Forsterite, Mg2SiO4, is the magnesium (Mg) end member of the olivine solid solution series, which runs from fayalite, the Fe end member to tephroite, the Mn end member to forsterite, the Mg end member. Forsterite exhibits an orthorhombic crystal structure with Mg occupying the 6 fold sites (octahedral). It is ionic in nature with a 2+ charge on the Mg-O bonds. The crystal structure displayed to the left is based on the fosterite-tephroite crystal structure refinement


Forsterite never occurs naturally in igneous rocks because olivine forms from melts, vapors, and solids and the magnesium occurs as ions. Because the components of the forsterite are impure, forsterite is impure.

Francis, C.A., and P.H. Ribbe. 1980. The forsterite-tephroite series: I. Crystal structure refinements. American Mineralogist 65:1263-1269