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Fluoro-potassic-pargasite, KCa2(Mg4Al)(Si6Al2)O22F2, from the Tranomaro area, Madagascar: mineral description and crystal chemistry

  • R. Oberti (a1), M. Boiocchi (a2), F. C. Hawthorne (a3), R. Pagano (a4) and A. Pagano (a4)...

Abstract

Fluoro-potassic-pargasite, ideally AKBCa2 C(Mg4Al) (Si6Al2)O22 WF2, 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 × 2 mm × 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° (in β obtuse), Y ‖ b, Z ^ c = 31.4° (in β acute). It is biaxial positive, α = 1.638(2), β = 1.641(2), γ = 1.653(2); 2Vobs = 49.6(4)°, 2Vcalc = 53.4°.

Fluoro-potassic-pargasite is monoclinic, space group C2/m, a = 9.9104(2), b = 17.9739(4), c = 5.3205(1) Å, β = 105.534(2)°, V = 913.11(6) Å3, Z = 2. The eight strongest lines in the X-ray powder-diffraction pattern are [d in Å(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 microp-robe 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 × Ti), is A(K0.69Na0.28Ca0.04)∑=1.01 BCa2.00 C(Mg3.64Fe0.24 2+Mn0.01Al0.79Fe0.27 3+Ti0.05)∑=5.00 T(Si5.80Al2.20)∑=8.00O22 W[F1.26(OH)0.74]∑=2.00. The mineral species and name have been approved by the IMA CMNMC (IMA 2009-091).

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References

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Hawthorne, F.C., Oberti, R., Ungaretti, L. and Grice, J.D. (1996) A new hyper-calcic amphibole with Ca at the A site: fluor-cannilloite from Pargas, Finland. American Mineralogist, 81, 9951002.
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Fluoro-potassic-pargasite, KCa2(Mg4Al)(Si6Al2)O22F2, from the Tranomaro area, Madagascar: mineral description and crystal chemistry

  • R. Oberti (a1), M. Boiocchi (a2), F. C. Hawthorne (a3), R. Pagano (a4) and A. Pagano (a4)...

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