Ferromagnesian silicates

Bums, R.G. (1993) Rates and mechanisms of chemical weathering of ferromagnesian silicate minerals on Mars. Geochim. Cosmochim. Acta 57 4555-4574... 

Ganguly J. (1982) Mg-Fe order-disorder in ferromagnesian silicates, 11 thermodynamics, kinetics and geological applications. In Advances in Physical Geochemistry, Vol. 2 (ed. S. K. Saxena), pp. 58-99. Springer-Verlag. 

Fe2+ many green ferromagnesian silicates such as peridot, diopside and actinolite, gillespite,... 

A review of the literature shows that there is a vast amount of crystal field spectral data for iron, the major transition metal in silicate and oxide minerals. The focus of this chapter, therefore, is mainly on ferromagnesian silicates. However, there is also a significant amount of information for chromium-, vanadium- and manganese-bearing minerals. The data are more sporadic for other cations. The optical spectra of the transition metal-bearing minerals enable semi-quantitative estimates to be made of the relative CFSE s of Fe2+, Cr3+, Mn3+, V3+, Ti3+, Ni2+ and Co2+ in many mineral structures. Note, however, that Mn2+ and Fe3+ in high-spin states acquire zero CFSE in oxides and silicates. The crystal field spectra of Mn(II) and Fe(III) minerals are described separately later in the chapter ( 5.10.6 and 5.10.7). 

In ferromagnesian silicates, the order of decreasing CFSE values is... 

Chapter 5 summarizes the crystal field spectra of transition metal ions in common rock-forming minerals and important structure-types that may occur in the Earth s interior. Peak positions and crystal field parameters for the cations in several mineral groups are tabulated. The spectra of ferromagnesian silicates are described in detail and correlated with the symmetries and distortions of the Fe2+ coordination environments in the crystal structures. Estimates are made of the CFSE s provided by each coordination site accommodating the Fe2+ ions. Crystal field splitting parameters and stabilization energies for each of the transition metal ions, which are derived from visible to near-infrared spectra of oxides and silicates, are also tabulated. The CFSE data are used in later chapters to explain the crystal chemistry, thermodynamic properties and geochemical distributions of the first-series transition elements. 

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). 

In ferromagnesian silicates, therefore, Ni2+ ions are expected to be enriched over Mg2+ in smallest octahedral sites, with the other divalent transition metal ions favouring larger sites in the crystal structures. Thus, based on the ionic radius criterion alone, the olivine Ml and pyroxene Ml sites would be expected be enriched in Ni2+, with the other divalent cations showing preferences for the larger olivine M2 and pyroxene M2 sites. Similarly, in aluminosilicates, all trivalent transition metal ions are predicted to show preferences for the largest [A106] octahedron. 

Several qualitative predictions and interpretations of the relative enrichments of transition metal cations in different coordination sites having octahedral, tetragonal, trigonal and lower symmetries may be made based on the theoretical stabilization energies summarized in table 6.4. The predicted and observed enrichments of transition metal ions in ferromagnesian silicate crystal structures are summarized in table 6.5. 

Figure 7.6 Ranges of CFSE of Fe2+ ions in ferromagnesian silicates. Legend to mineral symbols is given in table 5.16.

The predicted orders of relative Fe2+ enrichments expressed in eqs (7.18) and (7.19) are strictly valid only for mineral formation at 25 °C and atmospheric pressure, the conditions under which absorption spectral measurements producing the CFSE data shown in fig. 7.6 were made. In order to apply the CFSE data summarized in table 5.16 and fig. 7.6 to igneous and metamorphic assemblages, and in the absence of spectral data for each ferromagnesian silicate at elevated temperatures and pressures, it must be assumed that all phases show similar variations of CFSE with rising temperatures and pressures. 

Distributions of divalent transition metal ions between coexisting ferromagnesian silicates... 

A relationship between octahedral site preference energies (table 6.3) and distribution coefficients has been demonstrated for transition metal ions partitioned between olivine or pyroxene crystals and the groundmass of oceanic basalts, which is assumed to represent the composition of the magma from which the ferromagnesian silicates crystallized (Henderson and Dale, 1969 Dale and Henderson, 1972). Plots of In D against OSPE, such as those illustrated in fig. 7.8, show linear trends between the two parameters. 

Fe/Mg ratios in coexisting silicates. The CFSE s acquired by Fe2+ ions in ferromagnesian silicates obtained from spectral measurements may be used to explain Fe/Mg ratios in coexisting silicate minerals. The orders of decreasing CFSE between igneous mineral assemblages... 

Figure 8.5 A mechanism for the dissolution and hydrolysis of a ferromagnesian silicate. The figure is a two-dimensional representation of the reaction described in the text ( 8.7.1). Oxygens of two additional silicate groups lie above and below the plane.

---