Conglomerate of mica with staurolite and garnet. Taos, NM. Great for specimen collection and understanding how staurolites are formed.
What are Staurolites?
Staurolite is a red brown to black, mostly opaque, nesosilicate mineral with a white streak. It crystallizes in the monoclinic crystal system, has a Mohs hardness of 7 to 7.5 and the chemical formula: Fe2+2Al9O6(SiO4)4(O,OH)2. Magnesium, zinc and manganese substitute in the iron site and trivalent
iron can substitute for aluminium.
Staurolite often occurs twinned in a characteristic cross-shape, called cruciform penetration twinning. In
handsamples, macroscopically visible staurolite crystals are of prismatic shape. The mineral often
forms 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.
****These are from Taos, NM. They are formed in mica shist with garnet. Collected by myself and sometimes my crew, cleaned by myself****