Fluoropargasite, a new member of the calcic amphiboles from Edenville, Orange County, New York

Document Type

Article

Publication Date

1-1-2005

Abstract

Fluoropargasite (IMA 2003-050), ideally NaCa2(Mg4 Al)(Si6Al2)O22F2, is a new member of the calcic amphiboles (group 2). It was collected from the Franklin Marble, of Grenville age, at Edenville, Orange County, New York, the type locality. The holotype specimen is a 13 × 7 × 4.5 cm crystal associated with calcite, actinolite, titanite and phlogopite. The same mineral has also been identified from Russell, St. Lawrence County, and Monroe, Orange County, New York. The mineral is black, with a gray to greenish gray streak and a vitreous luster. It is transparent to translucent in very thin fragments, non-fluorescent in ultraviolet light, brittle with a conchoidal fracture, and has a perfect {110} cleavage, a Mohs hardness of ∼6, a Dmeas of 3.18 g/cm3 and a Dcalc of 3.19 g/cm3 for the empirical formula with Z = 2. Fluoropargasite is biaxial (+), with α 1.634, β 1.642, γ 1.654 (all ± 0.002), 2Vmeas 68°, 2Vcalc 79°, Y = b and Z Λ c = 24° (acute), dispersion r > v, weak, pleochroism X colorless to light brown, Y light brown, and Z brown. The Gladstone-Dale compatibility index is superior, with 1 - (KP/KC) = -0.010 for the calculated density, and -0.016 for the measured density. The mineral is monoclinic, C2/m, with refined unit-cell parameters a 9.8771(6), b 18.041(1), c 5.3092(3) Å, β 105.133(1)°, V 913.25(3) Å3, Z = 2, and a:b:c 0.5475:1:0.2943, determined from single-crystal X-ray-diffraction data using MoKα radiation. A refinement of the structure led to the determination of cation site-preferences and degree of order. The six strongest lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 8.44(100)(110), 3.13(80)(310), 3.28(41)(240), 2.345(41)(351), 2.810(32)(330), and 2.385(21)(350). The chemical composition determined by electron-microprobe analysis is SiO2 43.30, MgO 14.44, FeO 9.73, CaO 12.29, Al2O3 12.11, Na2O 2.88, TiO2 0.90, MnO 0.08, K2O 0.91, V2O3 0.18, Cr2O3 0.01, F 2.71, Cl 0.12, O = (F + Cl)-1.17, H2O (stoichiometry) 0.71, sum 99.20 wt%, which corresponds to the empirical formula: (Na0.75 K0.17)∑0.92 (Ca1.94 Na0.06)∑2.00 (Mg3.18 Fe2+1.18Al0.50Ti0.10 Fe3+0.02V0.02Mn0.01) ∑5.00(Si6.39Al1.61) ∑8.00O22[F1.26(OH)0.71 Cl0.03]∑2.00, with (OH + F + CI) = 2 apfu based on 24 anions. The name of the mineral is in accord with the IMA nomenclature for amphiboles.

Publication Title

Canadian Mineralogist

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