ALBERT

Description: Fluoro-potassic-pargasite, ideally AKBCa2C(Mg4Al)T(Si6Al2)O22WF2, a new amphibole species, has been found in a skarn in the Tranomaro area, Madgascar. The sample used for the description of the new mineral species is a large single amphibole crystal, 4 mm x 2 mm x 2 cm in size, brownish-black with brownish-yellow phlogopite lamellae adhering to one face of the crystal. Fluoro-potassic-pargasite is brittle, has a Mohs hardness of 6.5 and a splintery fracture; it is non-fluorescent, has perfect {110} cleavage, no observable parting, and has measured and calculated densities of 3.46 and 3.151 g cm-3, respectively. In plane-polarized light, it is pleochroic, X = colourless to very pale grey, Y = very pale grey, Z = colourless; X ^ a = 46.9{degrees} (in {beta} obtuse), Y || b, Z ^ c = 31.4{degrees} (in {beta} acute). It is biaxial positive, {alpha} = 1.638(2), {beta} = 1.641(2), {gamma} = 1.653(2); 2Vobs = 49.6(4){degrees}, 2Vcalc = 53.4{degrees}. Fluoro-potassic-pargasite is monoclinic, space group C2/m, a = 9.9104(2), b = 17.9739(4), c = 5.3205(1) A, {beta} = 105.534(2){degrees}, V = 913.11(6) A3, Z = 2. The eight strongest lines in the X-ray powder-diffraction pattern are [d in A(I)(hkl)]: 3.133(100)(310), 3.270(55)(240), 2.809(47)(330), 8.413(45)(110), 2.698(39)(151), 3.374(31)(131), 2.934(29)(221) and 1.647(29)(461). Electron microprobe analysis gives SiO2 40.20, Al2O3 17.61, TiO2 0.46, FeO 1.96, Fe2O3 2.51, MgO 16.95, MnO 0.05, CaO 13.18, Na2O 0.99, K2O 3.72, F 2.75, H2Ocalc 0.77, sum 99.99 wt.%. The formula unit, calculated on the basis of 24 (O,OH,F) with (OH + F) = 2 - (2 x Ti), is A(K0.69Na0.28Ca0.04){Sigma}=1.01BCa2.00C(Mg3.64[IMG]f1.gif" ALT="Formula" BORDER="0"〉Mn0.01Al0.79[IMG]f2.gif" ALT="Formula" BORDER="0"〉Ti0.05){Sigma}=5.00T(Si5.80Al2.20){Sigma}=8.00O22 W[F1.26(OH)0.74]{Sigma}=2.00. The mineral species and name have been approved by the IMA CMNMC (IMA 2009-091).