Olivine cation ordering

Annersten H., Adetnuji I, and Filippidis A. (1984). Cation ordering in Fe-Mn silicate olivines. Amer. Mineral, 69 1110-1115. 

Bish D. L. (1981). Cation ordering in synthetic and natural Ni-Mg olivine. Amer. Mineral, 66 770-776. 

Ottonello G., Della Giusta A., and Molin G. M. (1989). Cation ordering in Ni-Mg olivines. Amer. Mineral, 74 411 21. 

Redfern S.A.T., Henderson G.M.B., Wood B. J., Harrison R.J., and Knight K.S. (1996) Determination of olivine cooling rates from metal-cation ordering. Nature 381, 407-409. 

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). 

From a crystal chemical standpoint, the olivine structure is particularly interesting because it contains divalent cations in two distinct six-fold coordination sites having contrasting symmetries. Consequently, crystal field spectra of several transition metal-bearing olivines have been studied in attempts to detect cation ordering, including evidence for site preferences of Mn2+ ( 4.5) and Ni2+ (Hu et al., 1990), and to estimate CFSE data for Fe2+ and Ni2+ in each site in the olivine structure. 

These values still conform with other estimates (Wood, 1974 Bums, 1985a Hu et al., 1990) indicating that Ni2+ ions receive a higher CFSE in the olivine Ml site, accounting for the strong cation ordering of Ni2+ ions in this slightly smaller coordination site ( 6.7.1.2). 

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. 

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). 

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

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... 

Bums, R. G. (1970) Crystal field spectra and evidence for cation ordering in olivine minerals. Amer. Mineral., 55, 1608-32. 

Huggins, F. E. (1973) Cation order in olivines Evidence from vibrational spectra. Chem. Geol., 11,99-109. 

Ribbe, P. H. Lumpkin, G. R. (1984) Cation ordering in Ni-Fe olivines corrections and discussion. Amer. Mineral., 69,161-3 [see Annersten etal. (1982)]. 

Figure 14. Temperature dependence of non-convergent metal cation-ordering in several olivines and spinels, all measured by Rietveld refinement of nentron powder diffraction data.

Redfern, S.A.T., Dove, M.T., Wood, D.R.R. (1997b) Static lattice simulation of feldspar solid solutions ferroelastic instabilities and order/disorder. Phase Trans 61 173-194 Redfern, S.A.T., Knight, K.S., Henderson, C.M.B., Wood, B.J. (1998) Fe-Mn cation ordering in fayalite-tephroite (FexMni U2Si04 olivines a neutron diffraction study. Mineral Mag 62 607-615 Redfern, S.A.T., Harrison, R.J., O Neill, H.St.C., Wood, D.R.R. (1999) Thermodynamics and kinetics of cation ordering in MgAl204 spinel up to 1600°C from in situ neutron diffraction. Am Mineral 84 299-310... 

Redfern, S.A.T., Artioli, G., Rinaldi, R., Henderson, C.M.B., Knight, K.S., Wood, B.J. (2000) Octahedral cation ordering in olivine at high temperature. II An in situ neutron powder diffraction study on synthetic MgFeSi04 (Faso). Phys Chem Minerals (in press)... 

---