Fluoro-Potassichastingsite from the Greenwood Mine, Orange County, New York: a New End-Member Calcic Amphibole

TitleFluoro-Potassichastingsite from the Greenwood Mine, Orange County, New York: a New End-Member Calcic Amphibole
Publication TypeJournal Article
Year of Publication2009
AuthorsLupulescu, MV, Rakovan, J, Dyar, MD, Robinson, GW, Hughes, JM
JournalThe Canadian Mineralogist
Keywordscalcic amphibole, end member, fluoro-potassichastingsite, Grenville rocks, Hudson Highlands, New York, structure determination

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, Dmeas is 3.289 g/cm3, and Dcalc is 3.37 g/cm3. The {110} cleavage is perfect, and the fracture is conchoidal. It is biaxial negative, with α 1.668(2), β 1.688(2), γ 1.698(2), 2Vmeas 40–70°, 2Vcalc 70°, Y = b, Zc = 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 – (KP/KC) = 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) Å3, 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(K0.59Na0.25)∑0.84B(Ca1.87Na0.13)∑2.00C(Fe2+2.60Mg1.56Fe3+0.53Al0.26 Mn0.03Ti0.01)∑4.99T(Si6.36Al1.64)∑8.00 O22.68O3[F1.11(OH)0.73Cl0.16]∑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).