Orthopyroxene crystal structure

Figure 5.16 The orthopyroxene crystal structure. The figure shows the structure projected onto (001). Oxygen coordination polyhedra [(100) projections] and metal-oxygen distances in each site are indicated (pm). Ml M2 Si. Atomic coordinates and cell parameters from Ghose (1965).

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). 

In some cases, the CFSE attained by a transition metal ion in a regular octahedral site may be enhanced if the coordination polyhedron is distorted. This effect is potentially very important in most silicate minerals since their crystal structures typically contain six-coordinated sites that are distorted from octahedral symmetry. Such distortions are partly responsible for the ranges of metal-oxygen distances alluded to earlier, eq. (6.6). Note, however, that the displacement of a cation from the centre of a regular octahedron, such as the comparatively undistorted orthopyroxene Ml coordination polyhedron (fig. 5.16), also causes inequalities of metal-oxygen distances. 

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. 

Hawthorne, F. C. Ito, J. (1977) Synthesis and crystal structure refinement of transition metal orthopyroxenes. I. Orthoenstatite and (Mg,Mn,Co) orthopyroxene. Canad. Mineral., 15, 321-38. 

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). 

Pyroxenes have the general formula X(Y)(Si03)2. They crystallize in a monochnic (clinopyroxenes) or orthorhombic (orthopyroxenes) structure. The Y cations are located in the Ml sites which are moderately distorted octahedra whereas X represents cations in highly distorted sites with six-, seven- or eight-fold coordination according to the kind of mineral. Generally, the structure is orthorhombic when X is Mg or Fe " and is monoclinic when X is Ca or Na. 

---