Orthopyroxene cation ordering

Carpenter M. A. and Salje E. K. H. (1994a). Thermodynamics of nonconvergent cation ordering in minerals, II Spinels and the orthopyroxene solid solution. Amer. Mineral, 79 1068-1083. 

Ganguly J. and Domeneghetti M.C. (1996) Cation ordering of orthopyroxenes from the Skaergaard intmsion implications for the subsolidus cooling rates and permeabilities. Contrib. Mineral. Petrol. 122, 359-367. 

Ganguly J. and Stimpfl M. (2000) Cation ordering in orthopyroxenes from two stony-iron meteorites implications for cooling rates and metal-silicate mixing. Geochim. Cosmo-chim. Acta 64, 1291-1297. 

Other examples discussed later where changes of spectrum profiles across a solid-solution series correlate with cation ordering in the crystal structure include Ni-Mg olivines (Hu et al., 1990), in which Ni2+ ions are strongly ordered in the Ml sites ( 5.4.2.4), and Mg-Fe2+ orthopyroxenes mentioned earlier where strong enrichment of Fe2+ ions occurs in the very distorted M2 sites ( 5.5.4). 

Variations of extinction coefficients and spectrum profiles with changes in chemical composition of a mineral provide information on cation ordering in the structure. Examples involving Al3+-Mn3+ ordering in epidotes and andalusites are discussed in 4.4.2 and 4.5, and Mn2+-Fe2+ ordering in olivine is illustrated in fig. 4.8. Compositional variations of intensities of absorption bands in polarized spectra of orthopyroxenes described in 5.5.4. (fig. 5.15) have yielded Fe2+/M2 site populations (Goldman and Rossman, 1979), while similar trends in the crystal field spectra of synthetic Mg-Ni olivines described in 5.4.2.4 (fig. 5.12) have yielded site occupancy ratios of Ni2+ ions in the olivineMl and M2 sites (Hu etal., 1990). 

Therefore, the true configurational entropy is 0.95 J/(deg. mole) lower than the maximum value as a result of Fe2+-Mg2+ ordering in the orthopyroxene structure. The cation ordering found in other members of the enstatite—ferrosilite series, as well as the synthetic Mg2+-Ni2+, Mg2+-Co2+ and Mg2+-Mn2+ pyroxenes (Ghose et al., 1975 Hawthorne and Ito, 1977), shows that most transition metal-bearing orthopyroxenes are not ideal solid-solutions. 

The observed relative enrichments of Fe2+ ions in coordination sites within individual silicate minerals were discussed in 6.7 and cation ordering trends shown by olivines and orthopyroxenes were summarized in table 6.5. These intersite partitioning patterns are partially explained by the relative CFSE s attained by Fe2+ ions in each coordination site of the mineral structures ( 6.8.3.1). 

In pro-grade metamorphic reactions, on the other hand, effects of intersite cation ordering at elevated temperatures could influence Fe/Mg ratios of orthopyroxene-olivine assemblages in granulite facies rocks. For the olivine-orthopyroxene exchange reaction... 

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