BRADACZEKITE, NaCu4(AsO4)3, A NEW MINERAL SPECIES FROM THE TOLBACHIK VOLCANO, KAMCHATKA PENINSULA, RUSSIA

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Bradaczekite, NaCu 4 (AsO 4 ) 3 , was discovered in a fumarole in the North Breach of the Great fissure Tolbachik eruption (GFTE) in 1975–1976, Kamchatka Peninsula, Russia. Bradaczekite forms aggregates of dark blue plates elongate along [102]. The well-developed forms are {010}, {311}, {111} and {112}. The forms {301}, {001}, {321}, {101}, {100}, {021}, {110}, {131}, {111} and {341} are common. Associated minerals are hematite, tenorite, lammerite, urusovite, orthoclase and johillerite. The mineral has an adamantine luster and a light blue to white streak. No cleavage has been observed. The mineral is transparent, biaxial, optically negative, α 1.76(1), β 1.92(1), γ 1.96(1), 2 V calc 50.0°. The optical orientation is Z = b , X ∧ c = 23° in the obtuse β angle. Bradaczekite shows strong pleochroism: X violet-red, Y green, Z greenish blue. The mineral is monoclinic, space group C 2 /c , a 12.051(1), b 12.434(1), c 7.2662(7) A, β 117.942(1)°, V 961.8(2) A 3 , Z = 4, D calc = 4.77(1) g/cm 3 (for the empirical formula).The strongest eight lines of the X-ray powder-diffraction pattern [ d in A( I )( hkl )] are: 6.22(13)(020), 3.60(21)(202, 131), 3.43(100)(112, 310), 3.21(35)(002), 2.791(24)(402), 2.696(18)(330), 2.683(30)(240), and 2.665(17)(400). Electron-microprobe analyses yielded: Na 2 O 5.17 (4.37–5.78), K 2 O 0.35 (0.14–0.80), CuO 43.13 (41.31–45.22), ZnO 0.79 (0.19–1.18), Fe 2 O 3 0.38 (0.03–1.05), As 2 O 5 49.62 (48.61–50.66), V 2 O 5 0.13 (0–0.90), total 99.55 (97.69–101.56) wt.%. The empirical formula, normalized to O = 12 apfu (atoms per formula unit), is (Na 1.16 K 0.05 ) ∑1.21 (Cu 3.74 Zn 0.07 Fe 3+ 0.03 ) ∑3.84 (As 3.00 V 0.01 ) ∑3.01 O 12 , close to the ideal NaCu 4 (AsO 4 ) 3 confirmed by crystal-structure analysis. The mineral is named in honor of Hans Bradaczek, crystallographer at the Free University of Berlin.