Rocksday

Mylonite

A fine-grained, streaky rock formed deep in fault zones where rocks flow instead of breaking.

Mylonite specimen
Photo: Unknown, CC BY-SA 1.0, via Wikimedia Commons

How to identify mylonite

  • Fine streaks and stretched grains.
  • Larger crystals smeared into 'eyes'.
  • Found along major faults.

Often confused with

Where it's found

  • Moine Thrust, Scotland
  • Alpine Fault, New Zealand
  • Major fault zones worldwide

Uses

Clues to how faults move.

Rock tumbling

Can be tumbled with care.

Did you know?

The name comes from Greek mylon, 'mill' — the rock was ground down by fault movement.

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