First discovered about a decade ago, the largest known cave crystals—single hunks of gypsum as much as 11 meters long, 1 meter thick, and weighing 55 tons—could have taken up to 1 million years to grow, a new study suggests. The cavern in the Mexican silver and lead mine where the crystals were found was filled with mineral-rich waters until 1975, when it was drained to provide miners with access to new ore veins. Lab tests indicate that the gypsum hunks crystallized at temperatures between 54°C and 58°C, researchers report online today in the Proceedings of the National Academy of Sciences. By immersing a hunk of gypsum in a sample of the cave's waters and using a microscopic imaging technique that allowed the scientists to directly measure crystal growth, the team found that at 55°C, near the temperature at which the crystals would have grown most slowly, it would take around 990,000 years for a gypsum crystal 1 meter in diameter to form . At water temperatures of 56°C, the same crystal could have formed in about 500,000 years.
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