Polarised light micrograph of quartz phyllite from the Wurzenpass near Brixen, South Tyrol, Italy. This rock can be classified petrographically as granodiorite. It is a low-grade metamorphic rock of the metamorphic basement of the Southern Alps. It was formed from clay about 350-380 million years ago at temperatures of 300-400 degrees Celsius and pressures of 100-700 megapascals. These conditions are known as green schist facies. It is extremely fine-grained, very thin-leaved and has thin-schistosity with a silky sheen on its surface. The main components are sericite (fine-grained light mica) and quartz, with some biotite, ilmenite needles and graphite. Magnification: x30 when printed at 15 centimetres wide.