Forsterite crystal structure

Francis C. A. and Ribbe R H. (1980). The forsterite-tephroite series, I Crystal structure refinement. Amer. Mineral, 65 1263-1269. 

Hazen, R. M. (1976b) Effects of temperature and pressure on the crystal structure of forsterite. Amer. Mineral., 61,1280-93. 

Princivalle, F. Secco, L. (1985) Crystal structure refinement of 13 olivines in the forsterite-fayalite series from volcanic rocks and ultramafic nodules. Tschermaks Mineral Petrol. Mitt., 34,105-15. 

Figure 9.22 The crystal structure of forsterite, Mg2Si04, viewed in perspective along the [lOO] direction. The isolated Si04 tetrahedra alternately point up and down in rows alongthe a axis. The Mgl octahedra are at the corners of...

Another way that the choice of formulas can differ is that some choices can be multiples of other choices. This is most often seen in choosing solute species in solids, because there are no real species, just a crystal structure that is a solid solution. For example, the mineral oUvine is a soUd solution of two components, forsterite (Mg2Si04) and fayalite (Fe2Si04). The solution is represented by (Mg, Fe)2Si04, because the Mg and Fe atoms share the same positions in the crystal structure. 

These data fell on a line which also went through the O diffusion data for forsterite which had a similar crystal structure to that of mullite. 

Crystal field spectra of a chromium-bearing forsterite yielded bands at 23,500 and 16,900 cm"1 (Scheetz and White, 1972), indicating a CFSE of 20,280 cm-1 for Cr3+ in the olivine structure. Additional broad, asymmetric bands centred at 11,800 and 6,400 to 6,700 cm"1 were attributed to crystal field transitions in Cr2+ (Scheetz and White, 1972 Bums, 1974). Although EPR measurements of forsterite show slight enrichments of Cr3 in the Ml sites (Rager, 1977), polar-... 

The technique of channeling-enhanced X-ray emission (CHEXE) has enabled cation site occupancies to be determined in various minerals, including transition metal ions in spinels and ferromagnesian silicates (Taftp, 1982 Taftp and Spence, 1982 Smyth and Taftp, 1982 McCormick etal., 1987). The method, which is based on relative intensities of X-ray peaks measured on crystals with diameters as small as 50 nm under the electron microscope, is particularly useful for determining site occupancies of minor elements with concentrations as low as 0.05 atom per cent in a structure. The most important criterion for the determination of element distribution in a mineral by this technique is that the cation sites should lie on alternating crystallographic planes. In order to make quantitative site population estimates, additional information is required, particularly the occupancy of at least one element in one of the sites or in another site that lines up with one of the sites of interest (McCormick et al., 1987). For example, cation site occupancies by CHEXE measurements have been determined from X-ray peak intensity ratios of Si to Ni, Mn, Cr and Fe in forsterite, as well as thermal disordering of these cations in heated olivines (Smyth and Taftp, 1982). 

---