Cation ordering

Some of the above discussed precursor phenomena are also observed prior to diffusion driven phase transformations. A typical example are the conventional EM tweed images obtained in the tetragonal parent phase in high Tc superconductors and other ceramics. In a recent survey by Putnis St e of such observations it was concluded that in these cases the tweed contrast resulted from underlying microstructures fomied by symmetry changes driven by cation ordering. These symmetry changes yield a fine patchwork of twin related domains which coarsen when the transfomiation proceeds. However, in view of the diffusion driven character of the latter examples, these cases should be clearly separated from those in the field of the martensites. 

Spinels. There are limited experimental data on uranium and thorium partitioning between magnetite and melt (Nielsen et al. 1994 Blundy and Brooker 2003). Both studies find U and Th to be moderately incompatible. Blundy and Brooker s results for a hydrous dacitic melt at 1 GPa and 1025°C give Du and D h. of approximately 0.004. The accuracy of these values is compromised by the very low concentrations in the crystals and the lack of suitable SIMS secondary standards for these elements in oxide minerals. Nonetheless, these values are within the range of Djh of magnetites at atmospheric pressure 0.003-0.025 (Nielsen et al. 1994). It is difficult to place these values within the context of the lattice strain model, firstly because there are so few systematic experimental studies of trace element partitioning into oxides and secondly because of the compositional diversity of the spinels and their complex intersite cation ordering. 

Rebours et al. have investigated the structure of Mg/Al and Mg/Ga LDHs with interlayer carbonate anions [39]. The former did not reveal any evidence of cation ordering even for Mg2Al, but a supercell of dimension a /3... 

It has been suggested that powder XRD patterns of some mineral samples of LDH minerals show evidence of superlattice reflections [7,113] but there is no clear consensus [100]. It should also be borne is mind that, as discussed in Sect. 3.5, superlattice reflections may be due to anion, rather than cation, ordering although the latter may be an indication of the former. 

There have been a small number of theoretical studies of cation ordering in LDHs. First principles molecular dynamics calculations [43] on [Mg3Al(OH)8]Cl LDHs discussed in Sect. 3.2.6 suggested that structures with adjacent aluminum cations were energetically less favorable than one without, although the chosen arrangement for the latter lacked either hexagonal or rhombohedral supercell. 

The presence of local cation ordering in Mg2Ga and MgsGa - CO3 LDHs noted in Sect. 3.3.1 has been confirmed by means of both EXAFS and by calculation of the electron radial distribution function from the Fourier transform of the diffracted X-ray intensity. In each case the gallium was found to have six magnesium ions and no galhum ions as next-nearest neighbors [39]. 

Mossbauer spectroscopy is a potentially very useful way to study the cation ordering in LDHs containing iron [293-295], although care must be taken in order to avoid misinterpreting the results as has often happened... 

Figures 10—13 give further confirmation that the maximum energy for Mn occupation of tetrahedral sites in a ccp oxide occurs when the Mn valence is +4 (i.e., d filling), independent of cation ordering. Figures...

The possibility that Mn generally favors tetrahedral coordination as its valence approaches +2 (i.e., d ) is unlikely given that MnO has a rock-salt structure not zinc blende or some other structure with Mntet. Instead, the driving force for Mn movement out of the octahedral sites of 7-Lii/2Mn02 into neighboring Li layer tetrahedral sites appears to be due to the unique cationic ordering and associated cationic interactions that are present in 7-Lii/2Mn02. 

One energy contribution that is sensitive to cationic ordering given explicitly in eqs 3—5 is the y intercept AEMn7+. According to the proposed model AEiv[n7+... 

The other energy contribution that is sensitive to cationic ordering is implicitly part of d-orbital terms such as Et2 — Ee x — 3) in eqs 3—5. A coefficient such as Et2 — Ee, which gives the energy difference between the tetrahedral tz and octahedral eg orbitals, can be broken down into two parts. 

Since the LFSE term is determined by Mn valence and appears to be relatively insensitive to cation ordering in the Mn02 host structures, it would seem to be the easiest energy term to manipulate through chemical substitutions. This is because the exact placement of the chemical substitutions in the Mn sublattice would presumably be less important for the LFSE term than for the terms that are more sensitive to cationic ordering (i.e., AEb and AEm/+). 

Figure 5,54 (A) Cationic occupancies in tetrahedral positions in case of complete disorder (monoclinic structure upper drawing) and complete order (triclinic structure lower drawing). (B) Condition of complete order in microcline and low albite with AliSi = 1 3, compared with cationic ordering in anorthite (AhSi = 2 2). Note doubling of edge c in an-orthite.

Annersten H., Adetnuji I, and Filippidis A. (1984). Cation ordering in Fe-Mn silicate olivines. Amer. Mineral, 69 1110-1115. 

Bish D. L. (1981). Cation ordering in synthetic and natural Ni-Mg olivine. Amer. Mineral, 66 770-776. 

Carpenter M. A. and Salje E. K. H. (1994a). Thermodynamics of nonconvergent cation ordering in minerals, II Spinels and the orthopyroxene solid solution. Amer. Mineral, 79 1068-1083. 

Ernst W. G. and Wai C. M. (1970). Infrared, X-ray and optical stndy of cation ordering and dehydrogenation in natural and heat-treated sodic amphiboles. Amer. Mineral, 55 1226-1258. 

Ottonello G., Della Giusta A., and Molin G. M. (1989). Cation ordering in Ni-Mg olivines. Amer. Mineral, 74 411 21. 

Andreaozzi G.B. and Princivalle F. (2002) Kinetics of cation ordering in synthetic MgAl204 spinel. Am. Mineral. 87, 838-844. 

Carpenter M.A. and Putnis A. (1985) Cation order and disorder during crystal growth some implications for natural mineral assemblages. Adv. Phys. Geochem. 4, 1-26. 

Ganguly J. and Domeneghetti M.C. (1996) Cation ordering of orthopyroxenes from the Skaergaard intmsion implications for the subsolidus cooling rates and permeabilities. Contrib. Mineral. Petrol. 122, 359-367. 

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