![]() ![]() ![]() By default, the concentration of Mg in olivine rises. If it should be oxidized to Fe 3+, it will no longer fit in the olivine (fayalite) lattice, and is functionally expelled from (or not incorporated in) that structure. To be accommodated in fayalite, iron must remain in its Fe 2+ state. One apparent reason is the charge flexibility of Fe. Given the abundance of Mg and Fe in the upper mantle and crust, one might ask why the olivine series is so dominated by Mg, and not more equally apportioned between Mg and Fe. Typical aggregate of the forsterite grains from Lanzarote, Canary Islands, Spain. This is the reason why some minerals of the olivine group have only limited miscibility and others form complete solid solutions. The olivine structure contains two cation positions - usualy labeled as M1 and M2 - which are not exactly the same. Regular impurities include quite high amount of Fe, and significantly smaller amount of Ni, Co and Ca. Most forsterites form compact granular masses, only few select localities produce large, euhedral, prismatic stubby crystals with wedge-shaped terminations and striations parallel to the direction of elongation. Majority of natural forsterites contains 10-25 % of fayalite (Fe 2+) component, pure forsterite is rare. Its structure consist of isolated SiO 4 tetrahedrons. The forsterite crystallizes in the orthorhombic system and belongs to the nesosilicates. Minerals of the forsterite-fayalite solid solution are also present in many meteorites. Actually, rock-forming olivine is one of the most common minerals on Earth, but most of it is located inside Earth's upper mantle (as peridotite rock), so we do not see it that much on the surface. Other minerals from the olivine group are explicitly rare. 3 x 1.5 cm, photo: Albert Russįorsterite is the only common mineral from the olivine group, natural fayalite and tephroite are quite uncommon. Gemmy forsterite (peridot) crystals from Mogok, Myanmar. ![]()
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