• journal_title：American Mineralogist
• Contributor：Thomas J. Bujnowski ; Stephen Guggenheim ; Toshio Kato
• Publisher：Mineralogical Society of America
• Date：2009-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2009.3139
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：94
• issue：8-9
• firstpage：1144
• section：Articles

Anandite, a trioctahedral mica, has an ideal chemical formula of Ba(FeMg)₃(SiFe³⁺)O₁₀(OH)S and a microprobe-derived formula of (Ba_0.96K_0.03Na_0.01)_∑=1.00(Fe_2.01²⁺Mg_0.46Fe_0.28³⁺Al_0.10Mn_0.04³⁺Mn_0.04²⁺Ti_0.01)_∑=2.93(Si_2.60Fe_1.40³⁺)_∑=4.00O₁₀[(OH)_0.96S_0.84Cl_0.16F_0.04]_∑=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 (Si_0.61Fe_0.39³⁺ vs. Si_0.79Fe_0.21³⁺) 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.