How do I identify Mylonite?

Mylonite can be identified by its Variable color. Look for these key characteristics when examining specimens.

What color is Mylonite?

Mylonite typically appears in Variable, often grey to black. Color can vary depending on impurities and formation conditions.

Mylonite

A variety of Metamorphic

Uncommon

What is Mylonite?

Mylonites are a broad category of rock that may be made from many different mineral components. They are metamorphic and super fine-grained, and can take on a wide range of appearances. The rock is sometimes used as filler or in road construction.

Uses & Applications

Like many other strong and durable rocks, mylonite is often only associated with construction materials. When cut in large blocks, mylonite is particularly good in building foundations, paving, and walling. It is commonly used in gardens for decorative purposes, and may be crushed to be used as road aggregate.

Mylonite Market Value Calculator

Estimate the market value of Mylonite using size, quality, and finish. This preview calculator is for quick context and is not a formal appraisal.

Mylonite Localities Map

See where Mylonite is found with a localities map, collecting zones, and geology context. Generate a sample map preview below.

Map preview

North ZoneCentral RidgeSouth Basin

Key Characteristics

Rock Types of Mylonite

Blastomylonites are coarse grained, often sugary in appearance without distinct tectonic banding. Ultramylonites usually have undergone extreme grainsize reduction. In structural geology, ultramylonite is a kind of mylonite defined by modal percentage of matrix grains more than 90%. Ultramylonite is often hard, dark, cherty to flinty in appearance and sometimes resemble pseudotachylite and obsidian. In reverse, ultramylonite-like rocks are sometimes "deformed pseudotachylyte". Mesomylonites have undergone an appreciable amount of grainsize reduction, and are defined by their modal percentage of matrix grains being between 50 and 90%. Protomylonites are mylonites which have experienced limited grainsize reduction, and are defined by their modal percentage of matrix grains being less than 50%. Because mylonitisation is incomplete in these rocks, relict grains and textures are apparent, and some protomylonites can resemble foliated cataclasite or even some schists. Phyllonites are phyllosilicate(e.g. chlorite or mica)-rich mylonites. They typically have a well-developed secondary shear (C') fabric.

Formation of Mylonite

Mylonite are ductilely deformed rocks formed by the accumulation of large shear strain, in ductile fault zones. There are many different views on the formation of mylonites, but it is generally agreed that crystal-plastic deformation must have occurred, and that fracturing and cataclastic flow are secondary processes in the formation of mylonites. Mechanical abrasion of grains by milling does not occur, although this was originally thought to be the process that formed mylonites, which were named from the Greek μύλος mylos, meaning mill. Mylonite form at depths of no less than 4 km.