The Clays of the Yucatan Peninsula as A Key-Material in the Archaeometric Studies of the Mayan Blue Pigment

This research presents a multi-technique methodology for the characterization of clayey sediments found in sinkholes or cenotes from three locations in the north of the Yucatan Peninsula where the most recognized outcrops of palygorskite-type clays appeared in pre-Hispanic times and also today. According to historical and ethnographic evidence, these are, Sak lu’um (‘white earth’) cave in the community of Sacalum; a cave in the town of Muna, and Actun Hi cave, in the city of Ticul. The study is based on a macromorphological description of hand samples and a mineralogical characterization of the sediment.

The aim is to determine the mineralogy of the clays and their geological context and discuss the possible use of each of these three caves as a source of raw materials for producing Maya blue pigment. The multi-technique approach proposed includes optical microscopy (OM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The clay sediment samples extracted from Sak lu’um cave are mainly composed of palygorskite accompanied by dolomite and associated with sediments of endorheic lacustrine origin, which are located in the inner strata of the cave, mixed with montmorillonite-chlorite, coming from outer strata or paleosoils. This combination is due to the complex stratigraphy of the cave and the difficulty of sampling. A minor amount of dolomite and palygorskite is present in the Muna and Ticul caves, respectively. These clays are formed by detritus sedimentation, mainly restricted to polje mining areas scattered around Sierra de Ticul. The differences in the mineralogical composition of the clay sediments analysed, in light of the recent studies on the essential role displayed by the palygorskite clay in the preparation of the Maya blue pigment, indicate that the clay sediments from Sak lu’um cave were the most probable source of clay due to the lesser amount of palygorskite in the other two sites that should prevent the suitable attachment of the indigoid molecules to the clayey nanoparticles during the Maya blue preparation.

Keywords: Maya blue; Clay; Sak lu’um cave; Mineralogy; Muna cave; Ticul cave; palygorskite

Most of studies that have investigated the clays of the Yucatan Peninsula have focused on palygorskite deposits (called sak lu’um “white earth”, Yucatec Mayan), and have remarked on the importance of this clay as an ingredient in pre-Columbian culture for the manufacture of Maya blue [1-3].

Maya blue is a blue pigment that the Ancient Maya widely used for mural painting, sculpture, ceramics, textiles, among other artistic supports [4-14]. This pigment was predominantly used during the Classic and Post-classic periods, from the north of the Yucatan Peninsula to the highlands of Guatemala and Central Mexico [15], which shows that other pre-Hispanic cultures of Ancient Meso- America, including Teotihuacan used it. However, research carried out in the last decade has revealed that the Maya blue colour technology dates back to the Late Preclassic in the Maya area, as demonstrated by its physicochemical identification on the frieze of Substructure I at Calakmul [16]. Likewise, and even more recently, the ceramic cultures of Western Mexico have offered revealing results regarding the origin of this emblematic colour, which could have occurred in this region and cultures in Middle Preclassic times, according to the results obtained in 2019 in funerary contexts of Chupiacuro [17].

Shepard was the first to cite palygorskite as a component of Maya blue in the early 1960s [2] when the mineral was known as attapulgite [18]. Maya blue is an unusual pigment in that it has a molecular structure that combines an organic dye (indigo, obtained from the Indigofera suffruticosa plant, known locally as anyil or xiuquilitl and a clayey matrix of palygorskite [14,15]. Palygorskite possesses physical properties that distinguish it from other clays and this fact was known to the Maya long before the arrival of the first Europeans [3,13,18-33,34]. Recent studies on the chemistry of Maya blue have demonstrated that this pigment is a polyfunctional hybrid material whose properties depend on different indigoid components which form different topological redox isomers attached to the silicate framework of the clay [35-37]. In addition, these studies have demonstrated the essential role of the clay in obtaining a stable pigment and the development of the characteristic blue-greenish colour of the pigment due to its nano-channeled structure that favours the attachment of the indigoid molecules, and the occurrence of isomerization and redox tuning processes [36].

In one of the latest studies, after analysing 33 palygorskite samples chosen from ethnographic sources and extracted from the Yucatan Peninsula (Mexico) and El Petén (Guatemala), [15] determined that seven different clay deposits could have been exploited for the manufacture of the Mayan blue pigment: Yo’ Sah kab, Uxmal, Chapab, Sacalum, Maxcanú, Mama and in the south of El Petén. These results support the suggestion that palygorskite deposits may have been used to manufacture Maya blue in places other than those known in the north of the Yucatan Peninsula, thus supporting Littman’s hypothesis (1980) [34].

Following up on those results, in this research, eleven samples of clayey sediment are characterised from three sites where the most widely recognized outcrops of palygorskite-type clays appear, both in pre-Hispanic times and today, according to historical and ethnographic evidence.

The typology and origin of the clays used by the Mayans

Specifically, in the Yucatan Peninsula, three major groups of clays can be distinguished depending on their formation processes: pedogenetic, detrital, and primary clays [32,38].

I. Pedogenesis: Clay minerals formed by these processes can be found in all their stages of maturity. Younger clay minerals are formed as poorly crystallized such as kaolinite; there are also other associated minerals such as boehmite, talc and traces of chlorite. Clay minerals in an advanced phase of maturity are found mostly as well-crystallized kaolinite. In these complex processes of soil formation and its clay minerals, a fraction or part comes from the contributions by dissolution of different types of rocks, but other clay minerals are also added by alteration of siliceous impurities in the primary or parent limestone rock. The tonality of these clays ranges from red to reddish-brown.

II. Sedimentation of detritus: What is known as detrital clays are formed by the sedimentation of detritus. These are smectite clays associated with detrital quartz, magnetite, ilmenite, and kaolinite to a lesser extent. These clays are derived from volcanic detrital material (originating in the basins of southern Belize and central Guatemala) deposited in lakes or marginal lagoons that later become salt lakes due to the tectonic uplift of the peninsula. The tonality of these clays ranges from grey to black, with a blackish-brown colour in between. It is also noted that they are characterized by being highly plastic. Regarding the geographical distribution of these clays, they are infrequent in the north of the peninsula due to the lack of surface currents. They are confined to the eastern block of the Ticul fault, to areas of polje basins around the Sierra de Ticul, east of Campeche, and along the Escárcega-Chetumal highway (Quintana Roo).

III. Crystallization: These are primary clays formed by talc, chlorite, palygorskite-sepiolite, and mixed layers of kaolinite-montmorillonite. They belong to the generic type of clay found in the northeast region of the Yucatan peninsula in several towns located around the Sierra de Ticul, Umán, Muna, Sacalum, Ticul, Chapab, Uxmal. Talc and chlorite were formed by direct crystallization due to the diagenetic alteration of dolomitic rocks and soils in the north of the karstic platform of the peninsula. The whitish palygorskite-sepiolite clay was formed by direct crystallization of high-salinity seawater or by the diagenesis of dolomitic rocks. This clayey material occurs in isolation or as deposits within limestone rocks.

From the chemical-mineralogical point of view, the palygorskite is fibrous clay. Chemically, it is a hydrated magnesium aluminosilicate; the montmorillonite is laminar clay of the smectite family. Chemically, it is a hydrated sodium-calcium magnesium aluminosilicate; and the chlorite is an aluminosilicate of phyllosilicate type. In the Yucatan clayey materials studied, montmorillonite appears inter-stratified with chlorite (vide infra).

Samples of clayey sediments

The subdivision of the studied samples by location with their coding is as follows (Figures 1 & 2):

Clay sediment (C1) extracted from Sak lu’um (‘white earth’) cave at the Sacalum site north of Yucatan State, approximately 80 km south of Merida. Several studies have shown that this cave would have been an important source of clay from pre-Hispanic times. Nine clay sediment samples were extracted from this cave at different depths, which have been coded as follows: C1-1, C1-2, C1-3, C1-19, C1-33, C1-34, C1-35, C1-36, and C1-37.

Clay sediment (C2) extracted from the cave in the town of Muna, north of Yucatan State. Different studies refer to the use of this clay to produce colors for pottery. The clayey sediment sample has been coded as C2-7.

Clay sediment (C3) extracted from Actun Hi cave in Ticul, in the north of Yucatan State. Different studies describe the importance of these deposits as an ingredient for pottery from pre-Columbian times to the present. The clayey sediment sample has been coded as C3-18.

The criteria for sampling were of cultural and physical/geological type. Unlike the seminal work of Arnold et al. (2007) [15], who focused on the sources of palygorskite for both ceramic and painting use, the main interest was to find sources of this clay for specifically preparing Maya blue. Thus, not only physical properties (white colour) were considered, but also cultural motivations that relied on the magic and sacred properties attributed by the Mayas to the pigment with the mystic character of the caves, which were considered a gate that connected humans and God’s worlds. Therefore, other clay sources, like outdoor outcrops, were rejected. A second criterion was the geological characteristic of the cave. According to that, sampling points were selected in areas of the caves where geological differences were found. Therefore, one sample was taken in Ticul and Muna caves, where a small area inside the cave was accessible for sampling, and all the stony material exhibited uniform geology. In contrast, larger areas inside the cave could be accessible in Sak lu’um cave, where differences were observed in the geology of the rock from the cave’s entrance to the inner and back rooms. For this reason, nine samples were taken there.

From an ethnographic point of view, it is interesting to note that today these deposits and clays are still collected and used by local artists for the development of different activities, including ceramics and painting.

Instrumentation

Light microscopy (LM)

A Leica GZ6 (X10-X50) stereoscopic light microscope was used to select the samples for analysis and for morphological examination of the sediments. A Leica Digital FireWire Camera (DFC) with Leica Application Suite (LAS) software was used to acquire and process digital images.

Scanning electron microscopy (SEM)

The chemical composition of the minerals was obtained using a Jeol JSM 6300 scanning electron microscope operating with a Link- Oxford-Isis X-ray microanalysis system. The analytical conditions were: 20 kV accelerating voltage, 2 × 10−9 Å beam current, and 15 mm as working distance. Samples were carbon coated to eliminate charging effects.

X-ray diffraction (XRD)

XRD diffractograms were obtained using a Bruker D8 Advanced A25 diffractometer fitted with a Lynxeye fast detector. XRD patterns were collected covering 5-80º 2Ɵ with an exposure time of 0.8 s. Cu Kα radiation was used (40 kV and 40 mA). The minerals were identified by comparing the XRD diffractogram with the ICDD database (2007 release) using Diffracplus Evaluation software (Bruker 2007).

FTIR spectroscopy

The IR spectra in the ATR mode of the powdered samples were obtained using a Vertex 70 Fourier-transform infrared spectrometer with an FR-DTGS (fast recovery deuterated triglycine sulphate) temperature-stabilized coated detector and an MKII Golden Gate Attenuated Total Reflectance (ATR) accessory. A total of 32 scans were collected at a resolution of 4 cm-1 and the spectra were processed using the OPUS/IR software.

Morphological description

Light microscopy

The observed macromorphological characteristics can divide the samples into three large groups, C1, C2 and C3, corresponding to the sites from which the studied clayey sediments were extracted. The clayey fraction of the samples C1 of clayey sediments (Figure 3) extracted from Sak lu’um cave at the Sacalum site in the north of Yucatan State is characterised by being formed by ≤ 500 μm cylindrical particles and, to a lesser extent, by small acicular crystals adhered to the surface of the grains. Irregular lenticular fragments (arrow 1 in Figure 3), carbonate in composition, are abundantly found. They are white or translucent in colour, with a shape ranging from subangular to subrounded. In certain samples, tiny particles are also observed (arrow 2 in Figure 3) of a darker, reddish-orange hue, of ferrous composition (goethite and/or hematite minerals) (samples C1-2, C1-3, C1-19, C1-34, C1-35). The occasional presence of pinkish-colored particles, most likely sulphate-type crystals, is noted in sample C1-19 (arrow 3 in Figure 3).

The C2-type clayey sediment (C2-7 sample in Figure 3) extracted from the cave in the town of Muna is characterized by the presence of the same components as the C1 sediment samples. However, it differs in terms of its clayey fraction, composed mainly of laminar aggregates and some ≤ 500 μm cylindrical particles.

Finally, the C3-type of clayey sediment (sample C3-18 in Figure 3) extracted from Actun Hi cave in Ticul is characterized by its brownish color, which comes from the presence of cryptocrystal- line ferruginous nodules and fragments of rock or claystone. As in the other studied samples, fragments of carbonated composition are also observed. The clayey fraction of this sample is the same as C2-type clayey sediment.

Scanning electron microscopy

The study of clayey sediments by SEM indicates a close relationship between palygorskite and the presence of crystalline phases linked to dolomite produced by a diagenetic alteration process of dolomitic rocks. The secondary electron images of the C1-1 clayey sediment confirm the growth of palygorskite of fibrous-cylindrical morphology in rhombohedral dolomite (Figure 4). The left image of Figure 4, shows aggregates of palygorskite that cover and occupy intercrystalline positions (P) in rhombohedral dolomite (D) crystals of small size (2 μm) and the right image of Figure 4, shows palygorskite (P) with a high density of fibers that are arranged with a certain banding, encompassing dolomite (D) crystals.

Mineralogical characterisation

X-ray diffraction

Figure 5 presents the most characteristic X-ray diffractograms of the studied clayey sediment samples. The main mineral species identified from the diffractograms obtained from the selected clayey sediment are shown in the following Table 1.

Figure Abbreviations: M: montmorillonite; D: dolomite; P: palygorskite. The palygorskite sample that is used to perform the diffractogram and extract the crystalline spacings in [40] is from the mine at Lorenzo Pech on the Chapab road, near Ticul.

Table 1: Identification of minerals from the XRD pattern of the samples studied.

Table Abbreviations: major mineral species: +++; moderate mineral species: ++; minor mineral species: +; mineral specie not present: -; d: characteristic distances between the planes of the crystal lattice of the detected mineral species.

Theoretical structural formulas of the mineral species identified: palygorskite, Si_{gMg5O20(OH)2(H2O)4·4H2O [40] and (Mg,Al)2Si4O10(OH)·4H2O [41]; montmorillonite-chlorite, (Na,K,- Ca/2)0.34Mg0.8Al2.58(Si3.32Al0.68)O¹⁰(OH)5.2H2O, and dolomite (Ca,Mg)(CO3)2 [41].}

Fourier transform infrared spectroscopy (FT-IR)

The mineralogical composition of the clayey sediment samples is determined by interpreting their infrared spectra. The spectra reveal absorption bands in the characteristic regions of minerals in the clay family, confirming the results obtained by X-ray diffractometry. Specifically, the samples studied can be separated into two groups according to the prevalent clay mineral present. The first group includes samples C1 in which palygorskite is the prevalent clayey mineral. The second group comprises samples C2-7 and C3- 18, where montmorillonite-chlorite is the prevalent clayey mineral.

C1 clayey sediment samples (Sak lu’um cave)

The position of the bands identified in the spectra of the studied C1 clayey sediments is similar for all samples because they all share palygorskite as a characteristic clayey mineral species but with some differences in their intensities concerning the accompanying mineral species. Specially, the presence of dolomite in different proportions causes the intensity of the palygorskite to oscillate.

Figure 6 shows, as an example, the IR absorption spectra of samples C1-1, C1-35, and C1-33, representative of the different varieties of materials found in the group of clayey sediments C1 from the Sacalum site. The IR spectra are dominated by the absorption bands characteristic of palygorskite whose main diagnostic features are summarized in Table 2 for sample C1-35, as an example. According to [42-44], palygorskite exhibits absorption bands ascribed to the hydroxyl group (HO-) associated with different coordinated and zeolitic water present in the clay. IR band at 3612 cm-1 is assigned to the stretching vibration of the hydroxyl group attached to the different cations of the crystal lattice in octahedral coordination. Bands at 3580 and 3540 cm-1 correspond to coordinated water in the channels, and bands at 3372 and 3272 cm-1 are assigned to zeolitic water (physically absorbed water). The absorption band at 1654 cm-1 is assigned to the asymmetric bending vibration of the HOH group in coordinated and absorbed water. A second group of bands is related to silicate groups’ vibration modes. The absorption bands at 1191, 1110, 1087, 1036, and 972 cm-1 are assigned to the asymmetric stretching vibration associated with the O’-SiO3 terminal and Si-O-Si bridging groups. The absorption band at 910 cm-1 is assigned to the symmetric stretching vibration associated with the O-SiO3 terminal group and the stretching vibration associated with the terminal group Al2-HO and (Al, Fe)-HO. A third group of bands is ascribed to iron and aluminum in the network. The absorption band at 789 cm-1 is assigned to the bending vibration associated with the terminal group (Mg, Al)-HO, Fe2-HO, and (Mg, Fe)-HO. The absorption bands at 732 y 638 cm-1 conceal vibrations in octahedral coordination M-O, where M = Al, Mg. Bands occurring in the 600-400 cm-1 region are associated with the terminal groups Si-O-Si and O-SiO3.

Table 2: Diagnostic IR bands for palygorskite.

Table Abbreviations: Intensity: vs: very strong; s: strong; m: medium; w: weak; vw: very weak. Morphology: (sp): sharp; (b): broad; (sh): shoulder; (n): neck [43,44].

As can be seen in Table 3 and Figure 6, IR absorption bands of dolomite at 1413, 874, and 727 cm–1 ascribed to the v3 stretch and v2 and v4 bending of the carbonate group [45-46] are identified in sample C1-33 and, with lower intensity in samples C1-1, C1-2, and C1-3. In contrast, these IR bands are absent in samples C1-19, C1- 34, C1-35, C1-36, and C1-37.

Table 3: Diagnostic IR band for dolomite.

Table Abbreviations: (1). [45]; (2). [47]

Clayey sediment samples C2-7 (Muna cave) and C3-18 (Ticul cave)

Clayey sediment samples C2 and C3 exhibit similar IR spectra because they share montmorillonite-chlorite as the main clayey mineral species present in the material. Nevertheless, a distinct proportion of dolomite and palygorskite is present in each sample.

The two spectra of samples C2-7 and C3-18 (Figure 7) exhibit IR absorption bands characteristic of Al-Mg-montmorillonite-chlorite, as summarized in Table 4. Absorption bands between 1100 and 1000 cm-1 are assigned to the stretching vibration associated with the Si-O group. The IR band at 3621 cm-1 is associated with stretching OH- groups which are typical of Al-rich montmorillonites, and bands at 911 and 835 cm-1 are ascribed to vibration modes of (AlAlOH) and (AlMgOH) groups, respectively. The last two bands are related to the partial substitution in the octahedral coordination of Al to Mg. All those features confirm the presence of inter-stratified clay minerals of Al-Mg-montmorillonite-chlorite type [42,48]. The presence of small amounts of palygorskyte in both samples is recognized by a shoulder at 3240 cm–1 ascribed to stretching vibrations of OH- groups in this clayey species.

The two IR spectra of samples C2-7 and C3-18 present IR absorption bands corresponding to dolomite, at 1425, 874 and 727 cm–1, ascribed to the v3 stretch and v2 and v4 bending of the carbonate group [45-46].

Table 4: IR bands of Al-Mg-montmorillonite-chlorite

Genetic considerations of the studied clayey sediments

The significant content of palygorskite found in the samples studied suggests that these clayey materials were formed from lacustrine and saline endorheic environments where different types of carbonates are deposited and which generate magnesium-rich clays during diagenetic processes from dolomitic rocks. Montmorillonite- chlorite is also found in these samples as contributions from existing paleosoils in the upper part of the cave.

Figure 8 shows a ternary diagram palygorskite; montmorillonite- chlorite; dolomite. The samples analysed have been depicted according to their semi-quantitative mineralogical composition obtained by XRD and FTIR spectroscopy. Samples from the Sak lu’um cave, in which palygorskite is the major component (except for sample C1-33), are well discriminated from samples C2-7 and C3-18, extracted from the caves in Muna and Ticul, in which montmorillonite-chlorite is the major component. Three different compositions can be distinguished in the clayey sediments from Sacalum. Samples C1-1, C1-2, and C1-3, are mainly composed of palygorskite together with dolomite and, to a lesser extent, montmorillonite- chlorite. Samples C1-19, C1-34, C1-35, C1-36, and C1- 37 are primarily composed of palygorskite and, to a lesser extent, montmorillonite-chlorite. Sample C1-33 is mainly composed of dolomite together with palygorskite and montmorillonite-chlorite, to a lesser extent. These results confirm that these clayey sediment samples are part of the group of primary clays, the generic type of clay found in the Yucatan Peninsula, which is made up of talc, chlorite, palygorskite-sepiolite and mixed layers of kaolinite-montmorillonite [32,38].

Palygorskite should be formed by direct crystallization of high-salinity seawater or by diagenesis of dolomitic rocks [32,38]. The latter hypothesis would explain the presence or absence of dolomite in the studied samples studied, in different proportions, depending on the sample analysed. The samples that do not present dolomite could have undergone absolute diagenetic alteration. Thus, the material has been completely transformed into palygorskite (samples C1-19, C1-34, C1-35, C1-36, and C1- 37). The samples that present a certain proportion of dolomite are those in which the diagenetic alteration process is incomplete (samples C1-1, C1-2, and C1-3); and the samples that mainly present dolomite with respect to palygorskite are those in which the diagenetic alteration process is still in the initial phase (sample C1-33). The absence of detrital minerals confirms that the palygorskite has not undergone transport and that it has been formed by in situ processes, thus also corroborating the hypothesis that it was originated by direct crystallization. Also, palygorskite is a type of clay that appears in nature associated with other clayey mineral species, such as illite/smectite, smectite, illite and chlorite [38], as determined by the diffractograms of all these samples, where the presence of montmorillonite-chlorite is detected to a lesser extent. The origin of the chlorite is related to carbonate rocks and soils of the northern part of the karstic platform of the peninsula, associated with dolomitic rocks. It is formed by direct crystallisation, as a result of the diagenetic alteration of dolomitic rocks [32,38,49].

Clayey sediment samples C2-7 and C3-18, extracted from the caves in Muna and Ticul, respectively, share the mineral species detected in the diffractograms. These samples are mainly composed of montmorillonite-chlorite and, to a lesser extent, of dolomite and palygorskite. These results indicate that these samples of argillaceous sediment are part of the detrital clays that are formed by the sedimentation of detritus, mostly confined to the areas of polje basins around the Sierra de Ticul and made up of smectite clays associated with detrital quartz, iron oxides and, in lesser proportion, kaolinite. The presence, to a certain extent, of dolomite in their diffractograms is due to the carbonated nature of the rock substrate in the Yucatan Peninsula. Despite its high lithological variety [38,50- 53] and its exiguous proportion of palygorskite, it can be associated with the incipient diagenetic alteration of dolomitic rocks, as in the case of samples C1. Finally, the presence of detrital quartz is not detected.

The mineralogical composition found in the samples studied suggests that these clayey materials have diverse origins. Palygorskite should form the inner strata in the caves. These sediment layers should be formed from lacustrine and saline endorheic environments where different types of carbonates were deposited. In those geological contexts, diagenetic processes from dolomitic rocks originated these magnesium-rich clays. Interestingly, montmorillonite- chlorite is also found in these samples. Unlike palygorskite, these clays stem from the paleosoils located in the upper part of the cave.

Source of the studied clayey sediments and caves or sascaberas

As proposed by [54] in their study on the edaphic regionalisation of the Yucatan Peninsula, this research uses the term edaphic landscape to contextualize the studied clayey sediment samples within the edaphic distribution of Yucatan, since all of the samples come from of that state, specifically from sites at Sacalum, Muna and Ticul. This concept of edaphic landscape implies morphological and pedogenetic criteria and genetic, time, topographic, lithological, hydrological and climatic factors, from which it is possible to characterize and identify the existing edaphic diversity in the state.

A complex and diverse stratigraphy of soils and paleosoils product of the climatic evolution and its vegetation during the Quaternary on the Yucatan platform; to which are added the agricultural techniques of the Mayas, which also created new sedimentary deposits [54].

Therefore, by taking into account the Eocene period as the geological age of the lithological facies assigned to the three extraction sites of the clayey sediments, it can be determined that the edaphic landscape that was formed at this period or posterior on the karstic plain of Yucatan Peninsula was LV/CM/LP [54].

On his bases, the macromorphological descriptions of the clayey sediment samples, the diagnostic characteristics of the Mayan Soils of Classification (MSC) [55] and their correspondence with the World Soil Reference Base (WRB) [56] can be further refined with the contextualization of the edaphic landscape as follows [55, 57].

Sacalum correspond to lithic gleyic leptosol (calcareous) (LPglli( ca)) according to the WRG and to Sacalum according to the MSC: inorganic or mineral soils, with limited rooting due to shallow rockiness. Light grey soils, sandy clay, very shallow (3-17 cm), moderately well drained, on laminar limestone rock.

Ticul corresponds to luvisol (LV) according to the WRG and K’ankab lu’um according to the MSC. This type of soil concentrates iron oxides and minerals mixed with clays. For this reason, this type of soil acquires a reddish-brown color.

It is difficult to assign an edaphic landscape type to the Muna because its macromorphological characteristics, especially its white-greyish chroma, do not match the typological descriptions of the edaphic landscape for the area in question. However, what can be said is that mineralogical characterization determines that this sample is one of the inter-stratified, Al and Mg-rich montmorillonite- chlorite soils, detrital clay formed by the transport and sedimentation of detritus.

However, the study of the clay samples extracted from these sites or deposits does not correspond directly to the type of soils indicated, especially in the case of Sacalum. This is due to the fact that sascaberas are natural or artificial caves that penetrate to a certain depth in the ground below the level of the soils and the superficial calcareous stratum [58,59].

Figure 10 illustrates an idealised geological cross-section of a sascabera where the great stratigraphic diversity and sedimentary deposits that make up the walls of the extraction area can be observed [60].

Specifically, the Sacalum strata or deposits, from which the studied clay samples come from, have been extracted from inside the cave and therefore cannot be correlated with the distribution of soils indicated by the Geopedological map of the state of Yucatán [54]. Also note that although the clay samples come from caves and cenotes or poljes, they are not direct karstic deposits such as stalagmites and stalactites of carbonate composition.

Implications in the Maya blue preparation

As mentioned above, the different studies carried out on the role developed by the clay in the preparation of Maya blue (MB) pigment remark on the importance of the type of clay for achieving a satisfactory development of the colour during the preparation process of the pigment. Many studies are found in the literature in which synthetic MB-type specimens are prepared in an attempt to replicate the original method followed by the Maya artists with phyllosilicate-type clays such as the natural palygorskite, sepiolite, montmorillonite, kaolinite, and even the synthetic clay laponite, hardly used by Maya artists [61]. Nevertheless, it is convenient to note that presence of dehydroindigo, the oxidized form of indigo formed by redox-tuning during the heating of the pigment over 100ºC, only has been reported in specimens prepared by crushing indigo with palygorskite, sepiolite, montmorillonite and kaolinite [35-37].

It is worth noting that palygorskite, montmorillonite, and kaolinite are the main clayey minerals that compose the three studied clay sediments. Nevertheless, these minerals do not have the same ability to attach the indigo molecules to the silicate groups of the clay because a channelled structure of the micro- and nanoparticles is required to attach the indigo molecules properly. This peculiarity appears to be a crucial factor in the stability of the MB pigment and, therefore, it is unlikely that clayey minerals such as montmorillonite and kaolinite, which do not have a channelled structure in their particles, could be used for preparing MB pigment due to their low ability for attaching indigo molecules [13]. In addition, the size and shape of the channels determine of the physical and chemical mechanisms of attachment to the silicate and aluminosilicate groups as well as the reactive behaviour of the indigo molecules during pigment preparation. [36] have demonstrated, using MB simulants, that the ability to trap indigo during pigment preparation increases as the size of the channel decreases until a threshold below which the molecules cannot enter the channels due to their reduced size.

On the other hand, the ability to tautomerize and oxidize the attached indigo to indigo-enol and dehydroindigo in the clay channels is a crucial property that determines the characteristic blue-greenish hue of the MB pigment. Contrary to the trapping ability, the redox tuning ability and tautomerization properties increase with increasing the clay channels size. It is suggested that the best properties for the MB pigment should be obtained by achieving a compromise between the entrapping ability of indigo molecules in the clay channels and the ability to favour the formation of dehydroindigo from indigo, and these optimal characteristics are possessed by palygorskite.

In light of the differences found in the mineralogical composition of the series of clay sediments analysed, those from Sak lu’um cave should be the most likely source of clay in the preparation of Maya blue pigment due to the lower palygorskite content found in the other two sites. This low palygorskite content of the Ticul and Muna clays should prevent the suitable attachment of the indigo molecules to the clayey nanoparticles during the Maya blue preparation.

XRD and FTIR analysis combined with dependence on an interpretative framework derived from geology and edaphology is a suitable methodology for characterizing and discriminating samples of clayey sediments from the north of Yucatan Peninsula.

The clays from these caves do not correspond directly to the soils that currently overlie the Yucatan limestone platform. As has already been observed in other sascaberas on the coastal platform, they are natural caverns reworked by exploitation or artificial excavations that penetrate beneath the calcareous covering and the surface soils. These clay deposits are also not directly related to the formation of stalactites and stalagmites or the general precipitation of carbonates in the karstic cavities of the terrain.

The samples of clayey sediment extracted from Sak lu’um cave are mainly composed of palygorskite accompanied by dolomite and other minerals associated with sediments formed in endorheic lacustrine environments with a high saline level. Montmorillonite- chlorite is also identified in sample. Unlike, palygorskite, these clays come from different strata or from the paleosoils located in the outer strata of the caves. These results show that these samples are composed by rocks and minerals coming from different geological strata in the cave and, therefore, with multiple origins, due to the complex stratigraphy of the cave and the difficulty of sampling. Thus, a part of the sample is composed of the primary clay formed by direct crystallization, which is the generic type of clay found in the Yucatan Peninsula, composed of talc, chlorite, palygorskite- sepiolite, and kaolinite-montmorillonite mixed strata. The clay sediment samples extracted from different caves at Muna and Ticul differ from the former. They are composed mainly of montmorillonite- chlorite and, to a lesser extent, of dolomite and palygorskite, respectively. These results indicate that these clayey sediment samples are part of the detrital clays formed by sedimentation of detritus, mostly confined to the polje mining areas scattered around Sierra de Ticul, and composed of smectite clays associated with detrital quartz, iron oxides, and, in a lesser proportion, kaolinite.

It should also be noted that the abundant presence of palygorskite in the Yucatan limestone platform is due to the frequency with which favourable sedimentary environments have formed (high-salinity lacustrine environments and complex diagenesis), which have allowed the sediments of the environment in question (dolomitic sediments) to crystallise, giving rise to this type of clay. Although these sediments have an irregular distribution on the Yucatán limestone platform, the stratigraphic register indicates that they have been formed from the Eocene to the early Quaternary.

The results obtained agree with the common hypothesis that this clay is an important ingredient in pre-Columbian culture for the manufacture of pigments for mural painting, textiles, sculpture, and ceramics. It also confirms that these clays are confined to the northeast of the Yucatan Peninsula, located in sinkholes or cenotes, associated with the deposits of the walls and floors of these cavities, though not associated with the stalactites and/or stalagmites there.

On the other hand, it could also explain the importance that these clays had since pre-Hispanic times in the field of ritual, since in the ancient Mayan worldview, caves (and by extension cenotes) were the sacred space that connected the world of the gods with that of human beings; the sacred with the profane. Understood in this way, caves and cenotes were seen as a “sacred threshold”; a fertile matrix that produced, among other things, clays suitable not only for the arts but also in different daily ceremonies, where they could be used in different ways, but mainly two: as a drug (ingested), and as body paint, following what was common in many other Mesoamerican cultures.

The sacred and ritual character of the caves has conferred to their clays a symbolism as an exquisite material associated with the Gods and, thus, suitable for preparing such a noble and high-quality pigment as the Maya blue. Hence, the most relevant finding of this study is that in the light of the differences found in the mineralogical composition of the clay sediments from the three studied caves studied, Sak lu’um cave should be the most probable clay source for preparing Maya blue pigment due to the low content in palygorskite found in the other two sites that should prevent the suitable attachment of the indigo molecules and should result in an unstable pigment of low quality. Further research looking for possible sources of Maya blue clays should be done in light of the compositional requirements of this material for obtaining a suitable Maya blue pigment.

Finally, it is worth mentioning that the study carried out is of great interest as a field survey that lays the methodological grounds for identifying sources of suitable clays for Maya blue preparation. Despite the limited number of samples analysed in the Ticul and Muna caves, this study has put in evidence that technical criteria such as a suitable palygorskite-rich composition for the clay must be taken into account mandatorily in addition to other properties such as white colour or cultural criteria when archaeological or ethnographic studies on Maya blue preparation are performed.

aaGrant PID2020-113022GB-I00 funded by MCIN/AEI/ 10.13039/501100011033 and by “ERDF A way of making Europe”, by the “European Union”.

PGC2018-098904-B-C22 project: Mayan Art and Architecture. New technologies for their study and conservation, supported by the Ministry of Economy and Competitiveness.

I declare that there is no conflict of interest in the research

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