aragonite - "cave calcite" - El Potosi Mine, Santa Eulalia Mining District, Chihuahua - display specimen

aragonite CaCo₃

Aragonite is a pseudomorph of calcite, with the same chemical formula, calcium carbonate, but a different crystal structure. When carbonated groundwater contains calcium, it most commonly precipitates calcite when cold and aragonite, when hot.

Sometimes called "cave calcite," this aragonite comes from the Potosi Mine in the Santa Eulalia Mining District, Chihuahua, Mexico. The silver, lead, zinc and copper ores in this mine are associated with thick-bedded limestones overlain by a series of rhyolite flows and tuffs. In 1912, miners broke into a cavern lined with cave calcite forms similar to these specimens. In subsequent years, numerous cavities have been discovered, with at least some specimens extracted before the cavern was destroyed by mining.

As described in 1925 by mining engineer Carl Millikan, "one cave, the 'Gypsum' cave, when opened, presented a fairyland-like scene. Pure dazzling white, filled with crystals of all shapes and sizes, it was a bonanza for specimens of everything - but ore."

Photo under shortwave u/v follows the photo in white light for each specimen. Some fluoresce green weakly. Others have strong green fluorescence. These specimens are spectacular.

The pencil is 5.5" long, for scale. Click to select, click again to enlarge.

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