Formation of natural gypsum megacrystals in Naica, Mexico

Abstract

Contains 4 figures.Exploration in the Naica mine (Chihuahua, Mexico) recently unveiled several caves containing giant, faceted, and transparentud 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 whichud 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 thatud 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 mechanismud 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 aud mineral wonderland, a site of scientific interest, and an extraordinaryud phenomenon worthy of preservation.We gratefully acknowledge Compañía Peñoles for the facilities provided during the field studies performed in the Naica mine, and the Ministerio de Educación y Ciencia of Spain for financial support.Peer reviewe