minerals Review Evolution of the Astonishing Naica Giant Crystals in Chihuahua, Mexico Iván Jalil Antón Carreño-Márquez 1 , Isaí Castillo-Sandoval 2, Bernardo Enrique Pérez-Cázares 3, Luis Edmundo Fuentes-Cobas 2 , Hilda Esperanza Esparza-Ponce 2 , Esperanza Menéndez-Méndez 4, María Elena Fuentes-Montero 3 and María Elena Montero-Cabrera 2,* 1 Department of Engineering, Universidad La Salle Chihuahua, Chihuahua 31625, Mexico; [email protected] 2 Department of Environment and Energy, Centro de Investigación en Materiales Avanzados, Chihuahua 31136, Mexico; [email protected] (I.C.-S.); [email protected] (L.E.F.-C.); [email protected] (H.E.E.-P.) 3 Department of Computational Chemistry, Universidad Autónoma de Chihuahua, Chihuahua 31125, Mexico; [email protected] (B.E.P.-C.); [email protected] (M.E.F.-M.) 4 Department Physicochemical Assays, Instituto Eduardo Torroja de Ciencias de la Construcción, 28033 Madrid, Spain; [email protected] * Correspondence: [email protected] Abstract: Calcium sulfate (CaSO4) is one of the most common evaporites found in the earth’s crust. It can be found as four main variations: gypsum (CaSO4·2H2O), bassanite (CaSO4·0.5H2O), soluble Citation: Carreño-Márquez, I.J.A.; anhydrite, and insoluble anhydrite (CaSO4), being the key difference the hydration state of the Castillo-Sandoval, I.; Pérez-Cázares, sulfate mineral. Naica giant crystals’ growth starts from a supersaturated solution in a delicate B.E.; Fuentes-Cobas, L.E.; Esparza- thermodynamic balance close to equilibrium, where gypsum can form nanocrystals able to grow Ponce, H.E.; Menéndez-Méndez, E.; up to 11–12 m long. The growth rates are reported to be as slow as (1.4 ± 0.2) × 10−5 nm/s, taking Fuentes-Montero, M.E.; Montero- Cabrera, M.E. Evolution of the thousands of years to form crystals with a unique smoothness and diaphaneity, which may or may not Astonishing Naica Giant Crystals in include solid or liquid inclusions. Conservation efforts can be traced back to other gypsum structures Chihuahua, Mexico. Minerals 2021, 11, found prior to Naica’s. Furthermore, in the last two decades, several authors have explored the 292. https://doi.org/10.3390/ unique requirements in which these crystals grow, the characterization of their environment and min11030292 microclimatic conditions, and the prediction of deterioration scenarios. We present a state-of-the-art review on the mentioned topics. Beyond the findings on the origin, in this work we present the Academic Editors: Thomas N. current state and the foreseeable future of these astounding crystals. Kerestedjian, Tomasz M. Stawski and Alexander E. S. Van Driessche Keywords: Naica; gypsum; giant crystals; nucleation Received: 27 December 2020 Accepted: 6 March 2021 Published: 11 March 2021 1. Introduction Publisher’s Note: MDPI stays neutral Giant crystals are rare spectacles and can only be observed in unique places around with regard to jurisdictional claims in the globe. Crystals of different proportions have been reported. A beryl, Be3Al2(Si6O18), published maps and institutional affil- from Madagascar is one of the most outstanding examples with 18 m long, 3.5 m diameter, iations. and an estimated mass of 380,000 kg [1]. The Naica giant crystals, in Northern Mexico, are probably the largest and most impressive gypsum (CaSO4·2H2O) crystals on Earth. Among the variety of minerals found in nature, sulfates are ubiquitous. Calcium sulfate (CaSO4) and its various hydrated forms are widely distributed over the earth’s crust, absent only in volcanic regions. It has been in use by humans since the third Copyright: © 2021 by the authors. · Licensee MDPI, Basel, Switzerland. millennium before our era [2]. Gypsum (CaSO4 2H2O) is the main constituent of the White · This article is an open access article Sands dunes in New Mexico [3], while bassanite (CaSO4 0.5H2O) was found in Mount distributed under the terms and Vesuvius, Italy, for the first time [4]. Both phases have been detected in some regions of conditions of the Creative Commons Mars by independent teams [5,6]. Gypsum crystals of exceptional size can be found in the Attribution (CC BY) license (https:// Debar mine (Macedonia), which houses gypsum crystals up to seven meters long in caves creativecommons.org/licenses/by/ near the surface. They remain at room temperature and are surrounded by hydrothermal 4.0/). puddles [7]. These conditions resemble those found in Naica. There is also an old mine Minerals 2021, 11, 292. https://doi.org/10.3390/min11030292 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 20 Minerals 2021, 11, 292 2 of 20 the surface. They remain at room temperature and are surrounded by hydrothermal pud- dles [7]. These conditions resemble those found in Naica. There is also an old mine in Segobriga,in Segobriga, Spain, Spain, where where Romans Romans obtained obtained big gypsum big gypsum crystals. crystals. It was It employed was employed as win- as dowwindow material material before before the the technology technology to toproduce produce flat flat glass glass was was available available [8]. [8]. The The Chilean mine of El Teniente in RancaguaRancagua hosts euhedraleuhedral gypsum crystals of >4>4 m [[9].9]. Compared with Naica, these crystals are smaller but chemically similar. The efforts toto describedescribe thethe structural structural variants, variants, together together with with growth, growth, and and precipitation precipita- tionof calcium of calcium sulfate sulfate date backdate toback the to beginning the beginning of the 20thof the century 20th century [10]. In [10]. particular, In particular, growth growthkinetics kinetics and mechanisms and mechanisms for the precipitation for the precipitation of calcium of sulfate calcium as selenite,sulfate aas transparent selenite, a transparentgypsum habit, gypsum have habit, been establishedhave been establis in [11–hed13]. in It [11–13]. has also It beenhas also ascertained been ascertained that the thathydration the hydration of anhydrite of anhydrite (CaSO4) (CaSO is a crucial4) is a factorcrucial in factor the crystallization in the crystallization of selenite of selenite [14]. Its [14].dehydration Its dehydration in the presence in the presence of carbon of dioxidecarbon dioxide (CO2) has (CO been2) has studied been studied to understand to under- its standphase its transitions, phase transitions, which are which attributable are attributable to the better to the solubility better solubility or water or in water carbon indioxide carbon dioxide(CO2) than (CO in2) airthan [15 in]. air [15]. The present present contribution offers a comprehe comprehensivensive review of meaningful work towards understanding the evolution of the astonishing Naica crystals in Chihuahua, Mexico. 2. About About Naica Naica Naica is a town situated in the south-center region of Chihuahua State, in Saucillo municipality (Figure1). It is located at 27 ◦5102000 N, 105◦2700000 E coordinates, at 1330 m of municipality (Figure 1). It is located at 27°51’20’’ N,◦ 105°27’00’’ E coordinates, at 1330 m ofaltitude altitude over over sea sea level. level. Its Its mean mean temperature temperature is 18is 18C, °C, with with some some sporadic sporadic snow snow in in winter. win- Annual precipitation is about 200–400 mm, and it is considered to be scarce. ter. Annual precipitation is about 200–400 mm, and it is considered to be scarce. Figure 1. Naica, Chihuahua, Mexico. Mexico. On On the the left left (in (in green), green), the the si sizeze of of the the mine mine can can be be compared compared with with the the town town itself. itself. Adapted with an Open Data Commons Open Database License (ODbL). © OpenStreetMap contributors. Adapted with an Open Data Commons Open Database License (ODbL). © OpenStreetMap contributors. Mining is Naica’s primary economic activity since the arrival of the first first settlers in 1794. Some minerals present in the zone are quartz, calcite, and silica [16], [16], while relevant metals are lead, zinc, and silver. The geology of Naica consists of an Earl Earlyy Cretaceous carbonate sequence, composed mainly of limestone asas wellwell asas marlmarl and and dolomite dolomite stones stones in in minor minor quantities. quantities. The The recrystal- recrys- tallizationlization caused caused by by late late felsic felsic intrusions intrusions brought brought the the thermal thermal reactions reactions needed needed to drive to drive the thepresence presence of the of the metals metals (Ag-Pb-Zn), (Ag-Pb-Zn), and and therefore therefore the the ore ore deposits deposits in this in this mining mining district. dis- trict.The oreThe source ore source was produced was produced by a sub by horizontal a sub horizontal (~20◦) batholith-size (~20°) batholith-size intrusive intrusive magma magmabody located body 1000located m under1000 m the under surface the [surface17]. Nevertheless, [17]. Nevertheless, it has been it has suggested been suggested that the thatconcentration the concentration of several of several elements elements dissolved dissolved in Naica’s in Naica’s water water has variated has variated over over the cen- the turies [18] along with the temperature of the fluids. Three different stages of development can be distinguished, one with temperatures from 500 to 680 ◦C and high salinity, a second Minerals 2021, 11, x FOR PEER REVIEW 3 of 20 Minerals 2021, 11, 292 3 of 20 centuries [18] along with the temperature of the fluids. Three different stages of develop- ment can be distinguished, one with temperatures from 500 to 680 °C and high salinity, a stagesecond with stage temperatures with temperatures from 240 from to 490 240◦C, to and 490 a °C, third and one a third when one the when fluids the lowered fluids their low- temperatureered their temperature from 119 to from 379 ◦ 119C. These to 379 temperature °C. These temperature variations ledvariations to the development led to the devel- of calcite,opment quartz, of calcite, and quartz, anhydrite and deposits anhydrite [19 ].deposits [19].