Are the Naica Giant Crystals Deteriorating Because of Human Action? M

PROCEEDINGS PAPER Are the Naica giant crystals deteriorating because of human action? M. E. Montero-Cabrera ,1,a) I. J. A. Carreño-Márquez,1 I. Castillo-Sandoval,1 B. Pérez-Cázares,2 L. E. Fuentes-Cobas,1 H. E. Esparza-Ponce,1 E. Menéndez-Méndez,3 M. E. Fuentes-Montero,2 H. Castillo-Michel,4 D. Eichert,5 R. Loredo-Portales,6 J. Canche-Tello,1 M. Y. Luna-Porres,1 G. González-Sánchez,1 D. Burciaga-Valencia,1 C. Caraveo-Castro,1 G. Gómez-Méndez,2 L. Muñoz-Castellanos,2 and I. Reyes-Cortes2 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, Grenoble Cedex 9 38043, France 5Elettra Sincrotrone Trieste S.C.p.A., AREA Science Park, Basovizza (Trieste) 34149, Italy 6Universidad Nacional Autónoma de México, Hermosillo 83000, Mexico (Received 11 November 2019; accepted 14 April 2020) The giant gypsum crystals of Naica cave have fascinated scientists since their discovery in 2000. Human activity has changed the microclimate inside the cave, making scientists wonder about the potential environmental impact on the crystals. Over the last 9 years, we have studied approximately 70 samples. This paper reports on the detailed chemical–structural characterization of the impurities present at the surface of these crystals and the experimental simulations of their potential deterioration patterns. Selected samples were studied by petrography, optical and electronic microscopy, and laboratory X-ray diffraction. 2D grazing incidence X-ray diffraction, X-ray μ-fluorescence, and X-ray μ-absorption near-edge structure were used to identify the impurities and their associated phases. These impurities were deposited during the latest stage of the gypsum crystal formation and have after- ward evolved with the natural high humidity. The simulations of the behavior of the crystals in microclimatic chambers produced crystal dissolution by 1–4% weight fraction under high CO2 concentration and permanent fog, and gypsum phase dehydration under air and CO2 gaseous environment. Our work suggests that most surface impurities are of natural origin; the most significant anthropogenic damage on the crystals is the extraction of water from the caves. © 2020 International Centre for Diffraction Data. [doi:10.1017/S0885715620000287] Key words: gypsum crystals, Naica caves, elemental impurities, microclimatic chamber simulations, chemical–structural characterization I. INTRODUCTION of crystallization, occurs in the “selenite” variant (Van Naica is constituted by three minor mountain chains Driessche et al., 2019). located in the northern Mexican state of Chihuahua that The extraordinary conditions that occurred in Naica to form a large dome in the direction NW–SE. The GPS coordi- promote the formation of these selenite crystals were, for nates of the Naica mine are 27°51′3′′N and 105°29′47′′W. the most important ones, the presence of water containing a This mine produces lead, zinc, and silver. The Naica mine proper concentration of calcium sulfate; a constant temperature of about 55 °C; a permanent water supply, and very comprises several caverns, amongst them, the two main ’ caves, named the Cave of Swords and the Cave of Crystals, few movements of the earth s crust. These conditions lasted in which one can find the famous “giant crystals.” The Cave approximately 1 million years, time that mother Nature took to create this exceptional wonder (Van Driessche et al., of Swords contains gypsum (CaSO4⋅2H2O) single crystals of up to 1 m in length. It is found at a depth of 120 m and 2011). was discovered in 1910. The Cave of Crystals was discovered During mining activities, the groundwater is pumped out in the year 2000. It contains the most spectacular crystals in of the mine area. This action has resulted in lowering the water the world (Figure 1). The cave is at a depth of 290 m, at the table well below the level of the caves. Consequently, the giant end of one labyrinth in the mine. The crystals that have crystals are exposed to an atmosphere with humidity close to grown inside the cave are similar to the beams of buildings 100% and temperature of the order of 50 °C. Furthermore, and measure more than 10 m in length and 1 m in diameter. mining activities do not only require to carry out underground They are composed of transparent gypsum, a soft, hydrated explosions, but also produces gases such as carbon oxides calcium sulfate mineral which, under perfect conditions (COx) and nitrogen oxides (NOx) which, mixed with water, lead to acidic solutions. Likewise, methane is a common ground gas present in mines. a)Author to whom correspondence should be addressed. Electronic mail: It is well known that the crystalline gypsum structure [email protected] generates planes of water molecules (Van Driessche S15 Powder Diffraction 35 (S1), December 2020 0885-7156/2020/35(S1)/15/9/$18.00 © 2020 JCPDS-ICDD S15 Downloaded from https://www.cambridge.org/core. 28 Feb 2021 at 16:42:40, subject to the Cambridge Core terms of use. II. EXPERIMENTAL A. Samples A stock of about 70 individual samples were made avail- able for investigations to our laboratory: 30 samples are com- ing from the Cave of Swords (CS) and 40 samples from the Cave of Crystals (CC). Most samples are issued from private collections, universities, and state museums, where they were displayed in showcases. The largest one, a “blocky” type crystal from the CC, was a donation from the Naica mine manage- ment to research activities. From this stock collection, only samples presenting some surface impurities were studied. They were selected after visual inspection and optical microscopy: 10 originated from the CS and 20 from the CC. The characterization of CS samples has been described in Castillo-Sandoval et al.(2018) and is included here in the discussion for comparison. In Table I, the abbreviations for the 20 samples from the CC are given, together with the Figure 1. (Color online) View of the Cave of Crystals. In the background, fl one can distinguish the figure of two persons (encircled in red), which gives different methods used, which are brie y described in the cor- a scale of reference. responding sections. The four samples GYP001 to 004 (CC) are the first four samples, provided by the Faculty of Engineering of the Autonomous University of Chihuahua, in et al., 2017). The water associated with the gypsum can 2013. They were studied by optical microscopy at our labora- evaporate in gaseous environments, causing the dehydra- tory CIMAV and at Stanford Synchrotron Research Light tion of these planes. As a consequence, bassanite appears, source (SSRL) by 2D grazing incidence X-ray diffraction organizing the remaining water molecules into water chan- (GI–XRD) and micro X-ray fluorescence (μ–XRF). The four nels. The mining conditions may lead then to destabiliza- samples type CRYD001 to 004 (CC) were provided by the tion and dissolution of the gypsum crystals. Furthermore, Chihuahua Desert Museum, Ciudad Delicias, Chihuahua, in the CO2 produced by combustion and human breathing 2014. The four samples CRYD005 to 008 (CC) were provided may cause the deposition of calcium carbonate on the sur- by members of the Association of Mining Engineers, face of the crystals. These environmental changes have, Metallurgists, and Geologists of Mexico, also in 2014. therefore, the potential to generate both chemical and phys- These were studied only by optical methods at CIMAV. The ical processes, which may impair the integrity of the crystal eight samples CRYSN001 to 008 (CC) were collected by structures. The objective of the current project was to Castillo-Sandoval at the entrance of the cave in 2016. They clarify whether these impurities have a natural or anthropo- were studied at CIMAV, Elettra – Sincrotrone Trieste, and at genic origin. the European Synchrotron Radiation Facility (ESRF). Some At present, observations undertaken inside the Cave of representative samples of the studied specimens with surface Crystals show that these almost perfect crystals display only impurities, including the “blocky” type crystal, are presented a few impurities at the surface, which study may contribute in Figure 2. One of those samples was provided by the to understanding the processes the giant crystals were sub- Laboratory for Crystallographic Studies, University of jected to. The present work reports the detailed characteriza- Granada. This sample was observed by optical microscopy, tion of representative samples of these caves to clarify the and no surface impurities were found on or in it. origins and causes of the appearance of these impurities, which tend to darken the surface of some of the giant crystals and may reduce their beauty. Unraveling their chemical B. Methods composition and their phases, taking into consideration the In the present research, conventional laboratory tech- current environmental conditions of the caves, is the first niques such as petrography, optical and electron microscopy, aspect investigated in this paper. The second is the simulation or laboratory X-ray diffraction were combined to synchrotron of controlled experimental conditions that can lead to the radiation-based techniques. 2D GI-XRD, μ-XRF, and μ-X-ray deposition of impurities on selenite crystals and their absorption near-edge structure (μ-XANES) allowed identify- degradation. ing the elemental and structural composition of the impurities, TABLE I. Working names from 20 individual samples studied from the Cave of Crystals and used techniques. Samples names Optical microscopy LCM–DIM XRD GI-XRD SEM-EDS SR μ-XRF SR μ-XANES GYP001 to 004 YES YES YES CRYD001 to 008 YES YES CRYSN001 to 008 YES YES YES YES YES CRYD001 to 002 YES YES GYP001 YES YES YES CRYD001 YES YES YES S16 Powder Diffr., Vol.

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