MINERALOGICAL MAGAZINE

SUMMARY. Xocomecatlite and tlalocite were found at the Bambollita mine near Moctezuma, Sonora. Xocomecatlite is probably orthorhombic, and an indexed cell based on the powder data gave a 12.I4o/~, b I4"318, c I1.662. Based on this cell, the empirical contents are Cu3~. ~ Znl.7 (TeOa)12.5 (OH)51.e or CuzTeO4(OH)4. Tellurate was found both microchemically and by spectrophotometry. Colour ich green (R.H.S. 132A) with pale streak, H = 4. Optically negative with a = I '775,/3 = 1-9o0, = I'920. G = 4'65. Tlalocite is optically orthorhombic but the cell could not be indexed; only powder data are pre-sented. Chemical analysis howed both tellurite and tellurate and led to the formula Cul0Zn~(TeOz)(TeO~) 2CI(OH) 2~-27H20. Colour pale blue (R.H.S. I2IA) with a whitish streak; H = ~, gummy and sectile. G = 4"55. Plotting stability fields for various Te species at 25 ~ in terms of Eh and pH showed that TeO]--aq can exist in nature over a wide pH range, but only under extreme conditions of oxidation. Xocomecatlite Occurrence. Xocomecatlite was discovered in samples collected underground in the Bamboll ita Mine (La Oriental) by Peter Embrey and Pierre Bariand. Specimens containing this new species were found only in one drift, and it is such material that also provided carlfriesite and cesbronite. Xocomecatlite occurs on altered rhyolite adjacent o oxidized x(ein matter, or in the vein material (quartz-barite-altered-sulphides) as a late oxide-zone mineral. Physical properties. Xocomecatlite occurs as isolated or clustered spherules that seldom exceed oq5 mm in diameter. These spherules are composed of radially dis-posed needles that seldom attain 5/zm in width, and individual crystals are unknown. The colour is a rich green, near emerald green, and the best match is R.H.S. I32A. For such a rich colour, the streak is a remarkably pale green, almost white. The Mohs hardness is 4 and spherules are tough and brittle. No fluorescence was observed in 9 Copyright he Mineralogical Society.