Mylonite

Mylonite is a metamorphic rock produced by intense deformation along ductile fault and shear zones deep in the crust. Rather than shattering like a near-surface fault rock, the parent rock flows plastically, and its mineral grains are progressively reduced in size and smeared into a fine, banded fabric.

The result is a hard, streaky, often glassy-looking rock with a strong planar foliation and a distinct linear stretching direction. Larger surviving grains, called porphyroclasts, are wrapped by the finer matrix and often show tails that reveal the sense of shear.

Mylonites are central to understanding how the crust deforms in the warm, ductile regime below the brittle upper crust.

Mylonite forms in shear zones at depths and temperatures where rocks deform by ductile flow rather than brittle fracture, typically below about 10-15 km. Under sustained directed stress, mineral grains recrystallize into much smaller crystals through processes like dynamic recrystallization, dramatically reducing grain size while increasing strength of the fabric.

The rock develops a strong foliation parallel to the shear plane and a stretching lineation parallel to the slip direction. Geologists grade mylonites by how much of the rock has been reduced: protomylonite (partial), mylonite, and ultramylonite (almost entirely fine matrix).

Mylonite zones occur along major continental fault and thrust systems worldwide, including the Moine Thrust in Scotland and many exhumed shear zones in mountain belts.

Look for a fine-grained, hard, strongly foliated rock with a streaky, drawn-out fabric and often a silky sheen on foliation surfaces. Scattered larger grains (porphyroclasts) with tapering tails set in a fine matrix are diagnostic.

Unlike a simple schist, the foliation in mylonite results from extreme grain-size reduction and flow, and a stretching lineation is usually visible. Hardness depends on the parent rock, commonly 6-7 in quartz-feldspar types.

Look-alikes: cataclasite forms by brittle crushing and lacks the smooth ductile fabric; pseudotachylite is a dark glassy frictional melt; chert is fine-grained but lacks foliation and shear sense indicators.

Mylonite has limited commercial use but enormous scientific importance. Its fabric records the direction, sense, and intensity of movement along ancient faults, so structural geologists use mylonite zones to map crustal-scale shear and reconstruct tectonic histories.

As a hard, attractively banded stone, mylonite is occasionally cut for ornamental slabs or landscaping, where the swirling foliation can be striking. It is not an ore rock and carries no established metaphysical tradition.

Its main value lies in research and education, as a textbook example of how rocks flow rather than break deep in the Earth.