aragonite - "cave calcite" from the Potosi Mine, Santa Eulalia, Chihuahua - hand 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."

One of these fluoresces under shortwave ultraviolet. The photo under shortwave follows the normal lighting photo of the same specimen. The shapes don't match well, as the fluorescence was photographed in a different orientation. With the other, the fluorescence is weak or spotty.

The pencil is 5.5" long, for scale.

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