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Clays and Clay Minerals; June 2003; v. 51; no. 3; p. 301-308; DOI: 10.1346/CCMN.2003.0510306
© 2003 Clay Minerals Society
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THE CRYSTAL STRUCTURE OF ROSCOELITE-1M

Maria Franca Brigatti1,*, Enrico Caprilli1, Marco Marchesini2 and Luciano Poppi1

1 Department of Earth Sciences, University of Modena and Reggio Emilia, Italy
2 ENI-AGIP Towers, San Donato Milanese, Milano, Italy

* E-mail address of corresponding author: brigatti{at}unimo.it

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)O10(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) Å, ß = 100.98(2)° and space group C2/m, which indicate a 1M polytype. The crystal structure was refined on the basis of Fo2 for 846 unique reflections to R1 = 3.29% calculated using 746 unique observed reflections [|Fo| ≥4{sigma}(Fo)]. The mean tetrahedral cation–oxygen atom distance, <T–O> = 1.641 Å, 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 Å 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}= 2.3°), the lowest flattening of the basal oxygen surface ({Delta}z = 0.118 Å) and the narrowest interlayer separation (3.030 Å) in dioctahedral micas.

Key Words: Crystal Chemistry • Crystal Structure • Roscoelite • Vanadium Mica




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