The crystal structure of roscoelite-1M

Single-crystal X-ray diffraction experiments were carried out on roscoelite crystals from Reppia, Val Graveglia, Italy. Roscoelite [structural formula: (XII)(Ba0.006K0.994)(IV)(Si3.150Al0.850) (VI)(Al0.040Fe0.150Mg0.100Mn0.062V1.696Ti0.003)O-10(OH)(2)] shows a near-perfect three-dimensional stacking order with cell parameters a = 5.292(1), b = 9.131(2), c = 10,206(3) Angstrom, beta = 100.98(2)degrees and space group C2/m, which indicate a 1M polytype. The crystal structure was refined on the basis of F-o(2) for 846 unique reflections to R1 = 3.29% calculated using 746 unique observed reflections [\F-o\ greater than or equal to 4sigma(F-o)]. The mean tetrahedral cation-oxygen atom distance, <T-O> = 1.641 Angstrom, is close to the mean <T-O> value obtained for dioctahedral true micas from the literature, whereas the octahedral sheet is characterized by a larger cis-octahedral cation-oxygen atom bond distance <M2-O> = 2.020 Angstrom which, together with the mean electron count, is consistent with V occupancy. The presence of V within the octahedral sheet produces the smallest tetrahedral rotation (alpha = 2.3degrees), the lowest flattening of the basal oxygen surface (Deltaz = 0.118 Angstrom) and the narrowest interlayer separation (3.030 Angstrom) in dioctahedral micas.