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| The most breathtaking gypsum crystals ever found are located in Naica, Mexico. Found at 290m of depth in the Cueva de los Cristales, they form giant columns of selenite, measuring up to 11m in length, and have posed numerous questions to geologists since their discovery in 2000. Now a team including Professor Àngels Canals of the Department of Geology at the University of Barcelona has published a model explaining how the crystals formed, appearing in Geology, the journal of the Geological Society of America. The research team is led by Juan Manuel García-Ruiz, of the Andalusian Institute of Earth Sciences, part of the CSIC, Spain’s National Research Council. Also participating in the effort are Fermín Otálora (of the same institution), Carlos Ayora (of the Jaume Almera Institute of Earth Sciences, CSIC) and Roberto Villasuso (of Peñoles, the Mexican mining company).
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The Cueva de los Cristales is located at the Naica mine, which is operated by the Peñoles company in the state of Chihuahua, in the heart of the Mexican desert. Measuring thirty metres in length by ten metres in height, the cave was discovered by chance when some miners came across it while conducting explorations in 2000. Unusual geological conditions have created a favourable environment for the formation of giant selenite (dihydrate calcium sulphate) crystals of extraordinary size. Temperatures in the cave surpass 50 ºC and the humidity tops 90%. Further, the extraction pumps in the mine remove more than 55,000 litres of water per minute from the aquifer, which thus prevents flooding. How did the megacrystals of selenite form in the cave? According to the authors, the crystals grew within a very narrow range of temperature and chemical composition thanks to a stable self-feeding mechanism which, on the one hand, dissolved part of the mine’s anhydrate (anhydrous calcium sulphate) while, on the other hand, precipitating gypsum. As explained by Prof Àngels Canals, professor at the UB’s Department of Crystallography, Mineralogy and Mineral Deposits, “the giant crystals formed in an underwater cave in a mild solution of gypsum and a subsolution of anhydrite. At roughly 58 ºC, the solubility of the two minerals becomes equal. At higher temperatures, the anhydrite dissolves and the gypsum, or selenite, precipitates, letting megacrystals grow. To get the process started, a very high energy barrier had to be overcome, which is why there are few points of nucleation. What’s more, the system has remained stable for quite a long time to make self-feeding like this possible.” “Any parameter that we have studied regarding the crystals fits with the proposed model. All the mineralogical and geochemical data provided by nature confirm the model put forward in our article regarding the formation of the giant selenite crystals,” added Prof Canals, who is an expert in the field of fluid inclusions. One of questions posed to geologists is whether the growth of the megacrystals could be unlimited. Everything points to the geological system itself as the factor limiting the crystals’ continued growth. “Theoretically, if conditions were kept the same and there were enough anhydrite to supply the calcium and the sulphate,” noted Prof Canals, “the growth would be unlimited. The stability of the materials themselves may prove to be the limiting factor.” According to the authors, the cave’s highly unusual conditions may also be found elsewhere in the Naica site. “But as for columns of such an exceptional size,” said Prof Canals, “they could only be found in settings with similar geological conditions.” In the Geology article, the team of geologists, which is continuing the study of giant gypsum crystals in other parts of the world, also points out that the Cueva de los Cristales is a site of special scientific and geological interest which should be preserved for generations to come. From their standpoint, the conditions of humidity should be maintained and, if mining operations stop pumping out the water, a method may also need to be developed to ensure the integrity of the selenite columns, which, because of their size, could collapse. Black and white photographs are provided courtesy of Javier Trueba, Madrid Scientific Films.
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