Silicate spinels

Brown G. E. (1982). Olivines and silicate spinels. In Reviews in Mineralogy, vol. 5, P. H. Ribbe (series ed.), Mineralogical Society of America. 

The reflectance spectrum of Ni2Si04 spinel contains intense bands with absorption maxima of 9,150 cm-1, 14,780 cm-1 and 22,550 cm-1 at atmospheric pressure (Yagi and Mao, 1977). These led to initial estimates of the crystal field parameters for Ni2+ in silicate spinel of A0 = 9.,150 cm-1 and CFSE = 10,980 cm-1. However, there is also a prominent shoulder in the reflectance spectra around 8,000 cm-1 attributable to trigonal distortion of the octahedral site in the spinel structure (Bums, 1985a). This led to revised estimates for Ni2+ in Ni2Si04 spinel of... 

Thus, nickel should be enriched relative to pyroxenes in dense Upper Mande minerals (e.g., silicate spinel and olivine) and their low-temperature alteration products (e.g., gamierite). 

Finger, L. W., Hazen, R. M. Yagi, T. (1979) Crystal structures and electron densities of nickel and iron silicate spinels at elevated temperatures and pressures. Amer. Mineral., 64,1002-9. 

An X-ray analysis carried out on the insulator-insulator solids and on the insulator-semiconductor solids revealed that they were amorphous or badly crystallized. Since the preparation temperature was rather low, the presence of bulk compounds such as silicates, spinels and titanates is very unlikely although the incipient formation of disordered surface compounds cannot be excluded. Thus the solids should be considered intimate mixtures in which the chemical features of each component are not changed in a significant way. Indeed the I.R. measurements also support the idea that interaction between the components of the binary oxides is not strong. 

The enthalpy of formation from the oxides of Mg2Sn04 and Co2Sn04 were found by oxide solution calorimetry. Using thermochemical data for the formation of olivine, for olivine - spinel transitions and for the transformation of quartz to stishovite, pressures for the disproportionation of silicate spinels were calculated to be in the range 150 - 200 kbar. 

Natural iron oxides are commonly formed by weathering of iron-rich primary minerals (silicates, spinels) in an environment where other metal cations are also present in these processes. For this reason, natural iron oxides may contain different amounts of various cations incorporated into their structure. Due to their abundance in the Earth s crust as well as chemical and physical properties similar to those of iron, the most common metal found in natural iron oxides is Al [181-192], but the presence of many others, such as Mg [193,194], Si [195-197], Ti [194,198], V [199], Cr [ 186,192,200], Mn [ 192,194,200,201 ], Co [192,200], Ni [186,192,200,202,203], Cu [200], Zn [192,200], and Ge [204] has been observed. 

Navrotsky, A. Calculation of effect of cation disorder on silicate spinel phase boundaries. This volume (1976). 

Brady PV, Walther JV (1989) Controls on silicate dissolution rates in neutral and basic pH solutions at 25 °C. Geochim Cosmochim Acta 53 2823-2830 Brown GE (1980) Olivines and silicate spinels. In Ribbe PH (ed) Reviews in mineralogy 5. Ortho-silicates. Mineralogical Society of America, Washington DC, pp 275-333 Buffle J (1990) Complexation reactions in aquatic systems. Ellis Horwood, New York, 692 pp... 

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