Stalactitic rhodochrosite from the 25 de Mayo and Nueve veins, Capillitas, Catamarca, Argentina: Physical and chemical variations

[摘要] Capillitas is an epithermal vein-type deposit in Argentina known for its mineralogical diversity, with more than onehundred and twenty described minerals, including five new species, and for the presence of banded and stalactiticrhodochrosite. Stalactites occur as single or combined cylinders of different sizes, from a few cm to 1.36 m in length anddiameters up to 8 cm. Their cross-sections may show diverse aspects: from simple concentric banding to more intricatetextures, whereas their external surface can be smooth, with undulations or with a poker-chip-like texture. The colorof the stalactites varies from white to raspberry pink, with occasional brown bands toward the edges correspondingto a variety of rhodochrosite called “capillitite”. The contents of MnO range from 27.50 to 61.71 wt. % as it may besignificantly replaced by CaO, FeO, ZnO and MgO. Replacements are reflected in the various shades of pink and browndisplayed by this mineral. The different substitutions in the pink specimens exert only a minor influence on the unit cellparameters, whereas, in the brown variety, their size is significantly smaller with average values for pink rhodochrosite(n = 24): a 4.776 Å, c 15.690 Å and a cell volume of 310.3 Å3, whereas, “capillitite” unit-cell parameters (n = 7) are:a = 4.739, c = 15.558 with a unit-cell volume of 302.6 Å3. Conditions of formation of the banded rhodochrosite of the25 de Mayo vein, obtained from fluid inclusions data, indicate temperatures of 145 ° to 150 °C and salinities of up to4 wt. % NaCl(eq). The formation of the stalactites is explained by the infiltration of epithermal aqueous liquid, oversaturated with Mn and bicarbonate, into a transiently vapor-filled, isolated cavity.

[学科分类] 自动化工程

[关键词] rhodochrosite;stalactites;epithermal deposit;Capillitas;Argentina