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Geology; April 2007; v. 35; no. 4; p. 327-330; DOI: 10.1130/G23393A.1
© 2007 Geological Society of America
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Formation of natural gypsum megacrystals in Naica, Mexico

Juan Manuel García-Ruiz*,1, Roberto Villasuso2, Carlos Ayora3, Angels Canals4 and Fermín Otálora5

1 Instituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones Científicas-Universidad de Granada, Campus Fuentenueva, E-18002 Granada, Spain
2 Compañía Peñoles, Unidad Naica, Naica, Chihuahua, Mexico
3 Institut de Ciències de la Terra Jaume Almera, Consejo Superior de Investigaciones Científicas, Lluis Solé Sabarís s/n, E-08028 Barcelona, Spain
4 Departament de Cristal.lografia, Mineralogia i Dipòsits Minerals, Universitat de Barcelona, Martí Franquès s/n, E-08028 Barcelona, Spain
5 Instituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones Científicas-Universidad de Granada, Campus Fuentenueva, E-18002 Granada, Spain

Exploration in the Naica mine (Chihuahua, Mexico) recently unveiled several caves containing giant, faceted, and transparent single crystals of gypsum (CaSO4•2H2O) as long as 11 m. These large crystals form at very low supersaturation. The problem is to explain how proper geochemical conditions can be sustained for a long time without large fluctuations that would trigger substantial nucleation. Fluid inclusion analyses show that the crystals grew from low-salinity solutions at a temperature of ~54 °C, slightly below the one at which the solubility of anhydrite equals that of gypsum. Sulfur and oxygen isotopic compositions of gypsum crystals are compatible with growth from solutions resulting from dissolution of anhydrite previously precipitated during late hydrothermal mineralization, suggesting that these megacrystals formed by a self-feeding mechanism driven by a solution-mediated, anhydrite-gypsum phase transition. Nucleation kinetics calculations based on laboratory data show that this mechanism can account for the formation of these giant crystals, yet only when operating within the very narrow range of temperature identified by our fluid inclusion study. These singular conditions create a mineral wonderland, a site of scientific interest, and an extraordinary phenomenon worthy of preservation.

Key Words: gypsum • crystallization • mineral growth • anhydrite • Naica mine • nucleation






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