A major rock-forming mineral group belonging to the nesosilicates, forming a complete solid solution series between forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄); sp. gr. 3.22–4.39 increasing with increasing iron content; hardness 6–7; orthorhombic; usually olive-green, but white or yellowish in forsterite, brown or black in fayalite; colourless streak; vitreous lustre; crystals rare, short, prismatic, usually develops as granular aggregates; cleavage poor {010}; occurs in silica-poor, igneous rocks (e.g. basalt, gabbro, troctolite, and peridotite), extensively with pyroxene in dunites, and in stony meteorites and lunar basalts; alters readily to serpentine during weathering or hydrothermal alteration.