Olivine crystal structure

Hazen R. M. (1977). Effects of temperature and pressure on the crystal structure of ferromag-nesian olivine. Amer. Mineral, 62 286-295. 

The crystal radius thus has local validity in reference to a given crystal structure. This fact gives rise to a certain amount of confusion in current nomenclature, and what it is commonly referred to as crystal radius in the various tabulations is in fact a mean value, independent of the type of structure (see section 1.11.1). The crystal radius in the sense of Tosi (1964) is commonly defined as effective distribution radius (EDR). The example given in figure 1.7B shows radial electron density distribution curves for Mg, Ni, Co, Fe, and Mn on the M1 site in olivine (orthorhombic orthosilicate) and the corresponding EDR radii located by Fujino et al. (1981) on the electron density minima. 

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). 

Figure 5.8 The crystal structure of olivine, (a) The structure projected onto (100) showing serrated chains of octahedra running parallel to the c axis (b) oxygen coordination polyhedra projected about the Ml and M2 positions. Metal—oxygen distances in each coordination site are indicated. Cell parameters and interatomic distances are for fay-alite (from Smyth Bish, 1988).

Compositional variations of intensity of the absorption band at 23,400 to 24,000 cm-1 attributed to Ni2+/M2 site cations have provided site occupancy data for the Mg2+-Ni2+ olivines (Hu et al., 1990). The results obtained from crystal field spectra showing strong cation ordering of Ni2+ ions in the olivine Ml sites are in good agreement with estimates from crystal structure refinements (e.g. Bostrom, 1987 Ottonello etal., 1989) described later ( 6.7.1.2). 

A number of X-ray crystal structure refinements of lunar and terrestrial Mg2+-Fe2+ olivines spanning wide composition ranges have revealed that Fe2+ ions are often slightly enriched in the smaller Ml sites (e.g., Finger, 1970 Brown... 

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. 

Princivalle, F. Secco, L. (1985) Crystal structure refinement of 13 olivines in the forsterite-fayalite series from volcanic rocks and ultramafic nodules. Tschermaks Mineral Petrol. Mitt., 34,105-15. 

The crystal structure of Y-C2S is similar to that of olivine, (Mg,Fe>2Si04 (Fig. I.4D) (U4). The calcium is octahcdrally coordinated. The unit cell and the arrangement of and 8104 ions show some similarities to those of... 

The electronic structures of silicate minerals of polymerization intermediate between nesosilicates and tektosilicates have been studied to a lesser extent than have SiOj or the olivines. The complexity of their crystal structures makes calculation difficult, and their diversity in terms of local chemical environment makes phenomenological assignment of their spectra difficult. Nonetheless, some recent comparative studies have given valuable electronic structure information on such materials. 

Durham, W. B., Goetze, C., Blake, B. (1977). Plastic flow of oriented single crystals of olivine. 2 Observations and interpretation of the dislocation structures. J. Geophys. Res., 82, 5755-70. 

Most trace elements have values of D< C 1, simply because they differ substantially either in ionic radius or ionic charge, or both, from the atoms of the major elements they replace in the crystal lattice. Because of this, they are called incompatible. Exceptions are trace elements such as strontium in plagioclase, ytterbium, lutetium, and scandium in garnet, nickel in olivine, and scandium in clinopyroxene. These latter elements acmally fit into their host crystal structures slightly better than the major elements they replace, and they are therefore called compatible. Thus, most chemical elements of the periodic table are trace elements, and most of them are incompatible only a handful are compatible. 

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