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

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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

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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]. TheThe structural structural geological geological complexity complexity of theof mountainousthe mountainous system system where where the Naica the MineNaica isMine located is located has been has extensively been extensively studied [studie16]. Thed [16]. area isThe formed area byis formed three minor by three mountain minor ranges.mountain They ranges. constitute They an consti extensivetute an elongated extensive topographic elongated topographic dome, in the dome, NW–SE in direction, the NW– containingSE direction, a series containing of regional a series fractures of regional and faults fractures [20]. Theand mine faults opens [20]. atThe 1385 mine m overopens sea at level1385 inm the over northern sea level flank in the of thenorthern Naica flank mountain of the range. Naica The mountain different range. stages The of Naica’sdifferent developmentstages of Naica’s have development been controlled have by been a thermal controlled aquifer by a overthermal thousands aquifer over of years. thousands This evolutionof years. This led to evolution the formation led to ofthe fractures formation where of fractures selenite where crystals selenite have been crystals able have to grow. been Severalable to cavesgrow. containing Several caves selenite containing crystals selenite have been crystals found have due tobeen mining found activities, due to mining being theactivities, first discovered being the infirst the discovered early 20th in century. the early Among 20th century. the caves, Among four the stand caves, out: four “Cueva stand deout: las “Cueva espadas” de (Swordslas espadas” Cave), (Swords “Cueva Cave), de los “C cristales”ueva de (Crystallos cristales” Cave), (Crystal “Ojo de Cave), la Reina” “Ojo (Queen’sde la Reina” Cave), (Queen’s and “Cueva Cave), de and las velas”“Cueva (Sails de las Cave) velas” [19 ].(Sails A cross-section Cave) [19]. ofA thecross-section complex isof represented the complex in is Figure represented2. in Figure 2.

FigureFigure 2. 2.Cross-section Cross-section ofof Naica’sNaica’s complex.complex. (A)) Crystal Cave and ( (B)) Swords Swords Cave. Cave. Adapted Adapted and and published under a Creative Commons License https://creativecommons.org/licenses/by-nc/4.0/. published under a Creative Commons License https://creativecommons.org/licenses/by-nc/4.0/.

2.1.2.1. Water Water in in Naica Naica TheThe water water in in Naica Naica comes comes from from two two sources: sources: meteoric meteoric infiltrations infiltrations and and magmatic magmatic bodiesbodies [ 21[21].]. The The region, region, which which serves serves as aas drainage a drainage basin, basin, leads leads much much precipitation precipitation into theinto subsurfacethe subsurface [22]. [22]. Naica Naica aquifer aquifer consists consists of a carbonate of a carbonate sequence sequence up to up 3000 to m3000 deep m [deep23] and [23] itand has it been has suggestedbeen suggested that, in that, addition in addition to precipitation, to precipitation, the aquifer the aquifer could becould feed be in feed small in amountssmall amounts by the nearbyby the nearby rivers Conchos rivers Conchos and San and Pedro San[ 24Pedro]. [24]. Nowadays,Nowadays, underground underground water water presents presents a a temperature temperature gradient gradient that that ranges ranges between between ◦ 5050 and and 60 60 °C.C. TheThe deepdeep circulationcirculation ofof meteoricmeteoric waterwater hashas aa pHpH ofof aboutabout 7.37.3 andand approx-approxi- imatelymately 300300 ppmppm ofof totaltotal solidssolids dissolveddissolved [25[25].]. TheThe currentcurrent compositioncomposition ofof this this water water is is presentedpresented ins ins Table Table1[ 261 [26].]. Different hydrothermal compositions, which include Mg, Na, K, and Sr in addition to Ca and S [27], have been characterized, suggesting that at least three different compositions have been present in Naica water evolution [18]. The hydrological behavior was funda- mental to the development of the conditions in which the giant crystals later grew, since a stable composition of water is mandatory for a constant precipitation rate of calcium sulfate during crystallization [28].

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Table 1. Current composition of Naica water.

Solute Concentration (ppm) Calcium 1300 Magnesium 450 Sodium 100 Chlorine 51 Silicon 25 Bicarbonates 80 Carbonates 0 Hydroxides 0 Sulfates 1715

The calcium sulfate concentration in Naica’s water is close to both gypsum and anhydrite’s solubility, which is approximately 0.210 g of CaSO4 per 100 g of water in a 40–50 ◦C range [29]; being the difference within the error range of the used analytic techniques. Trace elements and isotope studies in “Cueva de los cristales” speleothems suggest that water circulation has varied between the last glaciation, 57 ka ago, and the Holocene in periods from 14.5 ± 4 to 7.9 ± 0.1 and after 7.9 ± 0.1 ka [23]. The study of these speleothems via U-Th dating allows estimating the phreatic level oscillations during the growth of the crystals [30]. Other isotopes and trace elements suggest that the more significant recharge of the aquifer took place during the last glaciation [28]. It is estimated that “Cueva de los cristales” was underwater at least until 1976, as its natural level would be around 130 to 140 m below the mine entrance. Then, water started to be extracted at a rate that reached 55 m3/min in 2012 to exploit deep ore bodies [31]. In January 2015, a rise in the phreatic level stopped exploration and exploitation of the mine.

2.2. Crystals The first description of giant gypsum crystals in Naica dates to 1912 [32]. It was in 1927 that William Foshag described for the first time the “Cueva de las espadas” in the American Mineralogist journal. This cave is located at 120 m under the main entrance of the mine, and it is described by the author as “marvelously beautiful” [33]. Today, the cave’s walls are utterly crowded with small crystalline gypsum structures of several centimeters in length with solid and liquid inclusions [34,35]. In the last decade, some efforts have contributed to deciphering the growth of Naica crystals. In 2010, the first multidisciplinary endeavor was carried out by “La Venta Explo- ration Team” [36] based on the mechanism proposed by García-Ruiz and later, by Otálora and García-Ruiz, to explain the slow growth of the giant crystals. According to this process, which the authors have called “self-feeding”, the water in the system is at a temperature slightly lower than that of equilibrium between anhydrite and gypsum, (53–58 ◦C), slowly dissolving the abundant hydrothermal and sedimentary anhydrite in the “Cueva de los cristales” [31,37]. The thermal conditions in which these structures have evolved have also been analyzed, putting into perspective the difficulty for these scenarios to be repeated somewhere else in the world [27]. “Cueva de las velas” structures, derived from the cave’s new particular hydric conditions, have been studied by Bernabei, Forti and Villasuso. They describe an atypical “sail” type growth, as seen in Figure3, in which, by capillarity, the calcium sulfate has been deposited on filaments protruding from its mother structure [38]. The variety of studies made in Naica includes the identification of biological entities present in the cave system. Bacteria have been found in different spots, such as crystals, iron oxide crusts and hot springs, being Firmicutes, Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria the ones present in the mentioned substrates. Some of these organisms have the ability to precipitate calcium carbonate [39]. Clay samples have been taken from cracks located between 700 and 760 m, and in the giant selenite crystals’ fluid inclusions. In these places, thermophilic microorganisms, such as Thaumarchaeota chemolithoautotrophs, have been isolated. The adaptive capacity of these creatures is Minerals 2021, 11, x FOR PEER REVIEW 5 of 20

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remarkable since there is low biomass availability [40]. Microbial communities are likely to be autochthonous from Naica, while some could have been introduced by human activities [39]. Moreover, ancient pollen from woody plants, herb plants, and spores has Minerals 2021, 11, x FOR PEER REVIEWbeen found within the crystals. It is suggested that the pollen was transported into the5cave of 20

by underground water streams [35,41].

Figure 3. Atypical growth in “Cueva de las velas”. Figure reproduced from Bernabei et al. (2007) [38] and published under a Creative Commons License https://creativecommons.org/licenses/by- nc/4.0/.

The variety of studies made in Naica includes the identification of biological entities present in the cave system. Bacteria have been found in different spots, such as crystals, iron oxide crusts and hot springs, being Firmicutes, Alphaproteobacteria, Betaproteobac- teria and Gammaproteobacteria the ones present in the mentioned substrates. Some of these organisms have the ability to precipitate calcium carbonate [39]. Clay samples have been taken from cracks located between 700 and 760 m, and in the giant selenite crystals’ fluid inclusions. In these places, thermophilic microorganisms, such as Thaumarchaeota chemolithoautotrophs, have been isolated. The adaptive capacity of these creatures is re- markable since there is low biomass availability [40]. Microbial communities are likely to

be autochthonous from Naica, while some could have been introduced by human activi- FigureFigureties [39]. 3. 3.Atypical AtypicalMoreover, growth growth ancient in in “Cueva “Cueva pollen de de lasfrom las velas”. velas”. wood Figure Fiygure plants, reproduced reproduced herb fromplants, from Bernabei Bernabeiand spores et al.et al. (2007) has (2007) been [38 ] [38] and published under a Creative Commons License https://creativecommons.org/licenses/by- andfound published within under the crystals. a Creative It Commons is suggested License that https://creativecommons.org/licenses/by-nc/4.0/ the pollen was transported into the cave by. nc/4.0/. underground water streams [35,41]. SinceSince 2000 2000 new new caves caves with with giant giant gypsum gypsum crystals crystals have have been been found found in in Naica’s Naica’s mining mining The variety of studies made in Naica includes the identification of biological entities complex.complex. TheseThese cavescaves areare 290290 mm underunder thethe mainmainentrance, entrance,being being“Cueva “Cueva dedelos loscristales” cristales” present in the cave system. Bacteria have been found in different spots, such as crystals, thethe most most famous famous due due to to its its 11 11 m m long long crystals, crystals, as as shown shown in in Figure Figure4. 4. iron oxide crusts and hot springs, being Firmicutes, Alphaproteobacteria, Betaproteobac- teria and Gammaproteobacteria the ones present in the mentioned substrates. Some of these organisms have the ability to precipitate calcium carbonate [39]. Clay samples have been taken from cracks located between 700 and 760 m, and in the giant selenite crystals’ fluid inclusions. In these places, thermophilic microorganisms, such as Thaumarchaeota chemolithoautotrophs, have been isolated. The adaptive capacity of these creatures is re- markable since there is low biomass availability [40]. Microbial communities are likely to be autochthonous from Naica, while some could have been introduced by human activi- ties [39]. Moreover, ancient pollen from woody plants, herb plants, and spores has been found within the crystals. It is suggested that the pollen was transported into the cave by underground water streams [35,41]. Since 2000 new caves with giant gypsum crystals have been found in Naica’s mining complex. These caves are 290 m under the main entrance, being “Cueva de los cristales” the most famous due to its 11 m long crystals, as shown in Figure 4.

Figure 4. Gypsum crystals from the Naica mine (Person as scale reference). Photo taken by Van Driessche (2010) [12] and published under a Creative Commons License https://creativecommons. org/licenses/by-nc/4.0/.

“Cueva de los cristales” and “Cueva de las espadas” are separated by approximately 170 m in the vertical axis and 1000 m horizontally, as seen in Figure2.

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Figure 4. Gypsum crystals from the Naica mine (Person as scale reference). Photo taken by Van Driessche (2010) [12] and published under a Creative Commons License https://creativecom- mons.org/licenses/by-nc/4.0/. Minerals 2021, 11, 292 6 of 20 “Cueva de los cristales” and “Cueva de las espadas” are separated by approximately 170 m in the vertical axis and 1000 m horizontally, as seen in Figure 2. 3. About Calcium Sulfate as a Crystal 3. About Calcium Sulfate as a Crystal Five variants of calcium sulfate are known depending on its hydration state: anhydrite Five variants of calcium sulfate are known depending on its hydration state:1 anhy- (CaSO4) I, II and III, hemianhydrite or, as it is geologically known, bassanite (CaSO4· 2 H2O), drite (CaSO4) I, II and III, hemianhydrite or, as it is geologically known, bassanite and gypsum (CaSO4·2H2O) [14]. The following chemical reaction governs the calcium sulfate(CaSO4∙ crystallization½H2O), and gypsum in the form (CaSO of4∙ gypsum:2H2O) [14]. The following chemical reaction governs the calcium sulfate crystallization in the form of gypsum: 2+ 2− Ca + SO4 + 2H2O → CaSO4·2H2O (1) Ca + SO + 2HO → CaSO ∙2HO (1)

Different habits can be found in nature, such as alabaster, crystallinecrystalline gypsum,gypsum, desertdesert rose, or gypsumgypsum sand.sand. These can be seenseen inin FigureFigure5 5.. The The differences differences between between hydration hydration states have been studied for decades. Howeve However,r, they are still not completely understood, and discrepancies in literature areare presentpresent [[11–14,27].11–14,27].

Figure 5. Different forms of calcium sulfate in nature. All pictures are published under a Creative Figure 5. Different forms of calcium sulfate in nature. All pictures are published under a Creative Commons License https://creativecommons.org/licenses/by-nc/4.0/. Commons License https://creativecommons.org/licenses/by-nc/4.0/.

Gypsum easily loses water when heated. The equilibrium of the endothermic reactions expressed in Equations (1) and (2) is displaced to decomposition by heating, while the addition of water to the hemihydrate and anhydrous compounds produces the inverse exothermic processes and forms polycrystalline hydrated gypsum.

CaSO4·2H2O ↔ CaSO4·0.5H2O + 1.5H2O(g) (2) Minerals 2021, 11, x FOR PEER REVIEW 7 of 20

Gypsum easily loses water when heated. The equilibrium of the endothermic reac- tions expressed in Equations (1) and (2) is displaced to decomposition by heating, while the addition of water to the hemihydrate and anhydrous compounds produces the inverse exothermic processes and forms polycrystalline hydrated gypsum.

CaSO ∙2HO ↔ CaSO ∙0.5HO + 1.5HO() (2)

CaSO ∙2HO ↔ CaSO + 2HO() (3)

3.1. Structure Minerals 2021, 11, 292 7 of 20 Although the structures of calcium sulfates portray essential differences, they are based on coordination polyhedrons that stabilize the Ca2+ ions. Each ion typically has a coordination number of eight or nine. The basic distribution of the atoms in anhydrite consists of alternating dodecahedralCaSO4·2H2 OCa↔2+ chainsCaSO4 that+ 2H share2O(g edges) with tetrahedral SO4(3)2-. Bassanite and gypsum represent small variations in anhydrite’s basic structure (Figure 6) 3.1.[42] Structure and, while calcium sulfate has been studied for several decades, no consensus has beenAlthough reached on the the structures balance it of represents calcium sulfates with the portray rest of its essential hydrated differences, variants [43]. they These are basedstructures on coordination have been studied polyhedrons with thatseveral stabilize techniques, the Ca 2+suchions. as neutron Each ion diffraction typically has and a syn- co- ordinationchrotron radiation number of [44]. eight or nine. The basic distribution of the atoms in anhydrite consists of alternatingThe gypsum dodecahedral structure Ca differs2+ chains from that anhydrite share edges in the with distance tetrahedral between SO42 the−. Bassanite chains. In andthe gypsumfirst one, represent it is greater small and variations water molecules in anhydrite’s occupy basic a place structure in the octa-coordinated (Figure6 )[42] and, Ca, whilewhich calcium results sulfate in a layered has been structure studied linked for several by hydrogen decades, nobonds. consensus In general, has been the reachedcompact- onness the of balance CaSO4 ithydrates represents structures with the depe restnds of on its the hydrated amount variants of H2O [present43]. These in the structures structure have[45]. been The studiedenergies with associated several with techniques, the hydr suchation as and neutron dehydration diffraction processes and synchrotron have been radiationwidely studied [44]. [46].

FigureFigure 6. 6.Anhydrite, Anhydrite, gypsum gypsum and and bassanite bassanite crystalline crystalline structures. structures. Pictures Pictures from from Janice Janice L. Bishop L. Bishop et al. et (2014)al. (2014) [42]. [42].

TheBassanite gypsum has structure two main differs forms, from α and anhydrite β, and both in the may distance present between different the lattice chains. param- In theeters first [47]. one, The it is first greater is obtained and water by moleculesdehydrating occupy gypsum a place in acidified in the octa-coordinated water or hydrother- Ca, whichmally results in the inpresence a layered of electrolytes. structure linked The bysecond hydrogen form bonds.is obtained In general, by dehydrating the compactness gypsum ofin CaSO water4 hydratesvapor at structurestemperatures depends above on 100 the °C. amount Some authors of H2O argue present that in the structureα form crystal- [45]. Thelizes energies in a monoclinic associated system, with the while hydration the β form and does dehydration so in a trigonal processes form have [48], been while widely others studiedargue that [46]. there is no structural difference [49]. Both forms have different chemical behav- iors,Bassanite such as their has two interaction main forms, withα water,and β ,as and well both as maydifferent present chemical different properties lattice parame- regard- tersing [ 47their]. The magnetic first is obtainedresonance by patterns. dehydrating It is ge gypsumnerally in accepted acidified that water bassanite or hydrothermally consists of a in the presence of electrolytes. The second form is obtained by dehydrating gypsum in water vapor at temperatures above 100 ◦C. Some authors argue that the α form crystallizes in a monoclinic system, while the β form does so in a trigonal form [48], while others argue that there is no structural difference [49]. Both forms have different chemical behaviors, such as their interaction with water, as well as different chemical properties regarding their magnetic resonance patterns. It is generally accepted that bassanite consists of a CaSO4 molecule associated with 0.5H2O[50], although other types of refinements show that varieties such as CaSO4·0.6H2O[51] or even CaSO4·0.81H2O[52] can be formed with relative frequency. As a rule, CaSO4·0.5H2O crystallizes in a hexagonal prism form, which is topped by rhombohedral faces, although epitaxy on a gypsum substrate is relatively standard. In such cases, bassanite growths on the (1,0,0) and (0,1,0) faces of gypsum, maintaining the direction 2+ 2− of the Ca and SO4 chains [53]. On the other hand, calcium sulfate dihydrate crystals Minerals 2021, 11, 292 8 of 20

(CaSO4·2H2O) are organized in monoclinic crystalline networks, which form exfoliable, low-hardness layers with a 2.3 g/cm3 density.

3.2. Nucleation and Growth Naica’s selenite giant crystals nucleated at low supersaturations, so the number of crystals produced in this system depends on the nucleation rate and the time the system remains in supersaturation [13,31,53,54]. According to the classical nucleation theory, the formation of nuclei is a one-step process that produces a nanoparticle with the same characteristics as the macroscopic crystal [55,56]. The nucleation rate depends on interfacial energy (σ), molecular volume (Vm), saturation state (Ω), and temperature (T) and is given by: ( ) βV2 σ3N f (θ) Js = A·e − m A (4) (RT)3ln2Ω

where A is a pre-exponential rate coefficient, β is a geometry factor, NA is Avogadro’s number, ƒ(θ) is a correction factor for heterogeneous nucleation, and R is the gas constant. Recent works [54] indicate that the classical approach to nucleation is not enough to explain the evolution of the Naica crystals, as gypsum growth involves one or more intermediate phases that interconvert over time (Figure7). This multi-stage approach is also denoted as “non-classical nucleation”, and there is no mathematical framework that satisfies all observations [57]. Different approaches have been proposed to deal with the problem of gypsum nucleation; from an induction time perspective, based on predictions from the classical nucleation theory and experimental observations [58], to a coupled model of basic crystal growth and nucleation rate laws [59]. Prior to nucleation, rod-shaped gypsum clusters are formed, with an anhydrous core and only surface water molecules. These structures are the precursor of larger gypsum crystals [60]. Due to the thermodynamic balance that the system must keep, the nucleation is highly unlikely for low supersaturation values. As these values increase, the probability of nucleation grows rapidly. The nucleation rate necessary to generate these crystals is narrowly close to the precipitation conditions. Outside this region, there would not be any precipitation at all, or massive precipitation would happen, so we can conclude that the growth of these crystals is highly unlikely. In the end, there is not a single pathway to gypsum growth, and an integration of a non-classical nucleation, at the nanoscale, and a classical nucleation approach is required to understand gypsum development [11,61–66]. This synergy is suggested by the use of synchrotron-based techniques. They allow studying calcium sulfate (and other minerals) nucleation at low supersaturation, enabling to experimentally confirm a multi- step process [67]. The kinetically most accessible pathway will prevail depending on physicochemical conditions [68], which can lead to massive evaporite deposits or crystalline structures [61]. Van Driessche [11] has studied different forms of bassanite, as nanorods or nanoparticles, produced from vacuum/solvent filtration or cryo-quenching techniques, as seen in Figure8. After nucleation, selenite crystals start to grow according to the mechanism proposed by García-Ruiz. This mechanism involves a low concentration solution with an isotopic composition compatible with a sulfate from the mine’s native anhydrite solution. His team started from three different saline concentrations and was able to establish the kinetics balance of the gypsum crystals, concluding that these required more than a million years at a temperature of 54 ◦C, to be able to grow to the size in which they are currently found in the “Cueva de los cristales” [31]. The combination of unusual geological conditions resulted in a self-sustained mechanism based on a solution with slow and smooth anhydrite-selenite phase transitions [55]. Conditions in the early stages of nucleation could imprint critical structural features at larger scales [69]. Minerals 2021, 11, x FOR PEER REVIEW 9 of 20 Minerals 2021, 11, x FOR PEER REVIEW 9 of 20

atat a atemperature temperature of of 54 54 °C, °C, to to be be able able to to grow grow to to the the size size in in which which they they are are currently currently found found inin the the “Cueva “Cueva de de los los cristales” cristales” [31]. [31]. The The comb combinationination of of unusual unusual geological geological conditions conditions re- re- sultedsulted in in a aself-sustained self-sustained mechanism mechanism based based on on a asolution solution with with slow slow and and smooth smooth anhy- anhy- Minerals 2021, 11, 292 drite-selenitedrite-selenite phase phase transitions transitions [55]. [55]. Conditions Conditions in in the the early early stages stages of of nucleation nucleation could could9 im- of im- 20 printprint critical critical structural structural features features at at larger larger scales scales [69]. [69].

(a) (b) (a) (b)

Figure 7. (a) Possible pathways for crystal growth according to a classical (upper) and non-classical (lower) theory. (b) As- Figure 7. (a) Possible pathways for crystal growth according to a classical (upper) and non-classical (lower) theory. (b) Associatedsociated energies energies for for a one-stepa one-step process process (solid (solid line) line) and and a multi-step a multi-step process process (dotted (dotted line). line). Figure Figure from from Van Van Driessche Driessche et al. et(2019) al. (2019) [69]. [69].

(a) (b) Figure 7. (a) Possible pathways for crystal growth according to a classical (upper) and non-classical (lower) theory. (b) Associated energies for a one-step process (solid line) and a multi-step process (dotted line). Figure from Van Driessche et al. (2019) [69].

FigureFigure 8. 8. HighHigh Resolution Resolution Transmission Transmission Electron Electron Microscopy Microscopy (HR-TEM) (HR-TEM) microphotographs microphotographs from from the bassanite the bassanite growth growth via vacuum/solventvia vacuum/solvent filtration filtration and cryo-quenching. and cryo-quenching. (A) Typical (A) Typical bassanite bassanite particle. particle. (B) Set (B of) Setbassanite of bassanite nanoparticles nanoparticles with rep- with resentativerepresentative 6.0 and 6.0 3.48 and Å 3.48 d-spacings. Å d-spacings. (C) small (C) smalland (D and) large (D) largebassanite bassanite nanorods. nanorods. (E) Bassanite (E) Bassanite nanorods nanorods oriented oriented prior toprior aggregation. to aggregation. (F) Bassanite (F) Bassanite nanorods nanorods and coexisting and coexisting nanoparticles. nanoparticles. Areas selected Areas for selected electron for diffraction electron diffraction(G), bassanite (G), enanorodsbassanite with nanorods reciprocal with lattice reciprocal in c-axis lattice (H in), and c-axis aggregated (H), and aggregated bassanite with bassanite reciprocal with lattice reciprocal along lattice c and alonga axes c ( andI). Figure a axes from Van Driessche et al. (2012) [11]. BNP = Bassanite nanoparticles; BNR = Bassanite nanorods; OAB = Oriented aggre- (I). Figure from Van Driessche et al. (2012) [11]. BNP = Bassanite nanoparticles; BNR = Bassanite nanorods; OAB = Oriented gatedFigure bassanite. 8. High Resolution Transmission Electron Microscopy (HR-TEM) microphotographs from the bassanite growth via aggregatedvacuum/solvent bassanite. filtration and cryo-quenching. (A) Typical bassanite particle. (B) Set of bassanite nanoparticles with rep- resentative 6.0 and 3.48 Å d-spacings. (C) small and (D) large bassanite nanorods. (E) Bassanite nanorods oriented prior As seen in Figure8F, several bassanite morphologies can coexist in specific conditions. to aggregation. (F) Bassanite nanorods and coexisting nanoparticles. Areas selected for electron diffraction (G), bassanite nanorods with reciprocal latticeSelf-assembly in c-axis (H bassanite), and aggregated nanorod bassanite aggregates with reci leadprocal to micrometer-sized lattice along c and gypsuma axes (I). crystals,Figure as seen in Figure9, and then to a larger crystal with some bassanite remanets.

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Minerals 2021, 11, 292 As seen in Figure 8F, several bassanite morphologies can coexist in specific 10condi- of 20 tions. Self-assembly bassanite nanorod aggregates lead to micrometer-sized gypsum crys- tals, as seen in Figure 9, and then to a larger crystal with some bassanite remanets.

FigureFigure 9.9. Gypsum crystallization. crystallization. (A (A) A) Alarge, large, oriented oriented aggregate aggregate of bassanite of bassanite nanorods. nanorods. (B–E ()B Gyp-–E) Gypsumsum crystals: crystals: C corresponds C corresponds to the to area the areamarked marked in B, inE corresponds B, E corresponds to the to area the marked area marked in D. F in and D. (FG,G are) are high-resolution high-resolution images images of of D D for for characteristic characteristic distances distances of of bassanite bassanite (6.0, 3.4,3.4, andand 3.13.1Å) Å) and and gypsum (3.1 and 2.8 Å). Figure from Van Driessche et al. (2012) [11]. gypsum (3.1 and 2.8 Å). Figure from Van Driessche et al. (2012) [11].

ThereThere isis no consensus consensus on on the the temperatur temperaturee equilibrium equilibrium of ofthe the gypsum–anhydrite gypsum–anhydrite sys- system,tem, which which can can go gofrom from 47 °C, 47 ◦throughC, through 53 °C 53 [70]◦C[ and70] andeven even reach reach 58 °C 58 [71].◦C[ Fluid71]. Fluidinclu- inclusionssions from from different different caves caves have have been been used used to to calculate calculate gypsum growthgrowth temperaturestemperatures demonstratingdemonstrating thatthat growthgrowth occurredoccurred atat differentdifferent temperaturestemperatures butbut withinwithin thethe reportedreported rangerange reported reported in in the the literature literature [ 72[72].]. For For the the mechanism mechanism to to be be viable, viable, the the system system must must meet meet twotwo requirements; requirements; there there must must be be enough enough anhydrite anhydrite in thein the area, area, and and the the solution solution temperature tempera- mustture must be close, be close, but lower, but lower, than the than transition the transition temperature temperature of the gypsum–anhydrite of the gypsum–anhydrite system (Figuresystem 10(Figure). Both 10). conditions Both conditions were met were in Naicamet in accordingNaica according to the informationto the information obtained ob- fromtained the from liquid the inclusions liquid inclusions found infound the crystals in the crystals [36]. Dating [36]. Dating using uranium,using uranium, and other and radioactiveother radioactive isotopes isotopes suggests, sugg althoughests, although the development the development of the crystals of the was crystals uninterrupted, was unin- theterrupted, crystal growththe crystal rate growth might haverate migh changedt have through changed time through [30,73]. time [30,73].

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The crystals’ isotopic composition shows that the waters in which they grew came Minerals 2021, 11, 292 from different sources: water enriched with sulfate ions from the anhydrite solution,11 of wa- 20 ter from mineral deposits, and water from higher levels. This complements the theory of a complex self-regulated mechanism [43].

FigureFigure 10. 10.Anhydrite–gypsum Anhydrite–gypsum relationshiprelationship inin aa supersaturatedsupersaturated solution (from Forti Forti 2017, 2017, fig. fig. 6). 6).

4. AboutThe crystals’ Research isotopic in Naica composition shows that the waters in which they grew came from4.1 Multidisciplinary different sources: Research water enriched with sulfate ions from the anhydrite solution, water from mineral deposits, and water from higher levels. This complements the theory of a Naica caves have been a passionate topic since their discovery last century. Several complex self-regulated mechanism [43]. groups around the world have contributed to the study of their crystals through constant 4.quality About investigation. Research in Naica Research has been relatively robust with in situ and ex situ, experi- 4.1.mental Multidisciplinary approaches Research[18,27,31,34,36,38]. In Figure 11, the unique proportions of the giant crystals can be observed. Naica caves have been a passionate topic since their discovery last century. Several groups around the world have contributed to the study of their crystals through constant quality investigation. Research has been relatively robust with in situ and ex situ, exper- imental approaches [18,27,31,34,36,38]. In Figure 11, the unique proportions of the giant crystals can be observed. Selenite is translucid and stable in standard conditions. However, nowadays, differ- ent phenomena can challenge its stability. From carbon dioxide (CO2) effects [74], crys- tal structure [75] and surface impurities characterization [76], the role of microclimatic conditions [77,78] and microbial diversity [35,39,40], to speleothems formation [34,38]. Conservation and protection against degradation is a relevant issue discussed by several authors. Some ideas have been proposed, from strict regulation to potential visitors to an ex-situ exposition [79]. Nevertheless, for the crystals, the eventual flood of the caves is the best scenario [80]. Naica’s research cycle has been intense over the last two decades, comprising the origin of the giant crystals as well as its mineralogy, geochemistry, and microclimatology [81]. Minerals 2021, 11, 292 12 of 20 Minerals 2021, 11, x FOR PEER REVIEW 12 of 20

FigureFigure 11.11. GiantGiant gypsum gypsum crystals crystals size size compared compared to tohu humanman size size (encircled (encircled in red). in red). Taken Taken from from Montero-CabreraMontero-Cabrera etet al.al. (2020).(2020).

MiningSelenite is was translucid a constant and stable activity in standard during severalconditions. decades However, in Naica’snowadays, complex. differ- Constantent phenomena exploration can challenge and excavation its stability. likely From produced carbon unusualdioxide (CO amounts2) effects of [74], carbon crystal ox- idesstructure (COx), [75] nitrogen and surface oxides impurities (NOx), sulphur characterization oxides (SOx), [76], amongthe role otherof microclimatic gases. Carreño- con- Mditionsárquez [77,78] et al. [ 74and] studied, microbial with diversity detail, carbon [35,39,40], dioxide to speleothems (CO2) effect. formation They reproduced [34,38]. some Con- plausibleservation environmental and protection conditions against degradation in a customized is a relevant climate fogissue chamber discussed with by intermittent several au- conditions,thors. Some simulating ideas have thebeen effect proposed, of people from entering strict regulation and leaving to potential “Cueva devisitors los cristales”, to an ex- assitu it exposition was the norm [79]. for Nevertheless, several years. for Nothe evidencecrystals, the of calcium eventual carbonate flood of the formation caves is due the tobest the scenario CO2 effect [80]. wasNaica’s found. research Nevertheless, cycle has been other intense natural over causes the last may two produce decades, carbon- com- ateprising precipitation the origin before of the andgiant after crystals water as extractionwell as its [mineralogy,31,34]. Several geochemistry, phases like and aragonite, micro- hematite,climatology galena, [81]. cuprite, and sphalerite were identified, but the origin of those was natural (non-anthropogenic)Mining was a constant [27,31,34 activi,36,76ty]. during several decades in Naica’s complex. Constant explorationAfter the and study excavation of more likely than produced 40 samples unusual with X-rayamounts based of carbon techniques oxides by (COx), Castillo- ni- Sandovaltrogen oxides et al. (NOx), [82], the sulphur elemental oxides and (SOx), structural among composition other gases. ofCarreño-Márquez the impurities wereet al. identified,[74] studied, as seenwith in detail, Figure carbon 12; concluding dioxide that(CO2 when) effect. the They gypsum reproduced supersaturated some plausible solution equilibriumenvironmental came conditions to an end, in impurities a customized started climate to accumulate fog chamber onthe with crystal’s intermittent surface. condi- The mosttions, frequent simulating elements the effect identified of people on theentering impurities and leaving are Ti, Mn, “Cueva Fe, Cu, de Zn,los As,cristales”, and Pb, as as it wellwas asthe hematite, norm for goethite, several andyears. Pb-Mn No evidence oxides [ 35of, 75calcium]. carbonate formation due to the CO2 effect was found. Nevertheless, other natural causes may produce carbonate precip- itation before and after water extraction [31,34]. Several phases like aragonite, hematite, galena, cuprite, and sphalerite were identified, but the origin of those was natural (non- anthropogenic) [27,31,34,36,76]. After the study of more than 40 samples with X-ray based techniques by Castillo- Sandoval et al. [82], the elemental and structural composition of the impurities were iden- tified, as seen in Figure 12; concluding that when the gypsum supersaturated solution equilibrium came to an end, impurities started to accumulate on the crystal’s surface. The most frequent elements identified on the impurities are Ti, Mn, Fe, Cu, Zn, As, and Pb, as well as hematite, goethite, and Pb-Mn oxides [35,75].

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µ FigureFigure 12. 12.(Left Left:) Micro Micro X-ray X-ray Fluorescence Fluorescence ( -XRF) (µ-XRF) elemental elemental mapping mapping of impurities of impurities from a selected from zone on a gypsum sample, and distribution of Pb according to a rainbow scale of intensity. (Right) a selected zone on a gypsum sample, and distribution of Pb according to a rainbow scale µ-XANES spectra of the Pb L3 edge from 14 different ROIs in four different samples. Each spectrum of intensity. Right: µ-XANES spectra of the Pb L3 edge from 14 different ROIs in four shows the identification of a lead compound. Data were acquired at the SSRL BL2–3. Scale bar: different samples. Each spectrum shows the identification of a lead compound. Data were 20 µm. Figure modified from Castillo-Sandoval et al. (2015) [75]. acquired at the SSRL BL2–3. Scale bar: 20 µm. Figure modified from Castillo-Sandoval et al. (2015)Due to [75]. the size and interconnections of the different galleries, temperature oscillations are normal within Naica. Air exchange and heat irradiation are causes of a particular Due to the size and interconnections of the different galleries, temperature oscilla- micrometeorology [83]. Constant microclimatic fluctuations may trigger a chain of events tions are normal within Naica. Air exchange and heat irradiation are causes of a particular that include surface alterations via a dehydration process and surface damage, as seen in micrometeorology [83]. Constant microclimatic fluctuations may trigger a chain of events Figure 13. Conditions like temperature (25 or 60 ◦C), exposure time (1, 3, 6, or 12 months) that include surface alterations via a dehydration process and surface damage, as seen in and environment (liquid or gas) were tested in a steady-state microclimatic simulation [76]. Figure 13. Conditions like temperature (25 or 60 °C), exposure time (1, 3, 6, or 12 months) Bassanite was identified by mid-infrared spectroscopy as a first approach. Later, these and environment (liquid or gas) were tested in a steady-state microclimatic simulation samples containing bassanite were analyzed with Grazing Incidence X-ray Diffraction [76]. Bassanite was identified by mid-infrared spectroscopy as a first approach. Later, (GI-XRD) from laboratory and synchrotron sources, and then compared to corroborate these samples containing bassanite were analyzed with Grazing Incidence X-ray Diffrac- the appearance of a new phase. In Figure 14, the signals from different spots of the same tion (GI-XRD) from laboratory and synchrotron sources, and then compared to corrobo- sample show the presence of gypsum and bassanite in different crystallographic planes. A rate the appearance of a new phase. In Figure 14, the signals from different spots of the semi-quantitative statistical analysis showed that (1) in short simulation times, physical damagesame sample is more show relevant the presence than any of chemical gypsum transformation; and bassanite (2)in indifferent long simulation crystallographic times, surfaceplanes. alterationsA semi-quantitative tend to disappear statistical due analysis to a dissolution/crystallization showed that (1) in short simulation process; and times, (3) bassanitephysical presencedamage is amore result relevant of gypsum than dehydration any chemical in transformation; a gaseous environment—precisely (2) in long simula- thetion condition times, surface taking alterations place right tend now to in disa the differentppear due caves to a ofdissolution/crystallization Naica. It has been observed pro- thatcess; condensation/dissolution and 3) bassanite presence produces is a result secondary of gypsum minerals dehydration related toin the a gaseous fluid inclusions environ- trappedment—precisely inside the the gypsum condition crystals taking [84 ].place right now in the different caves of Naica. It has been observed that condensation/dissolution produces secondary minerals related to the fluid inclusions trapped inside the gypsum crystals [84].

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Figure 13. Structural damage detected in microclimatic simulations by Carreño-Márquez et al. (2018) [77]. Further permis- sionsFigure related 13. 13. StructuralStructural to this figure damage damage should detected detected be direct in microc ined microclimatic to limaticthe American simulations simulations Chemical by Carreño-Márquez bySociety Carreño-M (ACS).á (rqueza et) Irregular al. (2018) et al. surface; [77]. (2018) Further [(77b)]. Slope; permis- Further (c) Well;sionspermissions (relatedd) Detachment; related to this tofigure ( thise) Minor figureshould slope should be direct be directeded to the to American the American Chemical Chemical Society Society (ACS). (ACS). (a) Irregular (a) Irregular surface; surface; (b) Slope; (b) Slope; (c) Well;(c) Well; (d) (Detachment;d) Detachment; (e) (Minore) Minor slope slope

Figure 14. Conventional and synchrotron Grazing Incidence X-ray Diffraction (GI-XRD) patterns Figure 14. Conventional and synchrotron Grazing Incidence X-ray Diffraction (GI-XRD) patterns with identified gypsum and bassanite peaks. Figure from Carreño-Márquez et al. (2018) [77]. Further withFigure identified 14. Conventional gypsum and and bassani synchrotronte peaks. Grazing Figure Incide from nceCarreño-M X-ray Diffractionárquez et al.(GI-XRD) (2018) [77]. patterns Fur- permissions related to this figure should be directed to the ACS. Gyp = Gypsum; Bas = Bassanite therwith permissions identified gypsum related andto this bassani figurete should peaks. beFigure directed from to Carreño-M the ACS. Gypárquez = Gypsum; et al. (2018) Bas [77]. = Bas- Fur- sanitether permissions Montero-Cabrera related to and this her figure team should have be studied directed more to the than ACS. 70 Gyp individual = Gypsum; Naica Bas = samples Bas- sanite provided by private collectors, universities, and museums performing chemical and struc- Montero-Cabrera and her team have studied more than 70 individual Naica samples tural characterizations. The first explorations of the samples were made by visual and opti- Montero-Cabrera and her team have studied more than 70 individual Naica samples providedcal methods. by private After determining collectors, universities, the most affected and museums regions in performing the samples, chemical different and character- struc- provided by private collectors, universities, and museums performing chemical and struc- turalization characterizations. techniques were The used, first including explorations petrography, of the samples electron microscopy,were made by and visual laboratory and tural characterizations. The first explorations of the samples were made by visual and opticalX-ray diffraction.methods. After They weredetermining combined the with most synchrotron affected regions radiation-based in the samples, techniques, different such optical methods. After determining the most affected regions in the samples, different characterizationas 2D Grazing Incidence techniques X-ray were Diffraction used, includ (GI-XRD),ing petrography, performed electron at Stanford microscopy, Synchrotron and characterizationResearch Lightsource techniques (SSRL) were (SLAC used, National including Accelerator petrography, Laboratory, electron Menlomicroscopy, Park, and CA,

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Minerals 2021, 11, 292 laboratory X-ray diffraction. They were combined with synchrotron radiation-based15 oftech- 20 niques, such as 2D Grazing Incidence X-ray Diffraction (GI-XRD), performed at Stanford Synchrotron Research Lightsource (SSRL) (SLAC National Accelerator Laboratory, Menlo Park, CA, USA); and µ-XRF, and µ-X-ray absorption near-edge structure (µ-XANES) USA); and µ-XRF, and µ-X-ray absorption near-edge structure (µ-XANES) measurements measurements at Elettra–Sincrotone Trieste (Trieste, Italy) and at the European Synchro- at Elettra–Sincrotone Trieste (Trieste, Italy) and at the European Synchrotron Radiation Fa- tron Radiation Facility (ESRF) (Grenoble, France). The microclimatic simulation was per- cility (ESRF) (Grenoble, France). The microclimatic simulation was performed in Instituto formed in Instituto de Ciencias de la Construcción Eduardo Torroja in Madrid, Spain, and de Ciencias de la Construcción Eduardo Torroja in Madrid, Spain, and proved to be an proved to be an efficient approach to understand the dynamics of the giant gypsum crys- efficient approach to understand the dynamics of the giant gypsum crystals deterioration. tals deterioration. In the considered study, it has been concluded that the main impurities producing In the considered study, it has been concluded that the main impurities producing opacity in the crystals are hematite (Fe2O3), sphalerite (ZnS), γ-MnO2, galena (PbS), as well opacity in the crystals are hematite (Fe2O3), sphalerite (ZnS), γ-MnO2, galena (PbS), as well as Pb oxides. Nevertheless, microclimatic conditions, especially in the gaseous environment, as Pb oxides. Nevertheless, microclimatic conditions, especially in the gaseous environ- can accelerate the crystal deterioration via dehydration of the crystal’s surface. ment, can accelerate the crystal deterioration via dehydration of the crystal’s surface. The human role in Naica has consisted of pumping out water and generating a gaseous environmentThe human to allow role in the Naica mining. has Consideringconsisted of pumping that water out extraction water and has generating been constant a gas- foreous several environment decades, to it allow is assumed the mining. that many Considering years have that passed water sinceextraction the giant has been crystals con- cavestant was for filledseveral with decades, water. it On is theassumed other hand,that many the current years have conditions passed have since changed the giant since crys- thetals mining cave was activity filled has with ceased. water. As On this the review other hand, is being the written, current accessconditions has stopped,have changed and therefore,since the themining imminent activity risk has to ceased. the crystal’s As this integrity. review New is being preventive written, measures access has should stopped, be takenand totherefore, avoid deterioration the imminent if the risk activity to the in cr Naicaystal’s is integrity. resumed. New preventive measures should be taken to avoid deterioration if the activity in Naica is resumed. 4.2.4.2 ParallelismParallelism withwith PulpPulpíí Geode Geode AnotherAnother paradigmatic paradigmatic case case of of selenite selenite is is located located in in the the town town of of Pulp Pulpíí (Almeria, (Almeria, Spain). Spain). SinceSince 1999, 1999, aa seriesseries of images images of of gypsum gypsum crys crystalstals found found in ina geode a geode began began to circulate to circulate pub- publiclylicly and, and, at atthe the moment, moment, a aformal formal request request has has been been filled filled byby locallocal authoritiesauthorities to to declare declare thethe geode geode a a protected protected area, area, according according to localto local news news media. media. With With an approximatean approximate volume volume of 3 11of m 11, m it3 contains, it contains several several selenite selenite crystals crystals of just of just over over a meter a meter (Figure (Figure 15)[ 15)85]. [85].

Figure 15. Interior of Pulpí geode. The average crystal size is 0.5 m × 0.4 m × 0.3 m. Photo pub- Figure 15. Interior of Pulpí geode. The average crystal size is 0.5 m × 0.4 m × 0.3 m. Photo published lished under a Creative Commons License https://creativecommons.org/licenses/by-nc/4.0/. under a Creative Commons License https://creativecommons.org/licenses/by-nc/4.0/.

TheThe geode geode is is located located 50 50 m m under under the the surface, surface, at at a a 3 3 km km distance distance from from the the sea. sea. Some Some studiesstudies on on the the topic topic of of the the geode’s geode’s genesis genesis propose propose that that its its growth growth occurred occurred in in several several stages:stages: (I)(I) thethe cavitycavity formationformation byby the removal of soluble soluble materials; materials; (II) (II) the the deposition deposition of offine fine celestine celestine needles needles on onthe thegeode geode walls; walls; and (III) and the (III) subsequent the subsequent infiltration infiltration of a mixture of a mixtureof meteoric of meteoric and seawater and seawater that mixed that with mixed hydrothermal with hydrothermal fluids, fluids,providing providing gradual gradual cooling cooling cycles [86,87]. Clearly, there is a parallelism with Naica. Although physicochemical conditions in Naica and Pulpí were different, conservation measures and experiences can be useful to protect Naica giant crystals.

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5. Final Remarks An overview of the physicochemical investigations on the giant Naica crystals has been presented, with a particular focus on the processes that have given rise to their current appearance. The growth process that has led to the sizes and shapes of the investigated crystals has been governed by classical nucleation and growth events, combined with multi-stage variations with respect to the classical model, of the type described by Van Driessche et al. [11]. Regarding the darkening events observed in some crystals, the work by Castillo et al. [76] shows the non-anthropogenic origin of the different compounds that give rise to opacity. The influence of the presence of man in the Naica caves has been detrimental due to the subtraction of samples and for having altered the original humidity of the environment. Carreño et al. [77] have clarified that this environmental alteration feasibly generates gypsum-basanite transformations in some crystals. Cave development is a complex phenomenon that involves several physicochemical processes over geological time, which may lead to different mineral formations. Results of the type considered in the current review provide valuable information about our planet’s dynamics that can help implement conservation and preservation policies and techniques [88]. As these formations are considered a natural heritage, there are remarkable efforts to evaluate risks and prevent damage of mineral specimens [89]. Preservation endeavors must be implemented in Naica as well. Understanding the composition and effect of the climatic environments [90], and the analysis of different scenarios of people affluence [91] should advance this purpose. Considering the current state of the giant crystals, the most viable option to approach them to the public is via an ex situ exhibition of a specimen, or the development of a virtual tour that compensates the desire to enter the caves. If the decision-makers promote visitors’ accessibility, it is recommended to limit their number and contemplate additional safety measures (to both humans and crystals). To continually measure changes in physical and chemical conditions is also advised.

Author Contributions: Conceptualization, I.J.A.C.-M. and M.E.M.-C.; Writing, reviewing and editing, I.J.A.C.-M., I.C.-S., B.E.P.-C., L.E.F.-C., H.E.E.-P., E.M.-M., M.E.F.-M. and M.E.M.-C. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The support provided by CONACYT Project No. 183706, the proposals SSRL 3939, the ESRF HG-77, Elettra Sincrotrone-Trieste 20155328, as well as by the International Centre for Theoretical Physics (ICTP) are acknowledged. Special thanks to Manuel Reyes-Cortés, who provided some of the key research samples and supported their selection and analysis. The authors thank the cooperation of the management of the Peñoles company, the Desert Museum of Ciudad Delicias, Chihuahua, the Harvard Museum of Natural History, and the Faculty of Engineering of the Autonomous University of Chihuahua for providing essential specimens for the study. The authors thank the anonymous referees for their helpful suggestions. Conflicts of Interest: The authors declare no conflict of interest.

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