Mineral
Forsterita
(Mg,Fe)₂SiO₄ · Silicatos (Nesosilicatos)
In the olivine group, the structural and chemical situation is straightforward: there is a continuous solid solution between two well-defined endmembers, forsterite Mg₂SiO₄ and fayalite Fe₂SiO₄. The Mg/Fe ratio varies, but the crystal structure does not. The fayalite–forsterite series, commonly referred to as "olivine", is a continuous solid solution between Fe₂SiO₄ and Mg₂SiO₄; intermediate names formerly used according to the Mg/(Mg+Fe) proportion are now obsolete, and only the two endmembers are distinguished, with the boundary at Mg/(Mg+Fe) = 50%: if magnesium predominates it is forsterite; if iron predominates, fayalite. Hence many specimens labelled simply as "olivine" are in fact forsterite with some iron, while peridot is the gem variety of forsterite. Tourmaline is also a group, but olivine is a line; tourmaline is a map.
Etymology
Named in honour of Adolarius Jacob Forster; within the olivine group it denotes the magnesium-rich end-member of the series with fayalite.