texture is the geometric relationship between crystals, glass and rock fragments. These include GRAIN SIZE, SHAPE, ORIENTATION, and GRAIN BOUNDARY RELATIONSHIPS. Mostly controlled by COOLING RATE and HISTORY.

Types of Igneous rock: Edit

  • Intrusive rocks - cool slowly from a magma underground and have a coarse grained texture (phaneritic).
  • Extrusive rocks - cool quickly from lava on the surface.
    • Aphanitic - fine-grained texture.
    • Glassy - cooled instantly.
    • Vesicular - some lava contain gases which escaped as the rocks cooled, leaving visible holes or bubbles in the rocks.
    • Porphyritic - a texture with two or more distinct sizes of grains due to two stages of cooling (Slow cooling suddenly rapid cooling). Larger

grains called phenocrysts and the material surrounding the phenocrysts is called groundmass or Matrix

Phaneritic texture - grain size ranges: Edit

  • <1 mm - fine grained
  • 1-5 mm - medium grained
  • 5-3 mm - coarse grained
  • >3 cm - very coarse grained

Porphyritic texture - grain sizes ranges of phenocrysts: Edit

  • 0.03 mm - microphenocrysts
  • 0.3-5 mm - phenocrysts
  • >5 mm - megaphenocrysts

Texture terminology Edit

  • Holocrytalline - totally crystalline.
  • Holohyalline - totally glassy.
  • Pegmatitic - very coarse grained (>5 cm)
  • Euhedral - perfect crystal shape.
  • Subhedral - partial crystal shape.
  • Anhedral - no crystal shape.
  • Granular texture - interlocking grains or roughly uniform size.
  • Graphic texture - very systematic intergrowth of quartz and feldspar.
  • Poikilitic - large grains that totally enclose smaller early formed minerals.
  • Ophitic - large clinopyroxene grains that enclose plagioclase.
  • Subophitic - simultaneous crystallization of clinopyroxene and plagioclase; enclose another.
  • Glomeroporphyritic - if phenocrysts are found to occur as clusters of crystals.
  • Amygdular or Amygdaloidal - is defined as a refilled vesicle.
  • Pumiceous - if vesicles are so abundant and the rock has a density of less than 1.
  • Scoriaceous - if vesicles are so abundant and the rock has a density of greater than 1.
  • Spherulitic - mostly found in glassy rhyolites wherein spherical intergrowths of radiating quartz and feldspar replace glass a recruit of devitrification.
  • Orbicular - consists of concentrically banded spheres wherein the bands consist of alternating light colored and dark colored minerals.

Some common grain shapes Edit

  • Tabular - grains with rectangular table shapes.
  • Equant - grains that have all of their boundaries of approximately equal length.
  • Fibrous - grains as that occurs as long fibers.
  • Acicular - grains that occur as long slender crystals.
  • Prismatic - grains that show an abundance of prism faces.

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