A special property of staurolite is that it often occurs twinned in a characteristic cross-shape, called penetration twinning. In handsamples, macroscopically visible staurolite crystals are of prismatic shape. They are often larger than the surrounding minerals and are then called porphyroblasts.
In thin sections staurolite is commonly twinned and shows lower first order birefringence similar to quartz, with the twinning displaying optical continuity. It can be identified in metamorphic rocks by its swiss cheese appearance (with poikilitic quartz) and often mantled porphyroblastic character.
Staurolite is a regional metamorphic mineral of intermediate to high grade. It occurs with almandine garnet, micas, kyanite; as well as albite, biotite, and sillimanite in gneiss and schist of regional metamorphic rocks.
It is the official state mineral of the U.S. state of Georgia and is also to be found in the Lepontine Alps in Switzerland.
Staurolite is also found in Fairy Stone State Park in Patrick County, Virginia. The park is named for a local name for staurolite from a legend in the area. Samples are also found in Island Park, Idaho, near henrys lake. Taos, New Mexico, near Blanchard Dam in Minnesota and Selbu, Norway.
7 - 7.5
1.745 - 1.762
3.74 - 3.83 meas. 3.686 calc.