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Texture of igneous rocks

Short answer

Texture refers to grain size, shape, and crystallinity. Igneous textures include holocrystalline (all crystals), glassy, and hypocrystalline; phaneritic (coarse, slow cooling), aphanitic (fine, rapid), porphyritic (two-stage); and vesicular with gas holes. Coarse interlocking textures are strong; glassy and vesicular ones are weaker.

Define rock texture. Describe the texture of igneous rocks.

Texture of a rock is the size, shape, arrangement and degree of crystallinity of its mineral grains, that is, the overall appearance of the rock on a small scale. In igneous rocks it depends mainly on the rate of cooling of the magma.

Textures of igneous rocks

  1. Based on degree of crystallinity
    • Holocrystalline: wholly crystalline (granite).
    • Hypocrystalline (hemicrystalline): partly crystals, partly glass.
    • Holohyaline (glassy): wholly glass, from very rapid cooling (obsidian).
  2. Based on grain size
    • Phaneritic: crystals visible to the naked eye, from slow cooling (granite, gabbro). Coarse, medium and fine grained.
    • Aphanitic: crystals too small to see, from rapid cooling (basalt, rhyolite).
    • Porphyritic: large crystals (phenocrysts) set in a fine-grained or glassy groundmass; two-stage cooling (porphyry).
    • Pegmatitic: very coarse crystals (pegmatite).
  3. Special textures
    • Vesicular: gas bubble holes (pumice, scoria).
    • Amygdaloidal: vesicles filled with secondary minerals (zeolite, calcite).
    • Ophitic: laths of plagioclase enclosed in pyroxene.
    • Graphic: intergrowth of quartz and feldspar.
  4. Based on grain shape: euhedral (perfect faces), subhedral, anhedral.

Significance

Coarse, interlocking crystalline rocks are strong; glassy and vesicular rocks are weaker and more porous.

✓ Answer checked by Claude Opus 5.5 (high effort)

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