Crystal structure and crystal chemistry of fluoro-potassic-magnesio-arfvedsonite from Monte Metocha, Xixano region, Mozambique, and discussion of the holotype from Quebec, Canada.

Fluoro-potassic-magnesio-arfvedsonite, ideally AKBNa2 C(Mg4Fe3+)TSi8O22 WF2, has been found in a dyke ~25 km southwest of Monte Metocha, Xixano region, northeastern Mozambique. Fluoro-potassicmagnesio- arfvedsonite and low sanidine form a fine-grained mafic, ultrapotassic, peralkaline igneous rock without visible phenocrysts. The amphibole is brittle, has a Mohs hardness of 6 and a splintery fracture; it is non-fluorescent with perfect {110} cleavage and no observable parting, and has a calculated density of 3.174 gcm3. In plane-polarized light, it is pleochroic, X = pale grey-green, Y = blue-green, Z = palegre y; X ^ c = 23.6¨¬ (in b obtuse), Y || b, Z ^ c = 66.4¨¬ (in b acute). Fluoro-potassicmagnesio- arfvedsonite is biaxial negative, a = 1.652(2), b = 1.658(2), g = 1.660(2); 2Vobs = 22.5(7)¨¬, 2Vcalc = 30.2¨¬. The unit-cell dimensions are a = 9.9591(4), b = 17.9529(7), c = 5.2867(2) A ¢ª , b = 104.340(1)¨¬, V = 919.73(10) A ¢ª 3, Z = 2. The nine strongest X-ray diffraction lines in the experimental powder pattern are: [d in A ¢ª (I)(hkl)]: 2.716(100)(151), 3.410(70)(131), 8.475(50)(110), 3.178(50)(310), 3.309(30)(240), 2.762(20)(3.31), 2.549(20)(260), 2.351(10)(3.51), 2.269(10)(331). Electron microprobe analysis gave: SiO2 54.25, Al2O3 0.03, TiO2 1.08, FeO 6.69, Fe2O3 8.07, MgO 13.99, MnO 0.32, ZnO 0.05, CaO 1.16, Na2O 6.33, K2O 5.20, F 2.20, H2Ocalc 0.74, sum 99.18 wt.%. Theformula unit, calculated on the basis of 24 (O,OH,F) with (OH+F) = 2(2 Ti), is AK0.98 B(Na1.81Ca0.18)S1.99 C(Mg3.07Fe2+ 0.83Mn0.04Al0.01Fe3+ 0.90Ti0.12Zn0.01)S=4.98 TSi8O22 W[F1.03(OH)0.73O0.24]S2.00 and confirms the usual pattern of cation order in the amphibole structure. The presence of a significant oxo component (locally balanced by Ti at the M(1) site) is related to the crystallization conditions. The presence of Fe3+ at the T sites, originally suggested for the holotype specimen, is discounted for this amphibole composition