The Naica mine in remote northern Mexico is famous for its abundance of crystals. It’s also one of Mexico’s most productive lead and silver mines, and not by coincidence, because the geological processes involved in creating lead and silver also provide the raw materials of crystals. Miners frequently hammer into impressive chambers of crystals, but in 2000, a pair of brothers drilled into what seemed like a child’s dreamscape: an enormous limestone cavern 300 metres underground, glittering with a forest of thick, luminous crystals up to 10 metres long and 600,000 years old. But how did they get so huge? Most caves and mines boast cool temperatures, but the Naica mines lie above an intrusion of magma 1.6 kilometres underground. In the Cave of Crystals, the temperature soars to 45 degrees Celsius. So, when calcium sulfate-infused groundwater flowed through the caves hundreds of thousands of years ago, it was heated until it reached a stable temperature where the minerals in the water formed selenite. This growth of selenite became the tiny bricks that built the vast crystal architecture. Temperature fluctuations in other caverns meant that the crystals stopped growing and so were smaller, but in the Cave of Crystals, where conditions remained unchanged for millennia, the crystals just kept growing.
This is a photo of chrome alum crystals in a chrome alum solution, at 20x magnification. Taken with my Canon SX110 IS, through a Novex stereo microscope with lighting from underneath. The crystals are grown from a saturated solution and are no more than 1 mm in diameter. Created myself, Paul, from www.paulslab.com