Crystal structure refinement of anandite-2Or, a barium- and sulfur-bearing trioctahedral mica

The structure of anandite-2or, ideally BaFel*1SirFe3*)O1o(OH)S, has been refined to R*: 6.40 using 1074 reflections. Unit cell parameters are a:5.439(1), b :9.509(2), and c : 19.878(6)A. The symmetry is reduced from the ideal Ccmm to Pnmn as a result of ordering of tetrahedral Fe3* and Si, octahedral F'e and Mg, and the anions OH and S. Mean bond lengths are T(lfo : r.620L, T(2Fo : r.7ggL, M(IFO,OH : 2.097 A, M(2FO,S : 2.n6L, M(3FO,S:2.2284, and M(4fO,OH:2.1204. Sulfur, which substitutes for OH, is present as 52and moves by 0.16A toward the interlayer Ba so that it serves as a 13th neighbor. This severely distorts the shapes of the Fe-rich transM(2) and cis M(3) octahedra. Fe3+-rich T(2) tetrahedra are linked to one another across the mirror plane, and small rings of four T(1) + two T(2) alternate with large rings of two T(1) + four T(2) in each tetrahedral sheet but are shifted to face one another in adjacent layers across the interlayer gap. The interlayer Ba moves deeper into the larger ring by 0.llalong c towards the sulfur. The 6-fold rings are essentially hexagonal (a : 0.9'). The usual instability of prismatic coordination around an interlayer cation resulting from anion-anion repulsion is counterbalanced in anandite-ZOr because (1) the divalent charge on Ba offsets more of the unsatisfied charges on the basal oxygens than would a univalent cation, (2) attraction exists between Ba2+ and St-, (3) alternating-wave corrugations in the basal oxygen surfaces are in-phase and do not lead to abnormally short contacts across the interlayer gap, and (4) alternation of the ordering pattern across the interlayer gap pairs the basal oxygen unique to hybrid T(2) with that unique to pure Si T(1). Several lines of evidence suggest the true symmetry is P2, (a-axis unique), but attempted refinement in that subgroup has not been successful. Introduction A second refinement of the 2Or structure has been carried out because of its unique nature and because of several Anandite is a trioctahedral brittle mica that to date has inconsistencies in the original refinement. The structure is been identified only from Wilagedera, Ceylon (now Sri of special interest because of the structural S, interlayer Ba, Lanka), where it occurs as thin bands and lenses within a and substantial tetrahedral Fe3*. Ordering has reduced the magnetite-barite-sulfide ore zone. The enclosing calcsymmetry from the ideal space group Ccmm to an apparent schists and calc-gneisses of Precambrian age have been subgroup Pnmn. It is the only known mica in which Csubjected to granulite grade metamorphism (Pattiaratchi, centering ofan orthohexagonal cell is violated and the only 1961). The simplified formula of anandite is Ba(Fe,Mg)3 ofl€ in which ordered tetrahedral R3+ cations do not alter(Si,Fe3*)4O1o(OH)S, wbich is unique because of the subnate with Si around each hexagonal ring. It is also the only stantial substitution of 52for (OH)-t.The essential presmica for which the 2Or polytype has been verified. The ence of structural sulfur was proven by the microprobe refinement by Giuseppetti and Tadini (1972) was of only analyses of Lovering and Widdowson (1968) and by the moderate accuracy, as indicated by the residual R: l3.l% first structural refinement by Giuseppetti and Tadini for 853 reflections (only 409 observed) and by inconsistency (1972). It is also confirmed in the present study. between certain bond lengths and the composition. For Three polytypes of anandite have been recognized. The example, tetrahedron T(l) was interpreted to be of compooriginal description of the mineral by Pattiaratchi et al. sition Sio nuAlo.ou despite its mean bond length of 1.68A. A (1967) listed a powder pattern that can only be indexed as more reasonable T-O bond would be 1.61-1.62A for that 2Mr.The structural refinement of Giuseppetti and Tadini composition. The mean T(2FO bond of 1.77A was inter(1972) was of the2Or polytype, as is the present study, and preted to give a composition Sio.ruFefi. *. rFefr.lr, and the this appears to be the dominant polytype. A refinement of possibility of tetrahedral Fe2* is suffrciently unusual that it a lM polytype is also in progress (T. Kato, personal comm. warrants further study. The present study confirms most of to S. W. Bailey, 1984). the major features of the Giuseppetti and Tadini structure,