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An olive-green, grayish-greenor brown orthorhombic mineral; (Mg,Fe)2SiO4. It comprises the isomorphous solid-solution series forsteritefayalite. (2) A name applied to a group of minerals forming the isomorphous system (Mg,Fe,Mn,Ca) 2SiO4, including forsterite, fayalite, tephroite and a hypothetical calcium orthosilicate. Specific gravity 3.27 – 3.37, increasing with the amount of iron present.