Oxyphlogopite K(Mg,Ti,Fe)3[(Si,Al)4O10](O,F)2: A new mineral species of the mica group

Oxyphlogopite is a new mica-group mineral with the idealized formula K(Mg,Ti,Fe)₃(Si,Al)₄O₁₀](O,F)₂. The holotype material came from a basalt quarry at Mount Rothenberg near Mendig at the Eifel volcanic complex in Rhineland-Palatinate, Germany. The mineral occurs as crystals up to 4 × 4 × 0.2 mm in size encrusting cavity walls in alkali basalt. The associated minerals are nepheline, plagioclase, sanidine, augite, diopside, and magnetite. Its color is dark brown, its streak is brown, and its luster is vitreous. D _meas = 3.06(1) g/cm³ (flotation in heavy liquids), and D _calc = 3.086 g/cm³. The IR spectrun does not contain bands of OH groups. Oxyphlogopite is biaxial (negative); α = 1.625(3), β = 1.668(1), and γ = 1.669(1); and 2V _meas = 16(2)° and 2V _calc = 17°. The dispersion is strong; r < ν. The pleochroism is medium; X > Y > Z (brown to dark brown). The chemical composition is as follows (electron microprobe, mean of 5 point analyses, wt %; the ranges are given in parentheses; the H₂O was determined using the Alimarin method; the Fe²⁺/Fe³⁺ was determined with X-ray emission spectroscopy): Na₂O 0.99 (0.89–1.12), K₂O 7.52 (7.44–7.58), MgO 14.65 (14.48–14.80), CaO 0.27 ((0.17–0.51), FeO 4.73, Fe₂O₃ 7.25 (the range of the total iron in the form of FeO is 11.09–11.38), Al₂O₃ 14.32 (14.06–14.64), Cr₂O₃ 0.60 (0.45–0.69), SiO₂ 34.41 (34.03–34.66), TiO₂ 12.93 (12.69–13.13), F 3.06 (2.59–3.44), H₂O 0.14; O=F₂ −1.29; 99/58 in total. The empirical formula is (K_0.72Na_0.14Ca_0.02)(Mg_1.64Ti_0.73Fe_0.30²⁺ Fe_0.27³⁺Cr_0.04)_Σ2.98(Si_2.59Al_1.27Fe_0.14³⁺ O₁₀) O_1.20F_0.73(OH)_0.07. The crystal structure was refined on a single crystal. Oxyphlogopite is monoclinic with space group C2/m; the unit-cell parameters are as follows: a = 5.3165(1), b = 9.2000(2), c = 10.0602(2) ?, β = 100.354(2)°. The presence of Ti results in the strong distortion of octahedron M(2). The strongest lines of the X-ray powder diffraction pattern d, ? (I, %) hkl]] are as follows: 9.91(32) 001], 4.53(11) 110], 3.300(100) 003], 3.090(12) 112], 1.895(21) 005], 1.659(12) −135], 1.527(16) −206, 060]. The type specimens of oxyphlogopite are deposited at the Fersman Mineralogical Museum in Moscow, Russia; the registration numbers are 3884/2 (holotype) and 3884/1 (cotype).