A new mineral species zincobradaczekite, NaCuCuZn2(AsO4)(3), and a new isomorphous series bradaczekite-zincobradaczekite in the alluaudite group

摘要

A new alluaudite-group mineral zincobradaczekite, a Zn-Cu ordered highly zincian analogue of bradaczekite, was discovered in sublimates of the Yadovitaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with bradaczekite, lammerite, lammerite-beta, borisenkoite, mcbirneyite, sanidine, hematite, tenorite, pseudolyonsite, lyonsite, starovaite, rutile, tripuhyite, pseudobrookite, piypite, langbeinite, calciolangbeinite, aphthitalite, alumoklyuchevskite, and palmierite. Zincobradaczekite forms oblique-angled prismatic crystals up to 0.02 x 0.02 x 0.08 mm(3)and equant or tabular crystals up to 0.05 mm. It is transparent, blue, greenish-blue, grey-blue or bluish-grey. The lustre is vitreous.D(calc)is 4.71 g cm(-3). Zincobradaczekite is optically biaxial (-), alpha 1.786(5), beta 1.846(8), gamma 1.90(1), 2V(meas.)80(5)o. Chemical composition (wt%, electron microprobe) is Na2O 4.07, K2O 0.53, CaO 0.01, MgO 0.71, MnO 0.01, CuO 19.89, ZnO 24.21, Al(2)O(3)0.02, Fe(2)O(3)0.90, TiO(2)0.03, P(2)O(5)0.41, V(2)O(5)1.05, As(2)O(5)47.10, SO(3)1.01, and total 99.95. The empirical formula based on 12 Oapfuis Na0.90K0.08Cu1.70Mg0.12Zn2.03Fe0.083+(As2.79S0.09V0.08P0.04)(sigma 3.00)O-12. The simplified chemical formula is NaCu2Zn2(AsO4)(3). Zincobradaczekite is monoclinic,C2/c,a12.0375(13),b12.4500(13),c7.2213(8) angstrom, beta 117.506(7)o,V959.90(18) angstrom(3)andZ4. The strongest reflections of the powder XRD pattern [d,angstrom(I)(hkl)] are 6.21(31)(020), 3.416(70)(310, - 112), 3.200(17)(002), 2.779(23)(- 402), 2.691(100)(- 132, 240), 1.841(20)(152) and 1.680(14)(- 642). The crystal structure was solved (single-crystal XRD,R = 4.38%). The simplified crystal chemical formula is(A(1))& x25a1;Cu-A(1)MODIFIER LETTER PRIME(A(2))& x25a1;(NaCuZn2)-Na-A(2)MODIFIER LETTER PRIME-Cu-M(1)-Zn-M(2)(AsO4)(3)(& x25a1;-vacancy) and the end-member formula is NaCuCuZn2(AsO4)(3). Zincobradaczekite and bradaczekite, ideally NaCuCuCu2(AsO4)(3), are isostructural. In the Yadovitaya fumarole, Mg-poor bradaczekite forms an isomorphous series with zincobradaczekite, whilst in the neighbouring Arsenatnaya fumarole only a Zn-depleted and Mg-enriched variety of bradaczekite occurs. Zincobradaczekite and bradaczekite differ from other anhydrous alluaudite-group arsenates by increased values of thecand beta unit-cell parameters due to the strong Jahn-Teller distortion ofM(1)O(6)octahedron predominantly occupied with Cu2+.