Olivines Electronic structures

A more complete view of the electronic structure of Mg2Si04 may be obtained by comparing its photoelectron spectra with Si, Mg, and O x-ray emission spectra (Al-Kadier et al., 1984). The only feature not seen in our previous analysis of the electronic structure of Si02 is the position of the Mg 3p-0 2p bonding orbitals. From the data of Urch (1985) shown in Fig. 5.7, we see that the Mg p character, as shown in the Mg x-ray emission spectrum, occurs both in the predominantly O 2s orbital set and 

Additional evidence for the effect of polymerization appears in the x-ray photoelectron spectral intensities of sihcates. DVM-Aa calculations on the energies and intensities of spectra by Sasaki and Adachi (1980a,b) satisfactorily reproduce relative intensities in the upper-valence-band region for SO/ [Fig. 5.8(a)] but seriously underestimate the intensity of the 5 i orbital feature of Si02 using a SiO/ cluster model [Fig. 5.8(b)]. This error may be a result of the influence of polymerization in SiOj, although the calculated spectrum is also somewhat different from that observed for olivine in Fig. 5.7. 

Information on unoccupied orbital electronic structure is available from the XANES of silicates. Although few data are available for olivines, studies of the anion series C104, S04, PO4 , as described in Bianconi (1988), indicate them to have structures similar to SiF4, with a, and L absorptions below threshold, and e and resonances in the continuum (Tossell et al., 1985a Tossell, 1987). Si XANES for Mg2Si04 are 

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Additional information on electronic structure may be obtained from the x-ray emission spectra of the SiOj polymorphs. As explained in Chapter 2, x-ray emission spectra obey rather strict selection rules, and their intensities can therefore give information on the symmetry (atomic or molecular) of the valence states involved in the transition. In order to draw a correspondence between the various x-ray emission spectra and the photoelectron spectrum, the binding energies of core orbitals must be measured. In Fig. 4.12 (Fischer et al., 1977), the x-ray photoelectron and x-ray emission spectra of a-quartz are aligned on a common energy scale. All three x-ray emission spectra may be readily interpreted within the SiO/ cluster model. Indeed, the Si x-ray emission spectra of silicates are all similar to those of SiOj, no matter what their degree of polymerization. Some differences in detail exist between the spectra of a-quartz and other well-studied silicates, such as olivine, and such differences will be discussed later. 

Electronic structures of silicates other than olivines and SiO, ... 

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. 

Langer, K., Taran, M.N., and Fransolet, A.-M. (2006) Electronic absorption spectra of phosphate minerals with olivine-type structures I. Members of the triphylite lithiophilite series, Mi[ lLi [ l(Fe, +Mni KPOJ. Eur. J. Miner., 18, 337 344. 

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. 

With a known mineral, as determined by electron diffraction or other technique (such as X-ray diffraction), determination of the stoichiometry and structural formula can be a suitable test for analytical precision of thin-film elemental analyses. This simple test follows the practice commonly employed for electron microprobe data in which the accuracy (and completeness) of an analysis is judged by the departure from stoichiometry calculated for a given mineral. Thus, thin-film analyses of olivines, pyroxenes, garnets, feldspars and many other common rock-forming minerals can be examined for internal consistency via a calculation of structural formulae. 

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 ... 

Transition metal ions most susceptible to large Jahn-Teller distortions in octahedral coordination in oxide structures are those with 3d4, 3d9 and low-spin 3(f configurations, in which one or three electrons occupy eg orbitals. Thus, the Cr2+ and Mn3+, Cu2+, and Ni3+ ions, respectively, are stabilized in distorted environments, with the result that compounds containing these cations are frequently distorted from type-structures. Conversely, these cations may be stabilized in distorted sites already existing in mineral structures. Examples include Cr2+ in olivine ( 8.6.4) and Mn3+ in epidote, andalusite and alkali amphiboles ( 4.4.2). These features are discussed further in chapter 6. 

The technique of channeling-enhanced X-ray emission (CHEXE) has enabled cation site occupancies to be determined in various minerals, including transition metal ions in spinels and ferromagnesian silicates (Taftp, 1982 Taftp and Spence, 1982 Smyth and Taftp, 1982 McCormick etal., 1987). The method, which is based on relative intensities of X-ray peaks measured on crystals with diameters as small as 50 nm under the electron microscope, is particularly useful for determining site occupancies of minor elements with concentrations as low as 0.05 atom per cent in a structure. The most important criterion for the determination of element distribution in a mineral by this technique is that the cation sites should lie on alternating crystallographic planes. In order to make quantitative site population estimates, additional information is required, particularly the occupancy of at least one element in one of the sites or in another site that lines up with one of the sites of interest (McCormick et al., 1987). For example, cation site occupancies by CHEXE measurements have been determined from X-ray peak intensity ratios of Si to Ni, Mn, Cr and Fe in forsterite, as well as thermal disordering of these cations in heated olivines (Smyth and Taftp, 1982). 

Tamada, O., Fujino, K. Sasaki, S. (1983) Structures and electron distributions of a-Co2Si04 and a-Ni2Si04 (olivine structure). Acta Cryst., B39,692-7. 

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).

Figure 8-35. BF image of the dislocations fonning a (100) tilt boundary in olivine viewed edge-on, and the electron diffraction pattern from the boundary re on. Note the fine structure of the 605 and 705 reflections shown in the inserts. (From Ricoult and Kohlstedt 1983.)... 

In this section, we have chosen to present the two alternatives to layered oxides currently commercialized (1) the manganese oxides with spinel structure that make a less expensive alternative to batteries requiring a limited lifespan and (2) the olivine LiFeP04 which by its development at the nanoscale overthrew all the concepts that had previously been established and thus opened the way to an entire field of research into new polyanionic materials with poor electronic conducting properties. 

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