Mylonite Identification Guide

What Mylonite Looks Like

Mylonite is a fine-grained, foliated metamorphic fault rock produced when pre-existing rock is deformed by intense ductile shearing in a fault or shear zone. Grains are reduced in size by recrystallization rather than fracturing.

• Color: varies with parent rock — grey, pink, greenish, banded
• Luster: dull to slightly silky on foliation surfaces
• Texture: very fine-grained matrix with scattered larger relict grains (porphyroclasts)
• Structure: strong planar foliation and a pronounced stretching lineation; often a streaky, ribboned, or "smeared" appearance
• Porphyroclasts: larger resistant grains (feldspar, garnet) wrapped by flowing matrix, sometimes forming sigma/delta tails that record shear direction

Step-by-Step Field ID Checklist

1. Look for a fine-grained, banded matrix that appears flowed or smeared.
2. Spot porphyroclasts — isolated larger grains surrounded by fine matrix, often with asymmetric tails.
3. Run a finger along the foliation to feel a lineation (a directional grain or grooving).
4. Confirm the rock is harder and more coherent than a soft fault gouge — mylonite is cohesive.
5. Note that foliation wraps around the porphyroclasts (flow texture), unlike simple bedding.

Key Diagnostic Tests

• Hardness: variable, inherited from parent minerals (often quartz/feldspar dominated, ~6-7).
• Cohesion: strongly cohesive (distinguishes from non-cohesive fault gouge).
• Fabric: foliated + lineated; recrystallized fine matrix is diagnostic under a loupe or microscope.
• Acid: generally no reaction unless derived from carbonate rock.
• Field context: found in mapped fault/shear zones — context is a key clue.

Common Look-Alikes and How to Tell Them Apart

• Cataclasite: another fault rock, but formed by brittle crushing (angular broken fragments) and is typically non-foliated; mylonite is foliated with recrystallized, flowed grains.
• Schist: foliated metamorphic rock too, but schist forms by regional metamorphism with visible aligned micas; mylonite has a finer, sheared, ribbon-like matrix and porphyroclasts with tails confined to shear zones.
• Gneiss: coarser banding from metamorphic differentiation; mylonite's banding is shear-induced and finer, with grain-size reduction.
• Pseudotachylite: glassy/dark melt veins from frictional melting; mylonite is crystalline, not glassy.

Where It Is Found

Mylonite forms at mid-crustal depths within ductile shear zones along major faults, thrusts, and orogenic belts worldwide — exposed in deeply eroded mountain belts and Precambrian shield terrains.