Pyroxenes cation ordering

The pyroxene structure is also of considerable interest to mineral spectro-scopists because, like olivine, it again contains distinguishable coordination sites yielding distinctive Fe2+ crystal field spectra. In contrast to olivine, however, Fe2+ ions in pyroxenes show strong intracrystalline cation ordering, so that there are major compositional variations of visible to near-infrared spectra. 

Table 6.5. Predicted and observed cation ordering in olivines and pyroxenes...

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. 

Only in calcic clinopyroxenes, in which Ca2+ ions completely fill the M2 sites and Fe2+ and other transition metal ions occur in the Ml sites alone, is ideal solution behaviour to be expected. This is because cation ordering is not possible in one-site atomic substitution in the pyroxene Ml site. Furthermore, there is an insignificant variation of the CFSE of Fe2+ across the diopside-hedenbergite series ( 5.5.3). 

Future work should emphasize calorimetric and structural studies on the same samples, these materials being well-characterized in terms of chemistry and teii5)eratiure of equilibration. Problems of cation ordering in pyroxenes are ripe for such study. 

Figure 5.3 The amount of order in silicates can vary dramatically. A. The crystalline backbone structures for olivine, pyroxene, and quartz. The charge of the silicon tetrahedra is neutralized by metal cations in olivine and pyroxene. B. Silicate melts contain a mix of unaligned crystalline structures with metal cations randomly distributed in the melt. C. Chaotic condensates have not formed silicate tetrahedra rather, they appear more like a frozen gas state. These materials are typically under-oxygenated and contain more metals than a glass. Annealing supplies the chaotic silicate with the energy needed to rearrange into the more stable silicate tetrahedra. D. The gas phase largely consists of SiO. Metals are typically present as atoms or simple monoxides while excess oxygen can be found as OH (Nuth et al. 2002).

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