Mylonite

About it

Fine-grained, streaky rock formed by ductile shearing in deep fault zones, where grains are ground down and smeared out around a few surviving larger crystals. Named from the Moine Thrust in the Scottish Highlands.

Type

Rock (metamorphic)

Group

Metamorphic (fault rock)

Colours

grey, black, dark green, streaked

Transparency

opaque

Cleavage and fracture

splits along a fine, streaky foliation

Typical values: real specimens vary. For more, see mindat.org.