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Metamorphic structures

Short answer

Metamorphic structures are either foliated, with parallel mineral alignment (slaty cleavage, schistosity, gneissose banding), or non-foliated, including massive granular textures (marble, quartzite) and cataclastic broken fabrics.

Describe the structures of metamorphic rocks.

The structures of metamorphic rocks are of two main kinds.

Foliated structures

Parallel arrangement of platy or elongated minerals due to directed pressure.

  • Slaty cleavage - very fine; the rock splits into thin sheets (slate).
  • Phyllitic structure - fine, silky sheen, often crinkled (phyllite).
  • Schistose structure - coarse flaky minerals in parallel layers (schist).
  • Gneissose structure - alternate light (quartz, feldspar) and dark (mica, hornblende) bands (gneiss).
  • Augen structure - lens-shaped (eye-shaped) large grains in a foliated matrix (augen gneiss).

Non-foliated structures

  • Massive/granulose - interlocking equigranular grains, no layering (marble, quartzite).
  • Hornfels - hard, fine grained, dense, from contact metamorphism.
  • Cataclastic - crushed and broken grains from dynamic metamorphism (mylonite, fault breccia).

✓ Answer checked by Claude Opus 5.5 (high effort)

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