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, β = 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| ≥4σ(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 (α= 2.3°), the lowest flattening of the basal oxygen surface (Δz = 0.118 Å) and the narrowest interlayer separation (3.030 Å) in dioctahedral micas.

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