FORSTERITE-FAYALITE SERIES

the most important minerals in the olivine family and possibly the most important constituents of the Earth's mantle. Included in the series are the following varieties: forsterite pure magnesium silicate, Mg2SiO4 0% to 10% Fe chrysolite 10% to 30% Fe hyalosiderite 30% to 50% Fe hortonolite 50% to 70% Fe ferrohortonolite 70% to 90% Fe fayalite pure iron silicate, Fe2SiO4 90% to 100% Fe These minerals are common as green to yellow, glassy crystals in many basic and ultrabasic rocks and are also abundant in chondrite meteorites. Forsterite is common in dunite, and chrysolite and hyalosiderite are common in gabbros, dolerites, basalts, and trachytes. Hortonolite and ferrohortonolite are less common but occur in dolerite, gabbro, and granophyre. Small amounts of fayalite are present in many volcanic rocks in which sodium is more common than potassium. The forsterite-fayalite minerals also occur in dolomitic limestones, marbles, and metamorphosed iron-rich sediments. These minerals are relatively infusible, not melting below 1,500 C (2,700F), and are sometimes used in the manufacture of refractory brick. For detailed physical properties, see olivine (table).