A mineral group including fayalite, Fe₂SiO₄; forsterite, Mg₂SiO₄; liebenbergite, (Ni,Mg)₂SiO₄; and tephroite, Mn₂SiO₄; orthorhombic; olive green, grayish green, brown, or black; members intermediate in the forsterite-fayalite crystal solution series are common rock-forming minerals in gabbros, basalts, peridotites, and dunites; alters hydrothermally to serpentine. Fayalite occurs in some granites and syenites, forsterite in thermally metamorphosed dolomites, and tephroite in iron manganese ore deposits and their associated skarns.