Site occupancy silicate minerals

The upper limit for tetrahedral Fe3+ I.S. in silicates is shown to be 0.25 mm/sec., whereas the lower limit for octahedral Fe3+ is 0.29 mm/sec. The correlations point to inconsistencies in Mossbauer spectral parameters and cation site occupancy assignments for clintonite, yoderite and sapphirine. New Mossbauer spectral data obtained for these minerals demonstrate that clintonites from skarn deposits contain tetrahedral Fe3+ and octahedral Fe3+ and Fe2+, with relative enrichment of Fe3+ in tetrahedral sites only octahedral Fe2+ and Fe3+ occur in sapphirines from granulite facies rocks and five-coordinated Fe3+ predominates over octahedral Fe3+ ions in yoderites from high grade metamorphic rocks. 

The crystal chemistry of many transition metal compounds, including several minerals, display unusual periodic features which can be elegantly explained by crystal field theory. These features relate to the sizes of cations, distortions of coordination sites and distributions of transition elements within the crystal structures. This chapter discusses interatomic distances in transition metal-bearing minerals, origins and consequences of distortions of cation coordination sites, and factors influencing site occupancies and cation ordering of transition metals in oxide and silicate structures, which include crystal field stabilization energies... 

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). 

Site occupancies in silicate minerals 6.7.1 Olivines 6.7.1.1 Fe2+ ions. 

Chapter 6 describes how crystal field stabilization energies influence the crystal chemistry of minerals containing the transition elements. Site occupancies of the cations in oxide and silicate structures are also discussed. 

Bums, R. G. (1972a) Mixed valencies and site occupancies of iron in silicate minerals from Mossbauer spectroscopy. Canad. J. Spectr., 17, 51-9. 

Application of Mossbauer spectroscopy to crystallography, mineralogy and geology is very similar to chemical applications, with the main emphasis on phase analysis and structure determination. In view of the great importance of iron in the earth s crust and the widespread occurrence of this element in rock-forming minerals, earth scientists have naturally focused attention on applications of - T e Mossbauer spectroscopy. One of the most important groups of rock-forming minerals are the silicates, in which particular lattice position is often occupied by more than two atomic species. In these cases, accurate site occupancy numbers for each species cannot be obtained by diffraction alone. Mossbauer spectroscopy has... 

In the dioctahedral 2 1 sheet-structure silicate with the occupied sites more than 85% occupied by Al, the structure seems to be able to compensate for the internal strain and can grow to a considerable size. The Al octahedral occupancy values of muscovite (>1.7) and the 2 1 dioctahedral clays (1.3—1.7) indicate that there is little overlap. It is likely that the decreased amount of tetrahedral twist induced by increasing the size of the octahedral cations and octahedral charge (decreasing Al) determines that a clay-size rather than a larger mineral will form. The R3+ occupancy value can be less than 1.3 when the larger Fe3+ is substituted for Al. When Al occupancy values are less than 1.3 (65%), in the absence of appreciable iron, the internal strain is such that growth is in only one direction. The width of the layer is restricted to five octahedral sites. Sufficient layer strain accumulates within this five-site interval such that the silica tetrahedral sheet is forced to invert to accommodate the strain. 

IHite/Smectite. Another common intergrowth of sheet silicates is the mixed-layering of illite and smectite. As discussed above, illite and smectite are clay minerals whose basic structures resemble the mica muscovite. Their compositions may differ significantly from muscovite, but they generally have a lower occupancy of the interlayer sites than mica. Numerous other compositional differences are possible for smectite however, this discussion will be restricted to a dioctahedral illite and a dioctahedral smectite containing potassium and vacancies in the interlayer sites as given above. 

DellaGuardia and Thomas reported similar decay profiles for [Ru(bpy)3] " bound to montmorillonite and attributed the components to two species, one at the outer smface and edges and one between the silicate sheets (intercalated) (79). Schoonheydt et al. also pointed out that [Ru(bpy)3] " is adsorbed on the external (edge sites) and interlamellar surfaces (planar sites), even at low loadings of [Ru(bpy)3] (80). This was first proposed by Tmro et al. (81). The occupancy of edge sites with respect to planar sites increases with decreasing particle size of the clay minerals. Nakamma and Thomas also reported two different adsorption sites for [Ru(bpy)3] on laponite (82). At low concentrations of laponite, [Ru(bpy)3] is adsorbed on outer layers and is in contact with the aqueous phase. 

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