7 ensp 0183 ensp Staurolite is a reddish 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 Fe 2 2 Al 9 O 6 SiO 4 4 O OH 2 Magnesium zinc and manganese substitute in the iron site and trivalent iron can substitute for aluminium Staurolite a brown glassy mineral consisting of iron aluminium silicate in the form of prismatic