Naica’s Giant Crystals: Deterioration Scenarios

The Cave of Giant Crystals of Naica (Chihuahua, Mexico) is a world geological treasure worth to be preserved. These crystals of up to 12 m in length are made of selenite, the macrocrystalline variety of gypsum (CaSO4·2H2O). They have grown for thousands of years until the cave was dried, which allowed the cave and the crystals to be accessible, but exposed their surfaces in contact with air. Gypsum crystals are fragile because of their trend to dehydrate, the possible replacement to CaCO3 upon reactions with atmospheric CO2 as well as their intrinsic mechanical properties. Several laboratory experiments, designed to study the deterioration of selenite crystals under different artificial atmospheric conditions, are presented. Four atmospheric compositions rich in CO2, CH4, NOx, and air were tested for 1 year at temperatures of 25 and 60 °C and in either liquid or gaseous environments. The surface evolution was monitored by optical microscopy, infrared spectrometry, and grazing incidence X-ray diffraction with two-dimensional detectors. Surface alteration and dissolution in a water environment were observed in short exposition times, as well as the formation of bassanite (CaSO4·1/2H2O). Neither anhydrite nor calcite was detected. The gaseous environment constituted the most detrimental conditions to the gypsum crystals integrity.
The authors would like to acknowledge Consejo Nacional de Ciencia y Tecnologia (grant number 183706); Industrias Peñoles for the support given to the experiments; A. Reyes-Rojas, D. Burciaga-Valencia, and E. Guerrero-Lastarjette for their continuous support in the realization of this project; and B. Aldea-Ballano and the team of the Chemical and Physicochemical Testing Unit in Instituto Eduardo Torroja de Ciencias de la Construcción in Madrid. The authors especially thank J. M. García-Ruiz for the suggestions of some experiments. Part of the experiments was performed (as part of Proposal No. 3939) at the Stanford Synchrotron Radiation Lightsource (SSRL), a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy, Office of Science, by Stanford University.