Bobmeyerite, a new mineral from Tiger, Arizona, USA, structurally related to cerchiaraite and ashburtonite

Abstract

Bobmeyerite, Pb₄(Al₃Cu)(Si₄ O₁₂)(S_0.5Si_0.5O₄)(OH)₇ Cl(H₂O)₃, is a new mineral from the Mammoth - Saint Anthony mine, Tiger, Pinal County, Arizona, USA. It occurs in an oxidation zone assemblage attributed to progressive alteration and crystallization in a closed system. Other minerals in this assemblage include atacamite, caledonite, cerussite, connellite, diaboleite, fluorite, georgerobinsonite, hematite, leadhillite, matlockite, murdochite, phosgenite, pinalite, quartz, wulfenite and yedlinite. Bobmeyerite occurs as colourless to white or cream-coloured needles, up to 300 m m in length, that taper to sharp points. The streak is white and the lustre is adamantine, dull or silky. Bobmeyerite is not fluorescent. The hardness could not be determined, the tenacity is brittle and no cleavage was observed. The calculated density is 4.381 g cm_-3. Bobmeyerite is biaxial (-) with α ≈ β = 1.759(2), γ = 1.756(2) (white light), it is not pleochroic; the orientation is X = c; Y or Z = a or b. Electron-microprobe analyses provided the empirical formula Pb_3.80Ca_0.04Al_3.04Cu²⁺_0.96Cr³⁺_0.13Si_4.40S_0.58O_24.43Cl_1.05F_0.52H_11.83. Bobmeyerite is orthorhombic (pseudotetragonal), Pnnm with unit-cell parameters a = 13.969(9), b = 14.243(10), c = 5.893(4) Å, V = 1172.5(1.4) ų and Z = 2. The nine strongest lines in the X-ray powder diffraction pattern, listed as [d_obs (Å)(I)(hkl)], are as follows: 10.051(35)(110); 5.474(54)(011,101); 5.011(35)(220); 4.333(43)(121,211); 3.545(34)(040,400); 3.278(77)(330,231,321); 2.9656(88)(141,002,411); 2.5485(93)(051,222,501); 1.873(39)(multiple). Bobmeyerite has the same structural framework as cerchiaraite and ashburtonite. In the structure, which refined to R₁ = 0.079 for 1057 reflections with F > 4σF, SiO₄ tetrahedra share corners to form four-membered Si₄O₁₂ rings centred on the c axis. The rings are linked by chains of edge-sharing AlO₆ octahedra running parallel to [001]. The framework thereby created contains large channels, running parallel to [001]. The Cl site is centred on the c axis alternating along [001] with the Si₄O₁₂ rings. Two non-equivalent Pb atoms are positioned around the periphery of the channels. Both are elevencoordinate, bonding to the Cl atom on the c axis, to eight O atoms in the framework and to two O (H₂O) sites in the channel. The Pb atoms are off-centre in these coordinations, as is typical of Pb²⁺ with stereo-active lone-electron pairs. A (S, Si, Cr)O₄ group is presumed to be disordered in the channel. The name honours Robert (Bob) Owen Meyer, one of the discoverers of the new mineral.