Staurolite
Staurolite is a redbrown to black mostly opaque silicate mineral with a white streak. It crystallizes in the monocline crystall system, has a hardness of 7 to 7,5 and a rather complex chemical formula: 2(Fe,Mg,Zn)O.AlOOH.4Al2O(SiO4), so it is an aluminosilicate. The brackets mean that iron, magnesium and zinc atoms can occur in arbitrary mixing but they are always related to the other groups in the same way.
A special property of staurolite is that it occurs often twinned in a characteristic cross-shape. The macroscopically visible crystals are of prismatic shape. They are often larger than the surrounding minerals and are then called porphyroblasts.
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