You've probably seen the pictures: Mexican miners dwarfed by enormous gypsum crystals more than thrice their size. For weeks, their size baffled scientists, but now it seems they've cracked the mystery. According to the BBC, "The scientists studied tiny pockets of fluid trapped in the crystals and conducted back-up lab experiments. They report in the journal Geology that the solution from which the crystals grew must have been kept in a very narrow, stable temperature range." This leads them to believe that there are even more -- and perhaps more spectacular -- crystal caves yet to be discovered.
An even more in-depth explanation, after the jump!
The crystals owe their origin to the volcanism which laid down the metal sulphides - the ores - that have proved so valuable. Copious amounts of calcium sulphate would also have been created towards the end of this mineralisation process more than 20 million years ago - but in the hot fluids that infused the cracks and cavities in the rock, this calcium sulphate would have taken the form of anhydrite.
Anhydrite has the same chemical formula as gypsum, except that it excludes water. Only as the magma chamber deep under the Naica mountain cooled did the hot fluids above start to fall to a temperature at which anhydrite could switch to gypsum.
Professor Garcia-Ruiz and colleagues say their studies indicate that the deeper of the two caves - Cueva de los Cristales - must have been kept just below the transition temperature for many hundreds of thousands of years. "The conditions were perfect. By maintaining the temperature just below 58 degrees for a very long time you get a few, very big crystals," said Professor Garcia-Ruiz.