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FLUORO-POTASSICHASTINGSITE FROM THE GREENWOOD MINE, ORANGE COUNTY, NEW YORK: A NEW END-MEMBER CALCIC AMPHIBOLE

Marian V. Lupulescu^1,, John Rakovan², M. Darby Dyar³, George W. Robinson⁴ and John M. Hughes⁵

¹ New York State Museum, Division of Research and Collections, 3140 CEC, Madison Ave., Albany, New York 12230, USA
² Department of Geology, Miami University, Oxford, Ohio 45056, USA
³ Mount Holyoke College, 405 Clapp, South Hadley, Massachusetts 01075–6419, USA
⁴ A.E. Seaman Mineral Museum, Michigan Technological University, Houghton, Michigan 49931, USA
⁵ Office of the Provost, The University of Vermon, Burlington, Vermont 05405, USA

E-mail address: mlupules{at}mail.nysed.gov

Fluoro-potassichastingsite, ideally KCa₂(Fe²⁺₄Fe³⁺)Si₆Al₂O₂₂F₂, is found in compact aggregates of crystals up to 1 cm size on the waste-rock dump at the Greenwood mine, in Harriman State Park, Tuxedo, Orange County, New York, USA. It is associated with magnetite, diopside, enstatite, pyrrhotite, chalcopyrite, pyrite and, rarely, phlogopite. Fluoro-potassichastingsite is transparent, black to greenish in very thin fragments, and non-fluorescent in ultraviolet light; it has a vitreous luster and a greenish gray streak. The Mohs hardness is 6, D_meas is 3.289 g/cm³, and D_calc is 3.37 g/cm³. The {110} cleavage is perfect, and the fracture is conchoidal. It is biaxial negative, with 1.668(2), β 1.688(2), 1.698(2), 2V_meas 40–70°, 2V_calc 70°, Y = b, Z c = 23°, and r < v. The observed pleochroism is X bluish green, Y green to brownish green, Z blue to light blue. The Gladstone– Dale compatibility index is superior, with 1 – (K_P/K_C) = 0.012 (based on empirical formula and calculated density). The mineral is monoclinic, space group C2/m, a 9.9480(3), b 18.1777(6), c 5.3302(2) Å, β 105.140(1)°, V 930.41(8) ų, and Z = 2. The strongest eight X-ray powder-diffraction lines [d in Å(I)(hkl)] are: 8.499(100)(110), 3.151(76)(310), 2.830 (53)(330), 3.299(32) (240), 2.722(23)(151), 2.402(17)(350), 3.401(11)(131), and 1.661(10)(461). Combined electron-microprobe and Mössbauer data yield: SiO₂ 40.49, TiO₂ 0.11, Al₂O₃ 10.29, V₂O₃ 0.03, Cr₂O₃ 0.01, Fe₂O₃ 4.49, FeO 19.80, CaO 11.13, MgO 6.68, MnO 0.20, K₂O 2.93, Na₂O 1.24, F 2.23, Cl 0.61, H₂O 0.70, O = (F+Cl) –1.08, sum 99.86 wt.%, which corresponds to the following crystal-chemical formula based on 24 [O + (OH) + F + Cl]: ^A(K_0.59Na_0.25)_0.84 ^B(Ca_1.87Na_0.13)_2.00 ^C(Fe²⁺_2.60Mg_1.56Fe³⁺_0.53Al_0.26 Mn_0.03Ti_0.01)_4.99 ^T(Si_6.36Al_1.64)_8.00 O_22.68 ^O3[F_1.11(OH)_0.73Cl_0.16]_2.00. The ICP–MS analyses on a crystal fragment different than the one analyzed for major elements yielded the following trace-element concentrations (ppm): Cu 278.9, Rb 162.5, Y 343.2, La 257, Ce 568, Nd 156. The new species and name have been approved by IMA–CNMMN (2005–006).

Keywords: fluoro-potassichastingsite, end member, calcic amphibole, structure determination, Grenville rocks, Hudson Highlands, New York.