558 MINERALS/Olivines Figure The olivine structure (view looking down the x axis of the unit cell): SiO4 tetrahedral units (yellow) point alternately either way in both the x and y directions, and there are two different cation sites, M1 (green) and M2 (red) CaMgSiO4, forming the mineral monticellite The Fe analogue, CaFeSiO4 (kirschsteinite) is known only in synthetic systems, but complete solid solution is possible between montcellite and kirschsteinite as Fe replaces Mg in the structure The most iron-rich naturally occurring examples in the CaMgSiO4– CaFeSiO4 series contain about 70% CaFeSiO4 component However, there is very limited solid solution between the montcellite-kirschsteinite series and the forsterite-fayalite series, because the size of the Ca2ỵ ion (ionic radius 1.00 A ) is substantially larger than Mg2ỵ and Fe2ỵ ions Substitution of the large Ca2ỵ ion into the olivine structure causes strain that distorts the SiO4 tetrahedra The olivine structure cannot tolerate the degree of distortion that would be produced by a random occupation of the cation sites by ions of such different sizes, so very little solid solution can exist between the Ca-olivines and the Mg, Fe-olivines, even at high temperatures In the Ca-bearing olivines, the Ca and Mg, Fe ions are completely ordered in the structure; Ca2ỵ occupies the (larger) M2 site, while Mg2ỵ and Fe2ỵ are randomly distributed on the M1 sites In many natural Mg-Fe olivines, particularly those rich in Fe, there is often a small proportion of Ca and Mn present in the structure The substitution of Mn2ỵ (ionic radius 0.83 in 6-coordination) for Fe2ỵ in fayalite also occurs when Mn is present during crystallization, and a complete solid solution series exists between Fe2SiO4 (fayalite) and Mn2SiO4 (tephroite) In natural occurrences, the low Mg-content of fayalitetephroite phases is more likely to be related to the limited amounts of Mg available in such crystallization environments, rather than the inability of the structure to tolerate differences between these cations sizes Zn-bearing tephroites have been reported, and CaMnSiO4 (glaucochroite), although rare in nature, has an olivine structure Other compositions that also possess the olivine structure, such as Ni2SiO4 and Mg2GeO4, have been prepared by laboratory synthesis Small amounts of Ni2ỵ (ionic radius 0.69 A˚ in 6-coordination) are commonly present in the structure of natural Mgrich olivines However, the presence of Cr3ỵ and/or Fe3ỵ (which would produce unfavourable charge imbalances in the olivine structure) is often found to be associated with exsolved sub-microscopic crystallites of chromite or magnetite inside the olivine Fe3ỵ is also likely to be present in the oxidation products commonly formed during the hydrothermal alteration of olivine Nomenclature Compositions within the forsterite-fayalite isomorphous solid solution series are the most abundant of the naturally occurring members of the olivine group, and the term olivine has come to signify compositions between these two end members, with general formula (Mg,Fe)2SiO4 In the past, different names have been assigned to specific ranges of Mg:Fe ratio (e.g., chrysolite, hyalosiderite, hortonolite, ferrohortonolite), but it is preferable to indicate the composition by giving the mole proportion of either the forsterite (Fo) or the fayalite (Fa) component in the solid solution This can be expressed in different ways; for example, an olivine with specific composition Mg1.8Fe0.2 SiO4 can be expressed as Fo90 (or Fa10), meaning that the olivine contains 90% of forsterite component (and 10% of fayalite component) Alternatively, the cation site occupancy in this olivine structure can be expressed as XMg ¼ 0.90, XFe ¼ 0.10, where XMg ẳ Mg/ (Mg ỵ Fe), XFe ẳ Fe/(Mg ỵ Fe), and XFe ỵ XMg ẳ This nomenclature can be extended to any number