Antigorite Antigorite crystals in serpentinite from Poland General Category Phylosilicate Serpentine-Kaolinite group Formula (repeating unit) (Mg, Fe)3Si2O5OH4 Strunz classification 9.ED.15 Crystal system Monoclinic Identification Color Green, Yellowish-green, Blueish-gray Crystal habit Massive or platy Cleavage (001) Perfect Fracture Brittle Mohs scale hardness 3.5-4 Luster Vitreous - Greasy Streak Greenish white Specific gravity 2.5-2.6 Optical properties Biaxial (-) Birefringence δ = 0.005-0.006