Crystal structure determination of anandite-2M mica

Anandite, a trioctahedral mica, has an ideal chemical formula of Ba[(FeMg).sub.3](Si[Fe.sup.3+])[O.sub.10](OH)S and a microprobe-derived formula of [([Ba.sub.0.96][K.sub.0.03] [Na.sub.0.01]).sub.[SIGMA]=1.00] [([Fe.sup.2+.sub.2.01][Mg.sub.0.46][Fe.sup.3+.sub.0.28][Al.sub.0.10] [Mn.sup.3+.sub.0.04][Mn.sup.2+.sub.0.04][Ti.sub.0.01].sub.[SIGMA]=2.93] [([Si.sub.2.60][Fe.sup.3+.sub.1.40]).sub.[SIGMA]=4.00][O.sub.10] [[(OH).sub.0.96][S.sub.0.84][Cl.sub.0.16][F.sub.0.04]].sub.[SIGMA]=2.00], where [S.sup.2-] primarily substitutes for [(OH).sup.-]. A single-crystal, X-ray structure determination of anandite-2M was refined to [R.sub.1] = 0.0443 and w[R.sub.2] = 0.1232 on [F.sup.2] in space group Am. Cell parameters are: a = 5.4431 (3), b = 9.4719(6), c = 20.042(1) [Angstrom], and [beta] = 95.046(1)[degrees]. 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 [beta] equaling ~95[degrees]. Subgroup symmetry of Am results from tetrahedral sheets within layers that are non-centrosymmetric with unequal compositions ([Si.sub.0.61][Fe.sup.3+.sub.0.39] vs. [Si.sub.0.79][Fe.sup.3+.sub.0.21]) and thicknesses (difference of 0.209 [Angstrom]), 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-20, include (1) alternation of smaller tetrahedral rings containing four Si-rich tetrahedra (T1a: 1.643 [Angstrom], T2b: 1.657 [Angstrom]) and two [Fe.sup.3+]-rich tetrahedra (T2a: 1.733 [Angstrom], T1b: 1.760 [Angstrom]) and larger rings containing four [Fe.sup.3+]-rich tetrahedra and two Si-rich tetrahedra within each layer; (2) nearly in-phase wave forms of basal O atoms across the interlayer ([DELTA]z = -0.110 and 0.011 [Angstrom] and across the interlayer [DELTA]z = -0.121 and 0.007 [Angstrom]); and (3) attraction that results in Ba being shifted toward S (0.070 [Angstrom]) and S being shifted toward Ba (0.117 [Angstrom] 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 [Fe.sup.3+]-rich tetrahedra and toward S-rich sites to achieve charge balance. Comparison of anandite-2M and anandite-20 shows that they possess unit cells (2M setting) that have nearly equal a axes, unequal b and c axes, and [beta] (anandite-2M is smaller by 0.0371 [Angstrom], larger by 0.08 [Angstrom], and smaller by 0.089[degrees], respectively). Moreover, anandite-20 exhibits larger [Fe.sup.3+]-rich tetrahedral rings than anandite-2M, which allow for a greater shift in Ba (difference of 0.03 [Angstrom]). 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. Keywords: Anandite mica, single-crystal X-ray diffraction, two-layer monoclinic mica polytypes, phyllosilicate, Ba-rich mica, S-rich mica, Fe-rich mica