Pyroxene structure

Figure 4.4 Infinite chain silicates (single, double, and sheet) (a) infinite single chain silicate with two corners shared per tetrahedron (pyroxene structure) (b) infinite double chain, with alternate two and three corners shared (am-phibole structure) (c) infinite sheet structure, with each tetrahedron sharing three corners (sheet silicates). (From Putnis, 1992 Figure 6.3, by permission of Cambridge University Press.)...

Figure 5,19 (A) Schematic representation of [Si03] chains of pyroxenes (a) projection on plane (100) (b) projection along axis Z (c) projection along axis Y (d) perspective representation. (B) Relative arrangement of tetrahedral chains in pyroxene structure (seen in their terminal parts with intercalation of Ml and Ml positions. From Putnis and McConnell (1980). Reproduced with modihcations by permission of Blackwell Scientific Publications, Oxford, Great Britain.

Figure 532 Structural features of amphiboles. (A) Double chain [T40n] seen along axis c (a) and in perspective (b). (B) Double chain seen from terminal part and various cationic positions (compare with figure 5.19, for analogies with the pyroxene structure).

Pyroxenes from extraterrestrial sources provide unequivocal examples of Ti3+ —> Ti4+ IVCT and Fe2+ —> Ti4+ IVCT bands. For example, the iron-free green titanian pyroxene in the Allende meteorite discussed in 4.4.1 is the one irrefutable example of a mineral showing a Ti3+ — > Ti4+ IVCT transition. The position of the band at 666 nm (15,000 cm-1) shown earlier in fig. 4.2 is insensitive to pressure, but it does intensify at high pressures (Mao and Bell, 1974a), consistent with it representing a Ti3+ —> Ti4+IVCT transition between adjacent Ti3+ and Ti4+ ions located in edge-shared Ml octahedra in the pyroxene structure (fig. 5.13). 

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. 

Figure 5.13 The calcic pyroxene structure projected onto the (001) plane showing the locations of the Ml and M2 cation sites (based on Cameron Papike, 1981). Note the chains of edge-shared Ml octahedra extending along the c axis.

Hazen et al 1978), in accord with the A a R 5 relationship (eq. 2.17). The positions of the 1 micron and 2 micron bands (cf. fig. 10.5) provide a powerful means of identifying pyroxene structure-types and compositions on surfaces of terrestrial planets by telescopic reflectance spectral measurements discussed in chapter 10. 

The spectra illustrated in fig. 5.15 show that absorption maxima of all spin-allowed CF bands move to longer wavelengths with increasing iron content of the orthopyroxene, forming the basis of composition determinative curves (Hazen et al., 1977b Adams, 1974) and enabling this pyroxene structure-type to be identified in telescopic reflectance spectra of surfaces of the Moon (Pieters etal., 1985 Bums, 1989a). 

The cause of colour in natural and synthetic chromium-bearing blue diop-sides has been widely debated and assignments of absorption bands in their polarized spectra remain controversial (Mao et al., 1972 Bums, 1975a,b Ikeda and Yagi 1977, 1982 Schreiber, 1977, 1978). One interpretation is that low-spin Cr3 ions in tetrahearal sites in the pyroxene structure are responsible for the colour and spectra of blue diopsides (Ikeda and Yagi, 1977, 1982). 

The MgSi03 pyroxene structured polymorphs are not well understood even though they are important rock-forming minerals and a significant component... 

The pyroxene structures simulated with this potential model are shown in Table 3.6. We have compared enstatite (Pbca) and low clinoenstatite (P2j/c) with observed structures at 300 K and 0 GPa. The protoenstatite structure is... 

Table 3.6 Comparison of simulated and experimental pyroxene structures. ...

Fig. 48. An a axis projection of LiScSi20j having pyroxene structure (Hawthorne and Grundy, 1977).

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