Pyroxene crystal structure

Table 5.33 lists the structural characters of various pure components of pyroxene crystallizing in the stable state, according to the synthesis of Smyth and Bish (1988). Note the doubling of cell edge a in the transition from monoclinic to orthorhombic structure, composition being equal (i.e., clinoenstatite-enstatite clinoferrosilite-ferrosilite). This effect may be represented by the equation... 

Tokonami M., Horiuchi H., Nakano A., Akimoto S., and Morimoto N. (1979). The crystal structure of pyroxene type MnSi03. Min. Jour., 9 424M26. 

The pyroxene group of minerals, where cations substitute one for another at interchain sites, also have a tetrahedral Si site that may contain Al. Some of these substitutions, especially those at M2, can distort the ideal crystal structures, as is depicted in Fig. 2.10. The small distortions are detected as variations in bond lengths between adjacent atoms during crystal structure analyses. These data, combined with accurate composition analyses, could indicate some of the conditions present during growth of the crystal and contribute to our understanding of why some mineral species have formed as fibers. Unfortunately, few detailed crystal structural analyses have been performed on pyroxene minerals with fibrous habits. 

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. 

The Ni2+, Cr3+ and low-spin Co3+ ions do not acquire additional stabilization in distorted octahedral sites. They are expected to favour smaller sites that more closely approximate octahedral symmetry than other available sites in the crystal structures. As noted in 6.8.2, the high octahedral CFSE s acquired by these three cations in small octahedral sites in silicate and oxide structures accounts for the observed relative enrichments of Ni2+ in the olivine Ml and orthopyroxene Ml sites, the sole occupancy by Cr3+ of pyroxene Ml sites, and the occurrence and stability of low-spin Co3+ in Mn(IV) oxides. 

Figure 17-2. Projection along [100] of the crystal structure of an amphibole and its modular slicing as modules of mica and pyroxene...

Figure 32. The crystal structures of foggite and whitmoreite (a) foggite projected onto (010) (b) foggite projected onto (001) (c) foggite projected onto (100), showing its similarity to the monoclinic pyroxene (calcium tschermaks) stmcture (d) whitmoreite projected onto (100) (e) whitmoreite projected onto (010). (Al( )6) octahedra are 4" -net-shaded, (Fe ( )6) octahedra are dot-shaded, Ca atoms are shown as small shaded circles, (H2O) groups are shown as large shaded circles, Fe atoms are shown as small shaded circles in (e).

FIGURE 7.11 The crystal structure of a pyroxene (spodumene) shows layers of Li+ (larger) and AP (smaller) ions in octahedra alternating with layers of Si in tetrahedra giving a nominal formula of LiAISisOe. 

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