Mylonite is a metamorphic rock formed by ductile deformation during intense shearing encountered during folding and faulting, a process termed cataclastic or dynamic metamorphism. This process involves nearly complete pulverisation of the parent rock so the original minerals are almost completely broken down and recrystallise as smaller grains which are tightly intergrown, forming a dense, hard rock. As a result of the shearing encountered during formation, recrystallised minerals grow preferentially along planes of foliation parallel to the direction of shear. Mylonite may also be characterised by the smearing, flattening or rotation of any porphyroblasts formed during metamorphism. Not surprisingly, the word mylonite is derived from the Greek word for mill.
Other specimens - Click the thumbnails to enlarge
Grain size - very fine grained; grains need to be observed under a microscope; sometimes contains porphyroblasts.
Hardness - hard.
Colour - variable, grey to black, but can form in a variety of colours dependent on parent rock composition.
Mineralogy - extremely variable, dependent on the original composition of the parent rock.
Other features - generally smooth to touch.
Uses - as aggregate in the construction and roading industries.
New Zealand occurrences - Paparoa Metamorphic Core Complex (northwest coast, South Island), sporadic occurrences along the Alpine Fault, Stewart Island.