Do the Naica Giant Deteriorate due to Human Action?

M. E. Montero-Cabrera1,*, I. Castillo-Sandoval1, I.J.A. Carreño-Márquez1, L. E. Fuentes-Cobas1, B. Pérez-Cázares2, H.E. Esparza-Ponce1, E. Menéndez-Méndez3, M.E. Fuentes-Montero2, H. Castillo-Michel4, D. Eichert5, R. Loredo- Portales6, J. Canche-Tello1, U. Salazar-Kuri7, M.Y. Luna-Porres1, G. González-Sánchez1, G. Herrera-Pérez1, J.M. Nápoles2, D. Burciaga-Valencia1, C. Caraveo1, G. Gómez2, L. Muñoz2, I. Reyes-Cortes2, M. Reyes-Cortés2

1Centro de Investigación en Materiales Avanzados (CIMAV), Chihuahua 31136, Mexico 2Universidad Autónoma de Chihuahua, Chihuahua 31125, Mexico 3Instituto Eduardo Torroja de Ciencias de la Construcción, Madrid 28033, Spain 4European Synchrotron Radiation Facility, 38043 Grenoble Cedex 9, France 5Elettra Sincrotrone Trieste S.C.p.A., AREA Science Park, Basovizza (Trieste) Italy 6Universidad Nacional Autónoma de México, Hermosillo, Mexico 7Benemérita Universidad Autónoma de Puebla, Puebla, México

*e-mail: [email protected]

The giant crystals of the Naica mine have fascinated scientists from around the world specially since the year 2000, when the "Cueva de los Cristales" was discovered. It houses crystals of (CaCO4 · 2H2O) of more than 11 m in length. Human presence has changed the microclimate conditions inside this cave, raising the question of whether anthropogenic action resulted in the deterioration of its single crystals and in the deposition of impurities on their surfaces. The present work provides a detailed characterization of representative samples of this cave and suggests an answer to the origin of the impurities on the surface of these nature-made large crystals. The investigation entailed two main aspects: first, the detailed chemical-structural characterization of these impurities, and second, the experimental simulation of possible deterioration.

Surface impurities of 40 samples from Cueva de los Cristales were studied. Conventional techniques such as petrography, optical and electronic microscopy, as well as laboratory X-ray diffraction were employed. Synchrotron radiation – based techniques, i.e. 2D grazing incidence X-ray diffraction, μ-X-ray fluorescence and μ-X-ray absorption near edge structure, played a crucial role in the phase identification of the impurities. The elements regularly observed in the impurities were Ti, Mn, Fe, Cu, Zn, As, and Pb. Twenty-eight crystalline and two amorphous phases were determined. They are consistent with the minerals present in the mine and in the walls of the cave. Hematite and iron oxyhydroxides prevail among crystalline impurities, while manganese and oxides are the amorphous phases. Among the impurities, calcium carbonate, product of incongruent dissolution, was not detected. A numerical model allowed to complete the geochemical model on conceptual and physicochemical bases. The surface impurities were deposited in the latest stage of the gypsum growth, due to the descent of the extraction cone of water from the mine. Subsequently, the impurities chemistries have evolved with this environment of high relative humidity and possible condensations produced when the cave is opened. There are some clay impurities mechanically deposited by man.

The effects of various microclimatic conditions were simulated to predict their impact on different potential scenarios. In experiments performed in a microclimatic chamber the formation of carbonates on the surface of the crystals was not detected. However, the dissolution of the crystals was observed, between 1 and 4 % by weight, caused by the combination of high concentration of CO2 and permanent fog. In another microclimatic simulation it was concluded that, with short exposure times, the modification of the appearance of the crystals arises. With longer exposure times, the surface defects tend to disappear, as the chemical equilibria are reached while dissolving and recrystallizing calcium sulfate. The dehydration of the gypsum phase was more likely observed in a gaseous environment. The atmospheres of air and CO2 produced greater dehydration proportions.

The general conclusions are: - The origin of most of the impurities at the surface is natural. - The greatest anthropogenic damage on the crystals is the extraction of water from the caves.

Acknowledgements: The support given by CONACYT Project No. 183706 and by SSRL, ELETTRA, ESRF synchrotron facilities is acknowledged. Authors are thankful to Cia. Peñoles and Naica Mine for providing samples.