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Bobmeyerite, Pb4(Al3Cu)(Si4O12)(S0.5Si0.5O4)(OH)7Cl(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 a & b = 1.759(2), g = 1.756(2) (white light), it is not pleochroic; the orientation is X= c; Yor Z= aor b. Electron-microprobe analyses provided the empirical formula Pb3.80Ca 0.04Al3.04Cu2+0.96Cr3+0.13Si4.40S0.58O24.43Cl 1.05F0.52H11.83. Bobmeyerite is orthorhombic (pseudotetragonal), Pnnmwith unit-cell parameters a= 13.969(9), b= 14.243(10), c= 5.893(4) Å, V= 1172.5(1.4) Å 3 and Z= 2. The nine strongest lines in the X-ray powder diffraction pattern, listed as [dobs (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 toR1= 0.079 for 1057 reflections withF> 4 sF, SiO4tetrahedra share corners to form four-membered Si4O12rings centred on thecaxis. The rings are linked by chains of edge-sharing AlO6octahedra running parallel to [001]. The framework thereby created contains large channels, running parallel to [001]. The Cl site is centred on the caxis alternating along [001] with the Si4O12rings. Two non-equivalent Pb atoms are positioned around the periphery of the channels. Both are elevencoordinate, bonding to the Cl atom on thecaxis, 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)O4group is presumed to be disordered in the channel. The name honours Robert (Bob) Owen Meyer, one of the discoverers of the new mineral. [ABSTRACT FROM AUTHOR]