Humite Humite specimen, Italy General Category Nesosilicates Formula (repeating unit) (Mg,Fe)7(SiO4)3(F,OH)2. Strunz classification 9.AF.50 Crystal system Orthorhombic Crystal class Dipyramidal (mmm) H–M Symbol: (2/m 2/m 2/m) Space group Pbnm Unit cell a = 10.24, b = 20.72 c = 4.73 [Å]; Z = 4 Identification Color White, yellow, dark orange, brown Cleavage Poor Fracture Uneven to subconchoidal Tenacity Brittle Mohs scale hardness 6-6.5 Luster Vitreous Streak White Diaphaneity Transparent Specific gravity 3.20 – 3.32 Optical properties Biaxial (+) Refractive index nα =1.607-1.643, nβ=1.619-1.653, nγ=1.639-1.675 Birefringence 0.0320 2V angle Calculated = 70-78°, measured = 68-81° Dispersion Weak, r > v References [1][2][3]