The packing in forsterite structure is dense. Peridot is the gemstone variety of forsterite olivine. It is polymorphous with the olivine phase forsterite (a magnesium iron silicate). Associated minerals include calcite, dolomite, grossular, wollastonite, talc, diopside, forsterite, cummingtonite, riebeckite and winchite. Associated minerals include rutile, spinel, clinohumite, perovskite, diopside, serpentine, forsterite, brucite, hydrotalcite, chlorite and calcite. These minerals include chromite, pyrope, diopside, picroilmenite, forsterite, orthopyroxene, as well as small diamonds. Magnesium-rich olivine (forsterite) favors production of magnesite from peridotite. Ringwoodite is polymorphous with forsterite, MgSiO, and has a spinel structure. As peridot is a magnesium-rich variety of olivine (forsterite), the formula approaches MgSiO. Nearly pure forsterite occurs in some metamorphosed serpentinites. Hence, the crystallizing olivine was Mg-rich, and igneous rocks rich in forsterite were formed. Olivine in mafic and ultramafic rocks typically is rich in the forsterite end-member. Pure forsterite is composed of magnesium, oxygen and silicon. Two polymorphs of forsterite are known&colon; wadsleyite (also orthorhombic) and ringwoodite (isometric). Olivine is a solid solution of forsterite, the magnesium-endmember, and fayalite, the iron-endmember.