Abstract

GARCIA-RUIZ, Juan Manuel et al. Formation of natural gypsum megacrystals in Naica, Mexico. Bol. Soc. Geol. Mex [online]. 2007, vol.59, n.1, pp.63-70. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2007v59n1a5.

Exploration activities in the mine of Naica (Chihuahua, Mexico) recently unveiled several caves containing giant, faceted and transparent single crystals of gypsum (CaSO4·2H2O) up to eleven meters in length. From a crystallization point of view, it can be expected that the formation of these large crystals occur at very low supersaturation, the problem being to explain how proper geochemical conditions can be sustained for long time without large fluctuations that would trigger substantial nucleation. Fluid inclusions analyses show that crystals grew from low salinity solutions at a temperature around 54°C, slightly below the one at which the solubility of anhydrite equals the one of gypsum. Sulfur and oxygen isotopic composition of gypsum crystals is compatible with the 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 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 inclusions study. These singular conditions create a mineral wonderland, a site of scientific interest, and an extraordinary phenomenon worthy of preservation.

Keywords : Gypsum; selenite; crystallization; mineral growth; megacrystals; anhydrite; Naica; nucleation kinetics; fluid inclusions.