Order-disorder spinels

Both ordered and normal spinels should disorder at high temperature, since the entropy of the random spinel is larger. The disordering reaction for a normal spinel can be described by a quasi-chemical reaction... 

Redfern, S.A.T., Dove, M.T., Wood, D.R.R. (1997b) Static lattice simulation of feldspar solid solutions ferroelastic instabilities and order/disorder. Phase Trans 61 173-194 Redfern, S.A.T., Knight, K.S., Henderson, C.M.B., Wood, B.J. (1998) Fe-Mn cation ordering in fayalite-tephroite (FexMni U2Si04 olivines a neutron diffraction study. Mineral Mag 62 607-615 Redfern, S.A.T., Harrison, R.J., O Neill, H.St.C., Wood, D.R.R. (1999) Thermodynamics and kinetics of cation ordering in MgAl204 spinel up to 1600°C from in situ neutron diffraction. Am Mineral 84 299-310... 

Wood BJ, Kirkpatrick RJ, Montez B (1986) Order-disorder phenomena in MgAl204 spinel. Am Mineral 71 999-1006... 

Lithium transport in all of these phases occurs via the shared faces of octahedra and tetrahedra. In normal spinels (e.g., Li2ZnCl4), wliert the tetrahedra are occupied by M, the conductivity is much louver. Diffuse order-disorder transitions in both Li2MX4 and LigMXg result in strong deviations from linearity on their Arrhenius plots. 

Gil] Gillot, B., Study by Infiared Absorption Speelia of die Factors Influencing the Order-disorder Transformation in -y Vacancy Ferrites Obtained from die Oxidation of Ferrous Spinels (in French), Mater. Chem. Phys., 10(4), 375-384 (1984) (Crys. Stracture, Experimental, 17)... 

Mossbauer spectroscopy in solid state chemistry under in situ conditions at high temperatures and at defined oxygen partial pressures have been made by Becker (2001) for order-disorder processes and their kinetics in nickel alu-minate spinel and magnetite. The study has also been carried out for heterogeneous solid olid and solid-gas reactions which relate to the formation of cobalt aluminate spinel and to redox processes in fayalite, Fe2Si04, respectively. The diffusion of the iron cations in Fc2Si04 has also been observed by means of Mossbauer spectroscopy. 

H. Cynn, S. K. Sharma, T. F. Cooney, and M. Nicol. High-temperature Raman investigation of order-disorder behavior in MgAl204 spinel. Phys. Rev. B, 45 500-502, 1992. 

T. Yamanaka and Y. Takeuchi. Order-disorder transition in MgAl204 spinel at high temperatures up to 1700 C. Kristallographie, 165 65-78, 1983. 

Hafner, S., 1961. Order/disorder and infrared absorption. IV. The absorption of some metal oxides with the spinel structure. Z. Krist, 155 331. 

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). 

Kaolinite is transformed into X-ray amorphous state when activated in air. According to authors [14,15], amorphization involves the destruction of bonds between tetrahedral and octahedral layers inside the package, till the decomposition into amorphous aluminium and silicon oxides. Other researchers [ 16,17] consider that amorphized kaolinite conserves the initial ordering of the positions of silicon atoms while disordering of the structure is due to the rupture of A1 - OH, Si - O - A1 bonds and the formation of molecular water. Endothermic effect of the dehydration of activated kaolinite is shifted to lower temperatures while intensive exo-effect with a maximum at 980°C still conserves. When mechanically activated kaolinite annealed at 1(X)0°C, only mullite (3Al20j-2Si0j) and X-ray amorphous SiOj are observed. In this case, the phase with spinel structure which is formed under thermal treatment of non-activated kaolinite is not observed thus, mechanical activation leads to the formation of other phases. 

Warren, M.C., Dove, M.T., Redfem, S.A.T. (2000a) Ab initio simulation of cation ordering in oxides application to spinel. JPhys Condensed Matter 12 L43-L48 Warren, M.C., Dove, M.T., Redfem, S.A.T. (2000b) Disordering of MgAl204 spinel from first principles. Mineral Mag 64 311-317... 

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