Abstract

Anandite, a trioctahedral mica, has an ideal chemical formula of Ba(FeMg)3(SiFe3+)O10(OH)S and a microprobe-derived formula of (Ba0.96K0.03Na0.01)∑=1.00(Fe2.012+Mg0.46Fe0.283+Al0.10Mn0.043+Mn0.042+Ti0.01)∑=2.93(Si2.60Fe1.403+)∑=4.00O10[(OH)0.96S0.84Cl0.16F0.04]∑=2.00, where S2− primarily substitutes for (OH). A single-crystal, X-ray structure determination of anandite-2M was refined to R1 = 0.0443 and wR2 = 0.1232 on F2 in space group Am. Cell parameters are: a = 5.4431(3), b = 9.4719(6), c = 20.042(1) Å, and β = 95.046(1)°. Layer stacking is analogous to a 1M stacking pattern (parallel unidirectional a/3 shifts within layers and octahedral set I only occupied), however cation order and (S,OH) positional disorder produce a two-layer repeat with β equaling ~95°. Subgroup symmetry of Am results from tetrahedral sheets within layers that are non-centrosymmetric with unequal compositions (Si0.61Fe0.393+ vs. Si0.79Fe0.213+) and thicknesses (difference of 0.209 Å), and there is positional and site-occupancy disorder (four sites with S of 0, 30, 52, and 58%) of (S,OH). Characteristics of anandite-2M, which are similar to those of anandite-2O, include (1) alternation of smaller tetrahedral rings containing four Si-rich tetrahedra (T1a: 1.643 Å, T2b: 1.657 Å) and two Fe3+-rich tetrahedra (T2a: 1.733 Å, T1b: 1.760 Å) and larger rings containing four Fe3+-rich tetrahedra and two Si-rich tetrahedra within each layer; (2) nearly in-phase wave forms of basal O atoms across the interlayer (Δz = −0.110 and 0.011 Å and across the interlayer Δz = −0.121 and 0.007 Å); and (3) attraction that results in Ba being shifted toward S (0.070 Å) and S being shifted toward Ba (0.117 Å average) along the c axis, relative to the ideal. Bond-valence calculations show that Ba is shifted toward the undersaturated, bridging-basal O atoms of the Fe3+-rich tetrahedra and toward S-rich sites to achieve charge balance. Comparison of anandite-2M and anandite-2O shows that they possess unit cells (2M setting) that have nearly equal a axes, unequal b and c axes, and β (anandite-2M is smaller by 0.0371 Å, larger by 0.08 Å, and smaller by 0.089°, respectively). Moreover, anandite-2O exhibits larger Fe3+-rich tetrahedral rings than anandite-2M, which allow for a greater shift in Ba (difference of 0.03 Å). The ordering and consequent absence of a twofold axis in anandite-2M allows the in-phase wave structure of basal O atoms, which was previously thought only possible in the orthorhombic P cell.

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