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 a rather complex chemical formula: Fe2Al9O6(SiO4)4(O,OH)2. Magnesium, zinc and manganese substitute in the iron site and trivalent iron can substitute for aluminium. 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...'
Contributions by Vsmith, Hadden, and Chris.urs-o.