Caves stumbled upon deep in the workings of the Naica mine in Chihuahua, Mexico, are famed for the huge, elongated crystals of selenite that they contain. The most spectacular such discovery, the transparently named Cueva de los Cristales, came in 2000.

Credit: J. TRUEBA/GEOL. SOC. AM.

The crystals found there — at 300 metres' depth, a temperature of just under 60 °C and 100% humidity — are up to 11 metres long (see picture). Juan Manuel García-Ruiz and colleagues suggest the very specific conditions that were required for such huge crystals to form (J. M. García-Ruiz et al. Geology 35, 327–330; 2007).

Selenite is the colourless crystalline form of calcium sulphate dihydrate (CaSO4·2H2O), better known as gypsum. Gypsum's solubility in water is highest at around 58 °C. Above that temperature, however, it is thermodynamically less stable than the anhydrous form of calcium sulphate, known simply as anhydrite.

According to the authors' ideas, then, the ambient conditions of the Cueva de los Cristales would have been ideal for maintaining low-salinity cave-water slightly undersaturated in anhydrite (which would have originated from hydrothermal processes during the formation of the surrounding rock) and slightly supersaturated in gypsum over long periods. This tiny equilibrium imbalance would have encouraged the slow, sparse formation of very long gypsum crystals.

The authors backed up their theory with various hydrochemical and geochemical analyses of samples from the cave. But they point out that the effulgent result of the unique natural circumstances that they describe might not be visible for future generations. The Naica mines today number among the most important for lead and silver in the world. Once their booty is exhausted, however, and water-pumping of the workings stops, nature might reclaim the Cueva de los Cristales, reverting it to its erstwhile fully flooded state.