Mylonite is a fine-grained, cohesive metamorphic rock formed by intense ductile deformation in shear zones deep within the Earth's crust. It is characterized by a strong foliation or lineation, often containing rotated porphyroclasts of harder minerals like feldspar within a fine-grained matrix. Collectors often look for these in tectonically active zones where rocks have been significantly crushed and sheared under high pressure.
Is this mylonite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch mylonite with a known reference. Mylonite sits at Mohs 5-7, softer than the next harder reference, harder than the previous one.
- 2Read the textureLook at a fresh break with a hand lens. Texture: Compact; formed by dynamic recrystallization in ductile shear zones. Grain size: Fine-grained.
- 3Pick out the mineralsComposition: Varies; mylonite is defined by texture rather than composition. Identify the two or three most abundant grains.
- 4Read the lusterHold the specimen under a strong light. Luster: dull to silky.
- 5Match the color rangeCompare against the expected color range: gray, black, white, brown.
Often confused with
Mylonite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside mylonite
Minerals reported to co-occur with mylonite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 5-7
- Density
- 2.6-2.8 g/cm³
- Luster
- Dull to silky
- Transparency
- Opaque
- Rarity
- Common
- Uses
- Geological study, Educational
- Host rock
- ductile shear zones
- Texture
- Compact; formed by dynamic recrystallization in ductile shear zones
- Grain size
- Fine-grained
- Composition
- Varies; mylonite is defined by texture rather than composition
Where rockhounds find mylonite
Classic worldwide localities
- Moine Thrust Zone (Scotland)
- San Andreas Fault (USA)
- Alpine Fault (New Zealand)
- Norway
- Himalayas
Field-hunting tip
Where it forms: ductile shear zones. If you start seeing quartz, feldspar, mica in float, you are in the right ground.
Common questions
How do you identify mylonite?+
Where is mylonite found?+
What rocks look like mylonite?+
What minerals are found with mylonite?+
What kind of rock does mylonite form in?+
What is mylonite used for?+
Find mylonite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
