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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 α 1.668(2), β 1.688(2), γ 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).