Olivine group forsterite

Other members of the olivine group have the same structure, but with Mg2+ ions partly or wholly replaced by Fe2+ or Mn2+. In olivine itself about io per cent of the Mg2+ ions of forsterite are replaced by Fe2+ fayalite and tephroite have the idealized compositions Fe2Si04 and Mn2Si04, respectively. 

Forsterite is a mineral of the olivine group, which possess orthorhombic symmetry and a structure consisting of independent Si04 tetrahedra linked by divalent atoms... 

The end members of the olivine group, the structure of which consists of individual Si04 tetrahedra linked by divalent ions in sixfold coordination, are forsterite (Mg2Si04) and fayalite (Fe2Si04). Those commonly found in soils are usually intermediate in composition and partially altered because of the ease with which olivine weathers. The structure is orthohombic. 

Fig. 5.9. Electron density distributions in olivines (a) Experimental difference density map of part of the forsterite structure showing the residual peaks around Si. Contours are at intervals of 0.1 electrons A negative contours being broken and zero contours dotted. Numbers in decimal fractions of the a length indicate the heights of the atoms. The tetrahedron formed by oxygen atoms around Si is shown (after Fujino et al., 1981 reproduced with the publisher s permission), (b) A comparison of a theoretical difference density map (i) of the O-Si-O group in the monosilicic acid molecule [Si(OH)4] with an experimental map (ii) of the same group in the monosilicate mineral andalusite (AljSiO,). Contours are at intervals of 0.07 electrons A in (ii). The region around the nucleus of each atom in the theoretical map represents the core region, where the data are not expected to be accurate (after Gibbs, 1982 reproduced with the publisher s permission).

The detailed structure of forsterite, Mg2Si04, an olivine-type material, has been studied by both vibration-rotation and e.p.r. spectroscopy. The i.r. and Raman spectra of various isotopically substituted species of forsterite have been measured new assignments of the bands have been obtained from a group-theoretical analysis of the olivine structure and the Mg— Mg and Si— Si isotopic shifts. The e.p.r. spectrum of low concentrations of Fe + cations in synthetic forsterite has been observed at 9 GHz and 35 GHz. It is concluded from the results that the Fe + cations are ordered in the octahedral M(2) sites of the olivine-type structure neither the valence state nor the site occupancy of the Fe cations is altered by heat treatment up to 110 C. 

Olivine sands are a mineral group which includes forsterite, fayalite and others. The characteristics of olivine sands are as follows ... 

Orthosilicates exhibit either no or limited polymerisation of the silica tetrahedra. The mineral forsterite (a variety of olivine) belongs to this group. 

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