Bradaczekite, NaCu4(AsO4)(3), a new mineral species from the Tolbachik volcano, Kamchatka Peninsula, Russia

Bradaczekite, NaCu4(ASO(4))(3), was discovered in a fumarole in the North B reach of the Great fissure Tolbachik eruption (GFTE) in 1975-1976, Kamchatk a Peninsula, Russia. Bradaczekite forms allregates of dark blue plates elon gate along [102]. The well-developed forms are {010}, {(3) over bar 11}, {1 11} and {(1) over bar 12}. The forms {301}, {001}, {(3) over bar 21}, {101} , {100}, {021}, {110}, {131}, {(1) over bar 11} and {341} are common. Assoc iated minerals are hematite, tenorite, lammerite, urusovite, orthoclase and johillerite. The mineral has an adamantine luster and a light blue to whit e streak. No cleavage has been observed. The mineral is transparent, biaxia l, optically negative, alpha 1.76(1), beta 1.92(1), gamma 1.96(1), 2V(calc) 50.0 degrees. The optical orientation is Z = b, X Lambda c = 23 degrees in the obtuse beta angle. Bradaczekite shows strong pleochroism: X violet-red , Y green, Z greenish blue. The mineral is monoclinic, space group C2/c, a 12.051(1), b 12.434(1), c 7.2662(7) Angstrom, beta 117.942(1)degrees, V 961 .8(2) Angstrom (3), Z = 4, D-calc = 4.77(1) g/cm(3) (for the empirical form ula). The strongest eight lines of the X-ray powder-diffraction pattern [d in Angstrom (l)(hkl)] are: 6.22(13)(020), 3.60(21)((2) over bar 02, (1) ove r bar 31), 3.43(100)((1) over bar 12, 310), 3.21(35)(002), 2.791(24)((4) ov er bar 02), 2.696(18)(330), 2.683(30)(240), and 2.665(17)(400). Electron-mi croprobe analyses yielded: Na2O 5.17 (4.37-5.78), K2O 0.35 (0.14-0.80), CuO 43.13 (41.31-45.22), ZnO 0.79 (0.19-1.18), Fe2O3 0.38 (0.03-1.05), As2O5 4 9.62 (48.61-50.66), V2O5 0.13 (0-0.90), total 99.55 (97.69-101.56) wt.%. Th e empirical formula, normalized to O = 12 apfu (atoms per formula unit), is (Na1.6K0.05)(Sigma1.21)(Cu3.74Zn0.07Fe0.033+)(Sigma3.84)(As3.00V0.01)(Sigm a3.01)O-12, close to the ideal NaCu4(AsO4)(3) confirmed by crystal-structur e analysis. The mineral is named in honor of Hans Bradaczek, crystallograph er at the Free University of Berlin.