Olivine structures

Recently, LiFeP04, which adopts the olivine structure, has been widely studied as a possible cheap cathode material.Although the specific capacities of this material and the related NASICON structures are relatively low (typically 100 mAhg ), these compounds are cheap and open up new avenues for research. Li MAS NMR studies of a series of olivines LiMP04 with M = Mn, Fe, Ni, and Co were carried out by Tucker et A single resonance with large... 

Figure 5.4 (A) Olivine structure dashed line contours elementary cell (axes c and b). (B) Details of coordination state and distortion of Ml-Ml sites 0(1), 0(2), 0(3) oxygens occupy nonequivalent positions distortion from perfect octahedral symmetry is more marked for Ml site.

Ganguly I (1977). Crystal chemical aspects of olivine structures. N. Jahrb Miner. Abh., 130 303-318. 

This (and high-temperature K2SO4, Sect. 2.6.1) is one of many compounds with the olivine structure - which we will now consider in detail. 

Figure 4.8 Polarized absorption spectra of manganiferous olivines of the fayalite-tephroite series, (a) Fayalite, Fa TejFop (b) knebellite, Fa69Te26Fo5 (c) tephroite, Fa31Te67Fo2 (d) tephroite, FajTe Foj. The reduced relative intensity of the absorption band at 1,078 to 1,120 nm is due to substitution of Fe2+ by Mn2+ in M2 positions of the olivine structure, [y spectra y=a.]...

From a crystal chemical standpoint, the olivine structure is particularly interesting because it contains divalent cations in two distinct six-fold coordination sites having contrasting symmetries. Consequently, crystal field spectra of several transition metal-bearing olivines have been studied in attempts to detect cation ordering, including evidence for site preferences of Mn2+ ( 4.5) and Ni2+ (Hu et al., 1990), and to estimate CFSE data for Fe2+ and Ni2+ in each site in the olivine structure. 

For the pseudo-tetragonally elongated Ml octahedral site in the olivine structure, an alternative method for obtaining the CFSE of Fe2+ ions in this site is to evaluate the ligand field splitting parameters in the equatorial plane and along the axial direction using eqs (3.14) to (3.16). The crystal field spectral data for the fayalite Ml site (see fig. 5.11a) yield values for 10 Dq(m) and 10 Dq(eq) of 8,172 cm-1 and 9,327 cm-1, respectively. The mean 10 Dq value of 8,942 cm-1 and the CFSE of 4,133 cm-1 compare favourably with A0 = 8,830 cm-1 and CFSE = 4,250 cm-1 for Fe2+ in the fayalite Ml sites determined by the baricentric method (fig. 5.11a). 

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-... 

Forbes, 1983), show that Cr3+ and Fe3+ ions preferentially populate the slightly larger A12 sites corresponding to the mirror plane acentric M2 sites of the olivine structure. The EPR measurements indicated that 78 per cent of the Cr3+ ions in a synthetic alexandrite occur in the A12 positions. This contrasts with the slight enrichment of Cr3+ in the Ml sites of forsterite (Rager, 1977). 

In the case of Cu2+ and Cr2+, compounds of which are susceptible to Jahn-Teller distortions ( 6.3 table 6.1), these cations are predicted to show strong preferences for the most distorted orthopyroxene M2 and amphibole M4 sites. A similar explanation accounts for the observed enrichments of Mn3+ ions in the distorted andalusite Ml, alkali amphibole M2, epidote M3 and, perhaps, epidote Ml sites (table 6.1). The presence of significant amounts of chromium in olivines from the Moon and as inclusions in diamond may be due to the presence of Cr2+ ions, and not Cr3+, in the distorted Ml and M2 sites of the olivine structure (Bums, 1975b), in which Jahn-Teller stability may be attained. A similar factor accounts for the stability and site occupancy of the Cr2+ ion in the orthopyroxene M2 site (table 6.1). 

Similarly, the synthetic liebenbergite (Ni051Mgo49)2Si04, in which site populations in the olivine structure were estimated to be (Bostrom, 1987 Ottonello et al., 1989)... 

This differential CFSE factor is demonstrated by the formation of hortono-lite, (Mgo5Fe05)2Si04, by the mixing of forsterite and fayalite components. To a close approximation, Mg2+ and Fe2+ ions may be assumed to be randomly distributed in the olivine structure eq., (7.12), so that 0.5 Fe2+ ions per formula unit occupy each of the Ml and M2 positions. The CFSE s of the Fe2+ ion in the Ml and M2 sites of hortonolite are approximately -53.2 and -52.0 kJ/mole, respectively, and in fayalite the corresponding CFSE are -50.9 kJ/mole (Ml site) and -51.2 kJ/mole (M2 site) (eqs (5.2) and (5.3) table 5.16). The formation of hortonolite may be represented as follows ... 

Because of the very large octahedral site preference energies of Ni2+ and Cr3+, their partitioning would be strongly biased in favour of the crystals. Hence, equilibria in eq. (8.22) were increasingly shifted to the right for Ni2+ and Cr3, with the result that these cations were preferentially expelled from the magma at similar rates. The Ni2+ ions entered the olivine structure whereas the Cr3 effectively controlled the precipitation of chromite. Irvine (1974, 1975) demonstrated experimentally that the fractionation path of a silicate melt... 

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

AB2X4 Compounds (Spinel and Olivine Structures). The AB2X4 spinels, based on the mineral MgAl204, and the inverse spinels, B[AB]04, are predominately ionic mixed oxides, containing a CCP-like array of X anions (Fig. 3.17). Most... 

The olivine structure is named after a series of A2BX4 minerals with two end members Fc2Si04 (fayalite) and Mg2Si04 (forsterite). The two minerals form a series with a structure consisting of a distorted HCP arrangement of oxygen anions. The A cations (Mg and Fe) reside in octahedral sites and the B cations (Si) in tetrahedral sites. The... 

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