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 mm62 mm62 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 cm3, respectively. In plane-polarized light, it is pleochroic, X = colourless to very pale grey, Y = very pale grey, Z = colourless; X ^ a = 46.9º (in b obtuse), Y || b, Z ^ c = 31.4º (in b acute). It is biaxial positive, a = 1.638(2), b = 1.641(2), g = 1.653(2); 2Vobs = 49.6(4)º, 2Vcalc = 53.4º. Fluoro-potassic-pargasite is monoclinic, space group C2/m, a = 9.9104(2), b = 17.9739(4), c =5.3205(1) A ? , b = 105.534(2)º, V = 913.11(6) A ? 3, Z = 2. The eight strongest lines in the X-ray powderdiffraction 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 (26Ti), is A(K0.69Na0.28Ca0.04)S=1.01 BCa2.00 C(Mg3.64Fe2+ 0.24Mn0.01Al0.79Fe3+ 0.27Ti0.05)S=5.00 T(Si5.80Al2.20)S=8.00O22 W[F1.26(OH)0.74]S=2.00. The mineral species and name have been approved by the IMA CMNMC (IMA 2009-091).
Fluoro-potassic-pargasite, KCa2(Mg4Al)(Si6Al2)O22F2, from the Tranomaro area, Madagascar: mineral description and crystal chemistry.
Oberti R;
2010
Abstract
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 mm62 mm62 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 cm3, respectively. In plane-polarized light, it is pleochroic, X = colourless to very pale grey, Y = very pale grey, Z = colourless; X ^ a = 46.9º (in b obtuse), Y || b, Z ^ c = 31.4º (in b acute). It is biaxial positive, a = 1.638(2), b = 1.641(2), g = 1.653(2); 2Vobs = 49.6(4)º, 2Vcalc = 53.4º. Fluoro-potassic-pargasite is monoclinic, space group C2/m, a = 9.9104(2), b = 17.9739(4), c =5.3205(1) A ? , b = 105.534(2)º, V = 913.11(6) A ? 3, Z = 2. The eight strongest lines in the X-ray powderdiffraction 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 (26Ti), is A(K0.69Na0.28Ca0.04)S=1.01 BCa2.00 C(Mg3.64Fe2+ 0.24Mn0.01Al0.79Fe3+ 0.27Ti0.05)S=5.00 T(Si5.80Al2.20)S=8.00O22 W[F1.26(OH)0.74]S=2.00. The mineral species and name have been approved by the IMA CMNMC (IMA 2009-091).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
