clipped from: www.kirchersociety.org   

Giant Crystals of Naica

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In 2000, geologists at the Naica zinc mine in Mexico discovered the largest crystals ever found in a cave 300 meters beneath the Chihuahuan Desert. Some of the the sword-like selenite (gypsum) crystals are as long as 36 feet and weigh up to 55 tons. Earlier this year, Spanish geologists published a paper in the journal Geology explaining the rare set of conditions that allowed the crystals to grow so extraordinarily large.
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“There is no limit to the size a crystal can reach,” García-Ruiz said.

But, he said, for the Cave of Crystals to have grown such gigantic crystals, it must have been kept just below the anhydrite-gypsum transition temperature for many hundreds of thousands of years.