Diopside, structure

Atomistic simulations have also been performed on pyroxenes and pyroxenoids by Catlow et al. (1982) and predict the stable Mg end member to have the diopside structure and the stable Ca end member the wol-lastonite structure, as observed. However, only pair potentials were employed in this study, and Post and Burnham (1986) have found that diopside structures are poorly described within such a model. It would clearly be of interest to repeat the simulations using the three-body silicate potentials, which have yielded accurate Si-O-Si angles in quartz (Catlow et al., 1985). 

X-ray photoelectron spectroscopy, 144 palladium complexes structure, 144 Diopside structure, 856 Dioxygen activation, 682 biology, 681-711 chemistry, 682 evolution... 

Figure 4.17 A clinogmphic view of the diopside structure, a clinopyroxene. The ideal density for diopside is 3.28 g/cc.

NaAs03 has an infinite polymeric chain anion similar to that in diopside (pp. 349, 529) but with a trimeric repeat unit LiAs03 is similar but with a dimeric repeat unit whereas /6-KASO3 appears to have a cyclic trimeric anion As309 which resembles the cyc/o-trimetaphosphates (p. 530). There is thus a certain structural similarity between arsenates and phosphates, though arsenic acid and the arsenates show less tendency to catenation (p. 526). The tetrahedral As 04) group also resembles PO4) in forming the central unit in several heteropoly acid anions (p. 1014). 

More than 90% of the rocks and minerals found in the earth s crust are silicates, which are essentially ionic Typically the anion has a network covalent structure in which Si044-tetrahedra are bonded to one another in one, two, or three dimensions. The structure shown at the left of Figure 9.15 (p. 243), where the anion is a one-dimensional infinite chain, is typical of fibrous minerals such as diopside, CaSi03 - MgSi03. Asbestos has a related structure in which two chains are linked together to form a double strand. 

Silicate lattices. The red circles represent oxygen atoms. The black dot in die center of die red circle represents the Si atom, which is at the center of a tetrahedron. (Left) Diopside has a one-dimensional infinite chain. (Right) A portion of the talc structure, which is composed of infinite sheets. 

Diol complexes, 9 211 d ion complexes, heavier, structure changes prior to ligand exchange, 34 258-259 Diopside, 4 61 crystal structure of, 4 51 p-Dioxane complexes, osmium, 37 312... 

Si04 units share two corners to form infinite chains (Figure 1.52(c)). The repeat unit is SiOs . Minerals with this structure are called pyroxenes (e.g., diopside (CaMg(S 103)2) and enstatite (MgSiOs)). The silicate chains lie parallel to one another and are linked together by the cations that lie between them. 

Proof that the anions of the substances mentioned really consist of long chains of P04 tetrahedra has been supplied by complete structural analyses of crystals. Four different types of such chain-like anions, differing in shape and inner periodicity, have so far been recognized. In the low temperature form of lithium polyphosphate, which has the same type of structure as lithium polyarsenate and diopside [Ca, Mg(Si03)2]x these are extended chains which (see Fig. 9a) as in enstatite have a Zweierperiodizit at (138) (see also ref. 66 in Table XVI). Rubidium polyphosphate has quite similar Zweierketten (55) (see Fig. 9b and ref. 63 in Table XVI) except that the P04 tetrahedra are somewhat rotated with respect to Form a. In the high... 

On the other hand, fine-grained CAIs consist of aggregates of nodules with a concentric structure consisting of spinel cores surrounded by melilite, anorthite, and diopside, or melilite cores surrounded by anorthite and diopside (Fig. 7.5). The sequence of minerals in the nodules is consistent with a condensation sequence. However, the textures of the overall objects are complicated and often indicate extended and multistage histories. For... 

Table III. PCM modelization of 170 efg tensors in diopside CaMgSi206 and forsterite Mg2Si04.Units for Vaa are V.A"2 and 1 - yj = 6.3 as for the corundum structure.

Fig. 9, from Warren s 1929 paper on the crystal structure and chemical composition of amphiboles 48>, is a physical picture of the silicon-oxygen chain in diopside. Large circles represent van der Waals-like domains of oxide ions (r = 1.40 A 2>) smaller, dashed circles represent van der Waals-like domains of silicon cations [r = 0.41 A 2>). 

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). 

Levien, L. Prewitt, C. T. (1981) High-pressure structural study of diopside. Amer. Mineral., 66, 315—23. 

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