Tetrahedral cation ordering

Ordering of tetrahedral cations is quite unusual in the common mica species such as muscovite-2Mi, phlogopite-lM and annite-lM (Bailey 1975, 1984c), whereas it is common in brittle micas. Margarite, bityite and anandite are examples of minerals with Si,Al (or Fe ) tetrahedral ordering (Guggenheim 1984). 

The possibility of tetrahedral cation ordering in kinoshitalite, characterized by a Si Al ratio close to 1, was addressed by Guggenheim and Kato (1984), Guggenheim 

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In the regression analysis, structures containing B, Be, and Ge in tetrahedral sites were not considered, as well as structures with symmetry lower than ideal owing to tetrahedral cation ordering (differences in values greater than 5o). Only structures containing tetrahedral Si, Al, and Fe were examined. 

Refinement using only A = 3 reflections gave the average structure of a lb type unit, which has monoclinic symmetry C2//m. In this specimen, the tetrahedra were found to be rotated by 5.0°. The average symmetry of the layer does not permit tetrahedral cation ordering, because there is only one independent tetrahedron. The average T-0 bond length is 1.67 A. Difference electron-density maps show 60% of the Fe,Mn to be concentrated in the octahedral portion of the 2 1 layer. 

Twin geometry may refer to just one set of atoms in the crystal rather than all. This type of twinning is met, for example, in cases where a fraction of octahedral or tetrahedral cation sites in a close-packed array of anions are occupied in an ordered fashion. The close-packed anion array remains unchanged by the twin plane, which applies to the cation array alone (Fig. 3.24). These boundaries are of low energy and are often curved rather than planar. In oxides such as spinel, MgAl204, in which cations are distributed in an ordered fashion over some of the octahedral and tetrahedral sites, boundaries may separate regions that are twinned with respect to the tetrahedral cations only, the octahedral cations only or both. 

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. 

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. 

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.

Fig. 21 a-c Crystal structure of Yb2 75C60 [76]. a Undistorted/cc structure of Yb3C60. The arrow indicates that 1/8 of tetrahedral cations is missing, b The doubled supercell of Yb2.75C6o, caused by the ordering of the tetrahedral vacancy. The octahedral cations are shifted towards the vacant tetrahedral site, c The vacancy and octahedral cation shift induce orientational order and distortion of C60 molecules. There are three inequivalent C60 molecules in the lattice... 

In earlier chapters, allusions were made to die effects of covalent bonding. For example, covalent interactions were invoked to account for the intensification of absorption bands in crystal field spectra when transition metal ions occupy tetrahedral sites ( 3.7.1) patterns of cation ordering for some transition metal ions in silicate crystal structures imply that covalency influences the intracrystalline (or intersite) partitioning of these cations ( 6.8.4) and, the apparent failure of the Goldschmidt Rules to accurately predict the fractionation of transition elements during magmatic crystallization was attributed to covalent bonding characteristics of these cations ( 8.3.2). 

Dyar (1987) A review of Mossbauer data on trioctahedral micas evidence for tetrahedral Fe3+ and cation ordering. Amer. Mineral., 72,101-12. 

Figure 23 Cation ordering (left) in M-SbA (A) and Li -exchanged M-SbA (B and C) and schematic coordination of lithium ions (right) to oxygen (B) regular octahedral (C) regular tetrahedral.

Order-disorder of Si and A1 in tetrahedral sites as a function of temperature was the object of several studies, with controversial results. The behavior of phengite-3r from Dora Maira at high T was studied by in situ single-crystal X-ray diffraction (Amisano-Canesi et al. 1994 Amisano-Canesi 1995) and powder neutron diffraction (Pavese et al. 1997, 2000). These authors found that this sample seems to exhibit cation ordering both on the tetrahedral and on the octahedral sites, supporting the hypothesis of Sassi et al. 

Spce goup Cllm and Ccmm. The minimum i-symmetry required by these two space-groups is C12/m(l). The O sheet contains the global twofold axis in both space groups. One tetrahedral and two octahedral independent sites are allowed and no ordering of the tetrahedral cations is possible. The IM and 20 polytypes show this symmetry. 

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