Forsterite, structure

Fig. 5.9. Electron density distributions in olivines (a) Experimental difference density map of part of the forsterite structure showing the residual peaks around Si. Contours are at intervals of 0.1 electrons A negative contours being broken and zero contours dotted. Numbers in decimal fractions of the a length indicate the heights of the atoms. The tetrahedron formed by oxygen atoms around Si is shown (after Fujino et al., 1981 reproduced with the publisher s permission), (b) A comparison of a theoretical difference density map (i) of the O-Si-O group in the monosilicic acid molecule [Si(OH)4] with an experimental map (ii) of the same group in the monosilicate mineral andalusite (AljSiO,). Contours are at intervals of 0.07 electrons A in (ii). The region around the nucleus of each atom in the theoretical map represents the core region, where the data are not expected to be accurate (after Gibbs, 1982 reproduced with the publisher s permission).

Such kind of emission, where the bands remain not quenched at low temperatures, was not detected in chromium activated forsterite, which was very carefully studied. Consequently its connection with other impurity may be supposed. Divalent magnesium in octahedron of the forsterite structure has ionic radius of 0.72 A, which is suitable for substimtion by especially taking into account that charge compensation is not needed in this case. Similar emission was found also in magnesium bearing sinhalite MgAlB04 (Fig. 4.177). 

Fayalite (melting point 1205°C) and forsterite (melting point 1890°C) form continuous solid solutions (the melting diagram of their binary mixtures correspond to the type exemplified in Fig. 2.28). Phase equilibria as a function of pressure, show, however, transition into the spinel structure. (cF56-MgAl204, spinel structural type). 

Francis C. A. and Ribbe R H. (1980). The forsterite-tephroite series, I Crystal structure refinement. Amer. Mineral, 65 1263-1269. 

These two structures have been described above (Sect. 2.6.3 and Sects. 2.8.1, 3.2) and are depicted in Figs. 16 and 29. We have already suggested (Sect. 4) that in forsterite (a-Mg2Si04, olivine type) the repulsive forces between O... O (d 2.6 A) are less than those between Mg... Mg (d = 3.0 A) which, in turn, are less than those between Mg... Si (d 2.7 A). One would therefore expect that in the phase transition a-Mg2Si04 (olivine) — y-Mg2Si04 (spinel) the principal result would be an increase in the last of these distances, in order to relieve the largest non-bonded repulsions . And this is indeed the case the mean non-bonded (next-nearest neighbour) distances are as shown... 

The nebulization was also employed to generate composite powders for specific applications, such as in ceramics, by hydrolyzing with water vapor droplets containing Al(5ec-OBu) and silicon methoxide in the atomic ratio Al/Si = 3. This ratio of alkoxides was chosen in order to produce mullite, which was achieved by calcination of the resulting amorphous particles at rather high temperatures (up to I400 C) (52). In another approach a mixed Al-Mg-Si ethoxide was first synthesized, and then nebulized and hydrolyzed as usual (77). Depending on the experimental conditions, the powders calcined at 500 C exhibited structures of pure cordierite, or mixed with forsterite. In all of these described cases the nebulization yielded spherical but polydisperse particles. 

Fig. 2.74 Structure of forsterite (Mg2SiO4), (2, 2), ci. of Mg. In the top left is drawn the arrangement of tetrahedral SiO4, in the bottom left the unit cell, in the centre the arrangement of (2, 2)ccp of Mg. This is a Nijin-type structure.

Fig. 2.76 Structure of clinohumite (4Mg2SiO4-Mg(OH)2), (3, 2 )(2ct of Mg. This structure can be interpreted as the intergrowth of norbergite and forsterite.

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.

SiC>4 tetrahedra around the divalent cation produces an octahedral configuration (see Section 10.3.1). Forsterite has oxide ions hep (P layers), one-eighth of the T sites occupied by Si, and half of the O sites occupied by Mg. Topaz, [A1(F, OH)]2SiC>4 (Section 10.3.2) contains SiC>4 tetrahedra and A1(0, F)6 octahedra. Orthosilicates (SiO ) are the most compact of the silicate structures. 

The occurrence of minerals which show CL is highly dependent on the type of meteorite. Possibly the most common phase which occurs is feldspar. Because this mineral accepts very little Fe into the structure, quenching is not a problem however, because the feldspar structure is quite open, the Na- and K-rich feldspars are easily damaged by electron beams. In contrast anorthite, the Ca rich variety, is quite stable. Pyroxene and olivine are common phases in meteorites but because they both usually contain iron, most do not luminesce. Only in the primitive meteorites do nearly pure enstatite and forsterite occur and both show brilliant CL. Other minerals are rare but include ... 

Crystal field spectra of a chromium-bearing forsterite yielded bands at 23,500 and 16,900 cm"1 (Scheetz and White, 1972), indicating a CFSE of 20,280 cm-1 for Cr3+ in the olivine structure. Additional broad, asymmetric bands centred at 11,800 and 6,400 to 6,700 cm"1 were attributed to crystal field transitions in Cr2+ (Scheetz and White, 1972 Bums, 1974). Although EPR measurements of forsterite show slight enrichments of Cr3 in the Ml sites (Rager, 1977), polar-... 

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