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Article

Insights into Gamma-Ray Spectrometry of Building Stones in the North Temple of the Great Ball Court, Archaeological Zone of Chichen Itza, Mexico

by
Alejandro Méndez-Gaona
1,
Vsevolod Yutsis
1,*,
Rubén Alfonso López-Doncel
2,
Claudia Araceli García-Solís
3 and
Alfredo Aguillón-Robles
2
1
Potosino Institute of Scientific and Technological Research, Camino a la Presa San José 2055, San Luis Potosí 78216, Mexico
2
Institute of Geology, Autonomous University of San Luis Potosi, Av. Dr. Manuel Nava 5, San Luis Potosí 78290, Mexico
3
Center INAH-Yucatán, Calle 10 310-A, Mérida 97119, Mexico
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(16), 2949; https://doi.org/10.3390/buildings15162949
Submission received: 26 June 2025 / Revised: 8 August 2025 / Accepted: 17 August 2025 / Published: 20 August 2025
(This article belongs to the Special Issue Advanced Research on Cultural Heritage)
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Abstract

Non-destructive tests are especially useful for the assessment of building stones and their deterioration in built cultural heritage. Gamma-ray spectrometry is a non-destructive test that has not been applied extensively in these types of constructions. Therefore, the purpose of this study is to show the results of gamma-ray spectrometry for limestone characterization and deterioration assessment. This study was conducted in the North Temple of the Archaeological Zone of Chichen Itza and several outcrops in the area. Gamma-ray spectrometry data were corrected for attenuation caused by the moisture content in rocks to calculate the real radioelements concentrations using linear regression, with interpretation based on their mobility resulting from chemical weathering processes. The results obtained with gamma-ray spectrometry were corroborated by laboratory analyses, demonstrating that stones from the North Temple are more weathered than rocks from the outcrops, and that some limestones have clasts derived from terrigenous sources, causing them to show slightly higher radiation, which can be distinguished easily with gamma-ray spectrometry, even when lithology cannot be recognized in plain sight. Gamma-ray spectrometry proved to be useful for limestone characterization, and data obtained can be correlated with parameters from other analyses.

1. Introduction

The deterioration of building stones is a common phenomenon that can be observed in historical monuments built with stone materials [1,2,3,4,5,6]. Recognition of deterioration patterns and building stone characterization helps make decisions about the best treatment options for conservation. Generally, sampling is necessary for the assessment of building stone durability, measuring density, porosity, hydric and strength properties in a laboratory [1,5].
However, non-destructive tests (NDTs) have the advantage of allowing for the assessment of stone deterioration and certain properties without sampling, an extremely valuable aspect when studying heritage buildings, where taking samples of original materials is restricted because it would contribute to their loss. NDTs are employed in built heritage to measure, for example, elastic properties [1,3], moisture [2,5,6], wave velocity [4] and magnetic susceptibility [7,8]. Elastic properties and wave velocity depend mainly on the density of materials, so these parameters can be used to evaluate the degree of deterioration based on changes in mechanical properties [1,3]. Moisture content can be determined by means of electrical resistivity, identifying areas where water accumulates within a building structure [2,5,6]. Magnetic susceptibility depends on the presence of ferromagnetic minerals [7], but also heavy metals, so specific deterioration processes, such as soiling, can be monitored [8]. On the other hand, gamma-ray spectrometry (GRS) is a type of NDT which is based on the detection of natural gamma rays generated by potassium, uranium and thorium radioactive decay. Since each of these radioelements decays with a specific energy or energy spectrum, each can be identified and their concentration determined.
Considering building stone characterization, radiation detection and GRS have been utilized to calculate dose rate from potassium, uranium and thorium distribution [9], although sampling of materials used for construction is more common to determine dose rate in the laboratory [10,11,12,13], and is not allowed in most heritage buildings.
Chemical weathering processes are known to alter the distribution of radioelements noticeable with GRS [14,15,16,17,18,19,20,21,22]. Nevertheless, the application of this technique to characterize building stones and their deterioration has only been used recently. Other studies have limitations: in [23] no methodological bases and data processing are described; in [24] the results are related to total radiation and little to radioelements concentrations; in [25] the amount of GRS data is low; and in [26], although some limitations of the previous studies were resolved, the proposed methodology was only tested in rhyolitic ignimbrites, which are rocks with high levels of radiation.
This study aims to demonstrate the effectiveness of GRS in built heritage, with a focus on its application for assessing stone deterioration and characterization, specifically limestones. This type of rock, due to its deposit environment and origin, contains very low radioelements concentrations. On average 0.3% of potassium, 2 ppm of uranium and 1.3 ppm of thorium is found in limestones [27]. Another objective considered was the possibility of correlating the information obtained by GRS with deterioration originating from carbonate dissolution processes, which would be reflected in radioelement loss.
The Archaeological Zone of Chichen Itza, Yucatán, Mexico (Figure 1) was selected as the study area. It should be noted that the Archaeological Zone of Chichen Itza was declared a cultural World Heritage Site by the United Nations Educational, Scientific and Cultural Organization (UNESCO) in 1988, highlighting its monumental architecture and carved stone relief [28]. Achieving a better understanding of the deterioration affecting its monuments depends on using adequate analytical techniques to find specific restoration and conservation measures. For this purpose, several studies have been carried out using geophysical methods, such as electrical resistivity tomography to detect structures beneath and in the interior of El Castillo [29,30], electrical resistivity in the polychromatic pillars of the Substructure of the Warriors to identify salts solubility related to moisture [31], and also NDT such as portable X-ray fluorescence equipment to identify the composition of the Throne of the Red Jaguar sculpture in the Substructure of El Castillo [32].
This investigation was focused on the west internal wall of the North Temple of the Great Ball Court of the Archaeological Zone of Chichen Itza, Tinum, Mexico (original name: Templo Norte del Gran Juego de Pelota) (Figure 2), which is an architectonic group composed of several buildings around the ball court, being the biggest in Mesoamerica [33]. Like all other buildings and sculptures in the Archaeological Zone of Chichen Itza, the North Temple was built with limestone and calcareous breccia that outcrop into the construction site of the prehispanic city; some quarries can even be distinguished containing pieces not finished or discarded near the buildings. In these outcrops, GRS data acquisition was also carried out to compare their radioactivity and radioelements concentrations with the North Temple stone blocks and, therefore, determine if a loss of radioelements exists due to weathering. Information about the historical context of the study area, as well as the geological framework of the region, is presented in the following section.

2. Historical and Geologic Context of the Study Area

Chichen Itza was an important prehispanic city of the Maya culture established in the north of the Yucatan Peninsula, specifically in the zone denominated as the northern Maya Lowlands, with great influence in the region.
Although there were settlements around Chichen Itza, it was in the Late Classic period (600–900 AD) that the construction of buildings in the style called Maya or Puuc began in the southern part of the site (in what has been called Old Chichen), in addition to the arrival of foreigners [32,34].
In the Early Postclassic period (900–1200 AD), the construction of monuments increased, which was carried out in the style called ‘toltec’ in the northern part of the site on a great artificial leveling [32,34]. It was in this period when the North Temple was built, being 10 m long and 6 m wide; its façade has only one bay and three accesses separated by two columns (Figure 2a) [35]. It was possibly built to celebrate the enthronement of a new ruler as internal walls and columns decorated with reliefs carving depicting ancestors, festivities and rites can be seen throughout the whole vault [34,35,36].
At the beginning of the Late Postclassic period (1200–1521 AD), the construction of large buildings ceased and a decrease in the city occurred, although Chichen Itza continued to be a pilgrimage site [34].
As previously indicated, the Archaeological Zone of Chichen Itza was built with limestone and calcareous breccia that in can be found in surrounding outcrops, and are part of the carbonate platform of the Yucatan Peninsula, so their use is widespread in Mexican Maya sites, with some sedimentologic and composition variants, as reported in the Uxmal, Tulum, Rio Bec, Dzibanche and Calakmul sites [37,38,39,40,41,42]. The Yucatan Platform (YP) is characterized by thick calcareous successions deposited during the Cretaceous and Cenozoic periods [43]. It has a karst relief with solution sinkholes locally called cenotes that are more common in the northwest part of the YP, with their distribution forming a ring related to the Chicxulub impact crater near the Cretaceous–Tertiary boundary [44,45,46].
Specifically, in the Archaeological Zone of Chichen Itza, limestone and calcareous breccia belong to the Chichen Itza Formation (Eocene) (Figure 3) composed of fossiliferous white yellowish limestones, and even reddish by the presence of colloidal iron oxides [43,47,48]. Stratigraphically it is above the carbonated sequence of the Icaiché Formation (Paleocene) and transitionally passes to bioclastic limestones of the Oligocene or discordantly to the Carrillo Puerto Formation (Miocene–Pliocene) (Figure 3) characterized by fossiliferous limestones deposited in nearshore environments [43,47,48].

3. Materials and Methods

As indicated at the beginning of this study, the objective of this investigation is to demonstrate the usefulness of the in situ GRS for the assessment of the building stones of the North Temple. However, we also decided to acquire GRS data for the outcrops to compare the results and determine possible changes in the radioelements contents due to weathering and deterioration processes. In addition, to support the interpretation of the GRS, it was decided to include other analyses of samples taken from outcrops only. The materials and methodology used are described below.

3.1. Gamma-Ray Spectrometry Data Acquisition

The GRS was performed using a Radiation Solutions Inc. (Mississauga, ON, Canada) RS-125 gamma-ray spectrometer, which records total radiation, dose rate, and concentrations of potassium, uranium and thorium. Using automatic calculations, the equipment corrects for background radiation and dead time, in addition to simultaneously calculating three radioelements’ concentrations. The spectrometer was calibrated by the manufacturer, and geochemical data were compared with its response to confirm its calibration. This data can be found in [26]. The calculated error for the concentrations found, with a maximum time of 300 s, is 0.06 for potassium, 0.39 for uranium, and 0.54 for thorium [50].
GRS data acquisition was performed similarly in the North Temple and in outcrops, with the spectrometer placed on the rock surface. Immediately after completing the GRS recording, moisture content was measured using a portable moisture meter (Duro trademark) with a percentage scale and an accuracy of 0.05%. This was performed to correct the data for radiation attenuation according to the procedure proposed by [26]. Another important correction was made for the influence of surfaces near the spectrometer. Under normal conditions, the spectrometer only records the counts from the surface it is in contact with, which is referred to as 2π geometry [51,52]. However, if the equipment is placed in proximity to other surfaces, it will record more radiation (>2π), while the opposite occurs if the measurement is made at the edges, recording less radiation (<2π). In the west internal wall of the North Temple, only 2π and >2π geometries occur.
Following data correction, the count rates of potassium, uranium and thorium were recalculated for 0.0% moisture content to avoid the effect of attenuation, then the real radioelements concentrations were calculated to interpret data according to geochemical mobility.
In the North Temple, a total of 119 points were measured in the stone blocks large enough to adequately place the spectrometer, and in the outcrops the number of measurements depended on their extension, resulting in a total of 24 points measured in limestone and calcareous breccia. The location of the outcrops can be seen in Figure 1.

3.2. Lithologic Description, Deterioration Patterns, and Weathering Conditions

To determine the correlation between the GRS data and the lithology of the North Temple stone blocks, it was necessary to determine the different rock types, which in this case were limestone and calcareous breccia. Lithological mapping was then performed with this data to visualize the results.
Similarly, to compare GRS with stone deterioration, the type and extent of deterioration patterns were recorded according to the illustrated glossary of [53], which aims to standardize the terms used for deterioration.
The lithological characteristics and weathering conditions of the outcrops were also detailed, although care was taken to ensure that they were not highly weathered.

3.3. Sampling, Petrography, and X-Ray Diffraction

As mentioned at the beginning of this section, no samples were taken from the North Temple, but only from the outcrops. A total of 12 representative samples were collected according to the lithological differences observed in the outcrops: 2 from calcareous breccia, 8 from limestone, 1 from red soil, and 1 from sascab, a local term for a poorly consolidated calcareous material [39,54,55].
From the 10 samples of calcareous breccia and limestone, 29 thin sections were prepared for petrographic analysis with an optical microscope to characterize their mineralogical composition and components using the classification of [56] modified by [57]. This analysis described the porosity type.
A fraction of all samples was pulverized for analysis by X-ray diffraction (XRD), which was performed with a Bruker D8 Advance X-ray diffractometer at the National Laboratory for Research in Nanosciences and Nanotechnology of the Potosino Institute of Scientific and Technological Research, San Luis Potosí, Mexico to determine the mineralogy of the samples.

3.4. Major Elements and Chemical Parameters Calculation

For all samples, major element concentrations were obtained at the National Laboratory of Geochemistry and Mineralogy, Institute of Geology, National Autonomous University of Mexico, Mexico City, Mexico using a wavelength-dispersive X-ray fluorescence spectrometer (model RIGAKU ZSX Primus II) according to the analytical technique proposed by [58]. The mean atomic weight (MAW) [59] was calculated from the major element concentrations to correlate with physical properties and the mafic–felsic–weathering index (MFW) from [60]. Although this index is primarily applied to igneous and clastic sedimentary rocks, it was useful for the assessment of limestone and calcareous breccia.

3.5. Petrophysical Tests

Petrophysical tests were conducted at the Institute of Geology of the Autonomous University of San Luis Potosi. Specifically, the matrix and bulk density, porosity, capillary water absorption (w value), and dry weight loss (DWL) due to salt crystallization were calculated for three limestone samples.
For these tests, the samples were cut into 6.5 cm cubes. Density was determined by weight under dry, saturated, and submerged conditions. Porosity was also calculated using these parameters. Capillary water absorption was measured by placing one face of the cubes in contact with the water surface, allowing capillary absorption to occur. This was performed on the X and Y axes of the samples due to the anisotropic characteristics of the rocks. Details of these tests can be found in [61].
Salt crystallization was measured using the DWL test, in which the cubes were immersed in a 10% Na2SO4 solution for 4 h and then dried in an oven. This process requires a 24 h cycle, and a total of 24 cycles were performed.

4. Results

4.1. Gamma-Ray Spectrometry

Since the GRS was conducted in limestone and calcareous breccia, the data were treated separately and are presented in counts per minute (cpm). Following the methodology proposed by [26], anomalous data in the North Temple, i.e., those taken in geometries >2π, were discarded to create a regression line. These discarded data were subsequently corrected with the equation of the modeled line and the calculated standard deviation. Figure 4 and Figure 5 show the correction and equations of the calculated lines for calcareous breccia and limestone, respectively.
To interpret the corrected data and calculate the real radioelement concentrations, the counts were recalculated for a moisture content of 0.0% to avoid the effect of a lower count rate caused by attenuation with higher moisture content. This yielded the concentrations of each radioelement.
According to Figure 4 and Figure 5, the calcareous breccias are slightly more radioactive than the limestones, which is especially evident in the potassium plots, while the uranium is more evenly distributed. Based on these GRS results, the differences between the calcareous breccia and limestone were interpreted as being due to a higher clay and/or iron oxides content in the former, which gives it a reddish color.
For the GRS in the outcrops, the same procedure was followed as for the west internal wall of the North Temple (Figure 6 and Figure 7). Anomalous readings made in geometries other than 2π were discarded to construct the regression line. Once this was completed, the standard deviation and the equation of the line were calculated to correct all the data. The radioelement concentrations were recalculated for a content of 0.0% for clearer interpretation.
A comparison of the recalculated concentrations for a moisture content of 0.0% between the GRS data from the North Temple and the outcrops shows that the latter exhibit slightly higher radiation than the North Temple stone blocks (Figure 8). This is very clear in the calcareous breccias for total radiation and the concentrations of potassium and thorium, but not for uranium. However, different trends are observed in the limestones: potassium and thorium are higher on average in the outcrops but have a broader distribution in the North Temple, while uranium is higher in the outcrops. These results suggest that radioelement mobility processes occur more frequently in the North Temple stone blocks than in the outcrops which are less weathered.

4.2. Lithologic Description, Petrography, and Mineralogy

As mentioned above, limestone and calcareous breccias stone blocks were encountered in the North Temple (Figure 9). In some cases, it was difficult to identify the lithology of the rocks because some limestones displayed orange hues like calcareous breccias. However, with the GRS both rock types could be easily differentiated because, as described in the previous section, breccias exhibit slightly higher radiation than limestones. This fact adds another advantage to in situ GRS, as it allows for the recognition of lithology, even when the surface of the stone blocks is not easily distinguished on plain sight.
The same rock types were found in the outcrops: limestone and calcareous breccia. The limestones are generally white, beige, and gray, without stratification, although they may be laminated due to the presence of algae. Some gastropod shells could be distinguished on plain sight. The calcareous matrix of the breccias is white, gray, and orange, with angular orange and reddish clasts measuring on average 3 cm, up to 5 cm. Some other clasts are calcareous of white and gray color.
As detailed in the Materials and Methods section, samples were collected only from outcrops for the petrographic description of thin sections. The results are shown in Table 1.
Petrographic analysis determined that the orange and reddish coloration of the calcareous breccias is due to rounded particles interpreted as colloids of terrigenous origin produced by paleosoils (Figure 10a). According to the classification of [56] modified by [57], the breccias were classified as floatstone with micritic matrix. Some angular crystals of altered quartz with fractures filled with the same reddish material were also identified (Figure 10b). In some portions, the sparite cement is partially dolomitized. The main type of porosity identified in the breccias was fenestral (bird’s eyes).
Limestones present more diverse characteristics. Some were classified as bindstone because they are formed by algal laminae that vary in thickness and color. The thickest algal laminae are dark brown (Figure 10c) and can reach several centimeters. Their porosity is generally much lower compared to the micritic matrix. Bindstone thinner laminae are light brown and tend to be millimeters (Figure 10d). Higher laminoid fenestral porosity is observed in this type of bindstone compared to the one with thicker laminae.
The matrix of most samples is micritic, and can be mainly classified as bindstones, although in some cases they can also be classified as mudstone, wackestone, or packstone (Figure 10e). Microfossils of gastropods, coralline algae, benthic foraminifera, and brachiopod fragments were identified.
Other limestones are characterized by orange bands and were classified as grainstone (Figure 10f). Their coloration is due to their composition of peloids with orange particles that, like breccias, have been interpreted as paleosoil particles.
Petrographic analysis and rock classification explain the results obtained using GRS: breccias with terrigenous input (floatstone) have higher concentrations of potassium and thorium, which are associated with this clastic supply, while limestones without terrigenous input (bindstone, mudstone, wackestone, packstone) have lower concentrations of these elements.
According to the XRD of all samples, calcite is the main component. The only exception is the red soil sample, which indicates the presence of halloysite, goethite, and quartz (Figure 11). These results indicate that the reddish particles distinguishable in the floatstone and grainstone can be interpreted as paleosoils.

4.3. Stone Deterioration Patterns

As mentioned in the Materials and Methods section, deterioration patterns are named according to [53]. A discussion of these terms can be consulted in [62].
Mainly features induced by material loss were found on the west internal wall of the North Temple (Figure 12): small depressions, cavities, and loss of original surface due to dissolution of the stone exposed to water run-off are the most widespread damages (alveolization, microkarst, and erosion); perforations are common, but their total extent is minimal compared to other damages; and millimetric cavities produced by biological colonization (pitting) are present [63]. Cracks are also found, but only in the stone blocks themselves, i.e., they do not penetrate the wall. Barely perceptible but extensive deterioration is seen in the area not covered by the vault, with a diffuse light gray coloration possibly caused by biological colonization (Figure 2b).
The features induced by material loss are related to carbonate dissolution, so it is expected that the physical properties of the North Temple stone blocks have changed, specifically their porosity and, therefore, their density. Comparing lithological mapping with deterioration patterns, it is observed that mostly breccias show these damages. This is likely due to their higher porosity, which has caused them to further deteriorate. It is also observed that most of the damage is in the upper and middle sections of the wall, which could indicate that water absorption occurs due to rainfall run off from the vault.
The patterns associated with biological colonization demonstrate that stone block surfaces are favorable for the growth of organisms. Since this depends on water availability, rain and high moisture content are the primary agents of deterioration in the North Temple. A detailed description and origin of stone deterioration patterns of the North Temple can be found in [42], and the study of the presence biodeterioration caused by microorganisms in several monuments of the Archaeological Zone of Chichen Itza in [64].
An important detail is that most stone blocks have a surface originating from dissolution and precipitation, which hampered lithological identification. However, as explained above, the GRS allowed for the identification between breccias and limestones, as the former exhibit slightly higher radiation. This is valuable when the stone surface is not recognizable, so the GRS can be used as a tool for predicting stone blocks prone to deterioration.
It is worth mentioning that the Archaeological Zone of Chichen Itza is located far from the main large cities, such as the capital, Mérida, so contamination and deterioration patterns commonly found in limestones in urban areas are not present in the study area, at least in the west internal wall of the North Temple.

4.4. Major Elements and Weathering

The results of the major element concentration analysis by X-ray fluorescence (XRF) as well as the MFW index and the MAW are shown in Table 2.
Major element geochemical analysis revealed that calcareous breccias (samples B1 and B2) contain relatively higher amounts of SiO2, Al2O3, Fe2O3, and K2O than limestones (samples C1, C2, C3, CL1, and CL2). Furthermore, limestones with peloids with terrigenous influence (L1 and L2) exhibit intermediate values between the first two. These oxides are therefore interpreted as associated with halloysite and goethite of the paleosoils observed in the petrography. Magnesium oxide is relatively higher in breccias, supporting the observations in thin sections of portions that were partially dolomitized.
Major element concentrations were used to calculate the MFW index proposed by [60]. As indicated above, this index was constructed for igneous and clastic sedimentary rocks. However, since breccias have a continental component, this index was chosen. One advantage is that it uses eight of the ten major oxides in its calculation. The results indicate that breccias (B1 and B2), grainstones (L1 and L2), and soil (CISUE) exhibit extremely high weathering values. These values are due to the fact that their terrigenous components are the end products of the weathering process, i.e., soils. This is consistent with the results of other analyses and with the interpretation of the GRS.
MAW is a chemical parameter correlated with rock density [59,61]. In this study, the values of this parameter were compared with the MFW index and a relationship was found between the two: rocks with a greater terrigenous contribution have a lower MAW, which implies a lower matrix density. These results are consistent with those of [42], who found that, in breccias, the matrix density values are lower than those of limestones. These parameters can be used to correlate geochemical and petrophysical properties, along with GRS data.

4.5. Petrophysical and Mechanical Properties

Results from petrophysical tests for density, porosity, capillary water absorption (w value), and DWL are shown in Table 3.
The matrix densities for all three samples are similar and, compared with the MAW, they have an excellent Pearson correlation of 0.93. Regarding porosity, sample C1 has the lowest value, followed by sample C2, and C3 sample has almost double the porosity of the other two samples. This parameter notably influences the capillary water absorption (w value; Table 3) and the capacity for the salt solution to be absorbed into the pore system (DWL; Table 3).
Capillary water absorption for sample C1 is higher in the X axis than in the Y axis. This can be explained because, as seen in the petrographic analysis, the thicker algal lamination has almost no fenestral porosity compared with the thinner algal lamination of sample C3, which, as expected, absorbed more water through capillarity in the Y axis, the same as sample C2.
Regarding the DWL, none of the three samples lost weight, but gained it according to their porosity: the sample with the highest porosity value (C3) absorbed more salt solution, and the sample with the lowest porosity value (C1) absorbed less [42]. It was found that limestones do not lose weight with the DWL test, because after 100 cycles there were no changes below the original weight value, but for calcareous breccia the sample experienced weight loss after 40 cycles. An important aspect of salt crystallization is that no efflorescence was observed in the North Temple stone blocks, perhaps because it is present as salt solution, although limestones are less affected due to their low porosity compared to calcareous breccias.

5. Discussion

The results obtained using GRS can be interpreted according to rock type and minerals [65]. However, it should be noted that rock properties are modified by weathering processes that ultimately produce geochemical changes, such as loss of certain elements, and petrophysical changes, such as increased porosity [66]. These aspects are discussed below.
Radiation in breccias and limestones of the Archaeological Zone of Chichen Itza is due to the presence of components of continental origin, specifically paleosoil particles and quartz. Of these, the former are characterized by the presence of clays and iron oxides that can adsorb potassium and thorium on their surface [67,68,69,70,71]. Uranium, on the other hand, is not associated with a specific mineral in the limestone but has a marine affinity and is associated with organic matter [72]. It is interpreted that, because the limestones formed in shallow marine environments and have an organic component such as algae, uranium is distributed over a wide range in both breccias and limestones.
Therefore, the distinction between breccias and limestone radiation is notable, as the former contain a greater content of terrigenous particles. Petrographic and geochemical analyses support this interpretation. Indeed, in situ GRS helped in determining lithology even when it was not distinguishable on plain sight, which will undoubtedly contribute to consolidating its use in the study of historical heritage.
In the case of potassium, it is easily removed from minerals due to its mobility [60]. The same is true for uranium, while thorium is generally considered an immobile element [65], being soluble in certain environments. Therefore, if GRS is performed on unweathered rocks and compared with identical weathered rocks, it will be observed that the radiation in the former is higher. On the other hand, if the outcrops are considered rocks with low weathering, they can serve as a reference for comparison with weathered rocks, as is the case with North Temple blocks. It is important to acknowledge that sometimes outcrops of the same rocks as the used for a building are not always known or accessible, so determining weathering/deterioration with stones only from the building should be interpreted with caution.
The GRS showed that outcrops in the Archaeological Zone of Chichen Itza exhibit relatively higher radiation levels than North Temple stone blocks, which is interpreted as an indicator of more dissolution and, therefore, higher radioelements loss. Potassium and thorium are released from terrigenous components and can be used as a measure of the degree of weathering, but uranium, not being associated with a mineral, is not sufficiently reliable to represent this deterioration process. However, considering that uranium is distributed over a wide range in breccias and limestones, it can certainly be used to specifically determine dissolution-related deterioration. Considering in this case the mobility of radioelements, then GRS can be used to describe weathering/deterioration no matter where they reside (crystal lattices or adsorbed) or lithology as demonstrated in other studies for other type of rocks [23,24,25,26].
Furthermore, as shown in Figure 8, breccias vary considerably between their unweathered states in outcrops and their deteriorated states in the North Temple. This is explained by their physical properties, such as their higher porosity than limestones, which reaches up to 25% [42]. Higher porosity can lead to more water entering the pore system, which in turn leads to greater dissolution. In the case of limestones, also shown in Figure 8, the differences between unweathered outcrops and those in the North Temple are less pronounced.
Physical weathering processes cause rock fragmentation, thereby increasing their porosity [61,66], which in turn allows more water and salts to enter in the pore system, if available. If there is more moisture content in the porous system, this can be determined primarily by total radiation [26]. With the petrophysical data of rocks from the Archaeological Zone of Chichen Itza, it is safe to assume that dissolution increases porosity, and this in turn causes the loss of radioelements. Breccias are the most deteriorated, indicating that they have higher porosity, which increases their capacity to retain water in the pores, causing greater dissolution and loss of material in solution [42]. Likewise, potassium and thorium, in addition to other elements, can be lost or released, which would result in lower radiation. So, GRS is useful to identify moisture content and to interpret weathering/deterioration in stone blocks, providing insights into some of their characteristics and targeting blocks that need conservation techniques.
In relation to this assessment of petrophysical and mechanical properties, it should be clarified that GRS, like any other NDT, must be complemented with other analyses for a better understanding of deterioration processes. Some considerations regarding the NDTs described in the Introduction section and GRS are detailed next. First, since elastic properties and wave velocity [1,3,4] depend on material density, a combination of NDTs and GRS can be used for comparison, specifically because dissolution of limestones affects the percentage of porosity and ultimately the bulk density. Moreover, the increased porosity can cause more water to reside in the stone blocks, causing radiation attenuation. Therefore, moisture measurements [2,5,6,26] and GRS are NDTs that can be well correlated, as demonstrated in this study. Another property measurable by means of NDTs is magnetic susceptibility; in particular, ref. [7] evaluated magnetic susceptibility and GRS to compare their responses in granites from different regions to determine provenance. This NDT can offer another tool to compare outcrop response with that of buildings to assess deterioration. Similarly, ref. [8] used magnetic susceptibility to assess soiling in facades, although further studies about this property and GRS should be carried out.
Since limestones with the same terrigenous input described here have been found in other buildings in the area [73], GRS may offer good results if this methodology is widely applied. It is also possible to compare the regression lines of the outcrops with those of the other monuments in the Archaeological Zone of Chichen Itza, since they were built with the same rock type. With these, their moisture content could be determined by relating it to lower radiation, which will also be related to porosity. Furthermore, considering that the breccias have higher porosity and are more deteriorated, the GRS could be used as an indicator of possible future damage when radiation readings are high, i.e., in breccias.
As described earlier, several studies carried out in the Archaeological Zone of Chichen Itza used electrical resistivity tomography to detect buried structures [29,30]. Unfortunately, GRS cannot be applied for this purpose because soil moisture causes radiation attenuation, and the maximum depth of investigation of GRS is approximately 1 m in special conditions, and 0.5 m normally [27]. However, there have been attempts to use GRS for buried structures recognition in zones with a thin soil cover [71,74]. On the other hand, the use of electrical resistivity to evaluate salt solubility and moisture [31] is an example of an NDT that would complement GRS data to assess moisture content and distribution. Finally, portable X-ray fluorescence equipment, such as that utilized to determine the chemical composition of the Throne of the Red Jaguar sculpture [32], may be useful in some materials because can it identify more chemical elements (Na to U) than GRS (K, U Th). However, considering that limestones are composed mainly of calcium carbonate (CaCO3), portable X-ray fluorescence equipment would be capable only of detecting its chemical components as the presence of other elements would be obscured. Inversely, GRS proved to be a reliable tool to identify the radioelements distributions in limestones.
According to the results obtained in this study and comparing them with those of [26] in volcanic tuff, it was verified that the in situ GRS methodology in buildings is applicable for different rocks, regardless of the concentration of radioelements, and monuments with different exposure times and in different environmental and climatic conditions. Nevertheless, some limitations regarding GRS should be pointed out. Because of the random nature of the nuclear disintegration of radioelements, the values obtained will never be exactly the same, but they will be similar (although this is easily overcome using the suggested measurement time between 2 and 3 min). Also, GRS can be affected by atmospheric radon, which has elevated concentrations after rain [27], so measurements must be taken hours or days later, allowing radon concentrations to reach normal levels. This situation represents a limitation in areas with frequent rain, although GRS could be planned during the dry season. Like any other NDT, the GRS should be compared with other analysis, and in the Archaeological Zone of Chichen Itza, the following investigations should be focused on correlating data from NDTs and GRS, particularly the ones providing resistivity and elastic properties data.

6. Conclusions

The gamma-ray spectrometry was successfully applied to assess the limestone and calcareous breccia used in the North Temple. By applying the methodology for in situ gamma ray spectrometry, it was found that radiation and radioelement concentrations are lower in the North Temple stone blocks than in the outcrops, meaning that gamma-ray spectrometry can be used to determine weathering. Potassium and thorium are the two most reliable elements for this purpose, as their loss can be related to clasts of terrigenous origin containing clay and iron oxides. Uranium, on the other hand, is only reliable in terms of general dissolution, as its concentration is not related to any mineral, but is distributed throughout the matrix and associated with organic matter. Considering these results, and as previous studies have shown, gamma-ray spectrometry can be used for building stone assessment in different types of rocks.
By means of the in situ gamma-ray spectrometry, it was possible to identify differences between breccias with terrigenous particles, which causes them to have higher radiation than limestones without the terrigenous particles. This allowed us to distinguish between the two rock types, although this is not easy to do on plain sight. This differentiation showed that the breccias are more deteriorated because they have lost more radioelements due to their greater porosity. Therefore, gamma-ray spectrometry allowed us to assess the relationship between porosity and deterioration in the stones from the Archaeological Zone of Chichen Itza and can be used in other Maya sites built with limestones. Furthermore, deterioration and the results of gamma-ray spectrometry can be compared to limestones in cities like Mérida, capital city of Yucatán to assess differences in radioelements mobility due to industrialization.
The advantage of in situ gamma-ray spectrometry is that it is a non-destructive test, and this article demonstrates that it has great potential to become a routine test for assessing building stones, even when their radioelement concentration is very low. Also, considering the results obtained in this research, gamma-ray spectrometry can be applied in different conditions, in this case, a hot and tropical climate.
Gamma-ray spectrometry, like any other non-destructive test, should be complemented with laboratory analysis and compared to data from non-destructive tests, since no technique alone can assess all building stone properties or deterioration processes. A combination of all these tests permits a better understanding of past and ongoing deterioration processes to make it possible to supervise the conditions of building stones in cultural heritage and prevent their decay.

Author Contributions

Conceptualization, methodology, A.M.-G. and V.Y.; validation, A.M.-G., V.Y., R.A.L.-D., C.A.G.-S. and A.A.-R.; formal analysis, A.M.-G.; investigation, A.M.-G. and V.Y.; resources, V.Y., R.A.L.-D., C.A.G.-S. and A.A.-R.; data curation, A.M.-G.; writing—original draft preparation, A.M.-G.; writing—review and editing, visualization, A.M.-G., V.Y., R.A.L.-D., C.A.G.-S. and A.A.-R.; supervision, V.Y., R.A.L.-D., C.A.G.-S. and A.A.-R.; project administration, V.Y., R.A.L.-D.; funding acquisition, V.Y. and R.A.L.-D., C.A.G.-S. and A.A.-R. All authors have read and agreed to the published version of the manuscript.

Funding

The APC was partially funded by the Instituto Potosino de Investigación Científica y Tecnológica, A.C. The first author acknowledges the financial support for PhD studies (CVU 926119) by the Secretaría de Ciencia, Humanidades, Tecnología e Innovación.

Data Availability Statement

Data sets generated during the current study are available from the corresponding author on reasonable request.

Acknowledgments

The authors acknowledge the Instituto Potosino de Investigación Científica y Tecnológica, and the Instituto de Geología, Universidad Autónoma de San Luis Potosí for the use of their laboratories and equipment; and the National Institute of Anthropology and History for allowing these studies at Chichén Itzá. The authors acknowledge Lidia Miroslava Lerma-Pérez for field assistance, and Erasmo Mata-Martínez for laboratory support. The authors acknowledge the four anonymous reviewers for the valuable observations that allowed the improvement of this manuscript.

Conflicts of Interest

The authors declare no conflicts of interest. The authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript.

Abbreviations

The following abbreviations are used in this manuscript:
NDTNon-destructive test
GRSGamma-ray spectrometry
YPYucatan Platform
XRDX-ray diffraction
MAWMean atomic weight
MFWMafic-felsic-weathering
DWLDry weight loss
cpmCounts per minute
XRFX-ray fluorescence

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Buildings 15 02949 g001
Figure 1. Localization of the Archaeological Zone of Chichen Itza and its main monuments. The yellow circle represents the North Temple of the Great Ball Court. The red circles indicate the studied outcrops.
Figure 1. Localization of the Archaeological Zone of Chichen Itza and its main monuments. The yellow circle represents the North Temple of the Great Ball Court. The red circles indicate the studied outcrops.
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Figure 2. (a) Main facade of the North Temple; (b) west internal wall where the GRS was performed.
Figure 2. (a) Main facade of the North Temple; (b) west internal wall where the GRS was performed.
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Figure 3. Regional geologic map of the YP around the study area from Eocene (Chichen Itza Formation) to Miocene–Pliocene (Carrillo Puerto Formation). Modified from [49].
Figure 3. Regional geologic map of the YP around the study area from Eocene (Chichen Itza Formation) to Miocene–Pliocene (Carrillo Puerto Formation). Modified from [49].
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Figure 4. Plots on the left represent the original calcareous breccia data recorded in the North Temple. The discarded data are in shown dark red and in orange the data used for modeling the regression line are shown in orange. Plots on the right show the corrected data, also including the previously discarded values which were corrected according to the regression line.
Figure 4. Plots on the left represent the original calcareous breccia data recorded in the North Temple. The discarded data are in shown dark red and in orange the data used for modeling the regression line are shown in orange. Plots on the right show the corrected data, also including the previously discarded values which were corrected according to the regression line.
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Figure 5. Plots on the left represent the original limestone data recorded in the North Temple. In dark blue are the discarded data and in light blue the data used for modeling the regression line. Plots on the right show the corrected data, also including the previously discarded values which were corrected according to the regression line.
Figure 5. Plots on the left represent the original limestone data recorded in the North Temple. In dark blue are the discarded data and in light blue the data used for modeling the regression line. Plots on the right show the corrected data, also including the previously discarded values which were corrected according to the regression line.
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Figure 6. Plots on the left represent the original calcareous breccia data recorded in the outcrops. The discarded data are shown in dark red and the data used for modeling the regression line are shown in orange. Plots on the right show the corrected data, including also the previously discarded values which were corrected according to the regression line.
Figure 6. Plots on the left represent the original calcareous breccia data recorded in the outcrops. The discarded data are shown in dark red and the data used for modeling the regression line are shown in orange. Plots on the right show the corrected data, including also the previously discarded values which were corrected according to the regression line.
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Figure 7. Plots on the left represent the original limestone data recorded in the outcrops. The discarded data are shown in dark blue and in the data used for modeling the regression line are shown in light blue. Plots on the right show the corrected data, including also the previously discarded values which were corrected according to the regression line.
Figure 7. Plots on the left represent the original limestone data recorded in the outcrops. The discarded data are shown in dark blue and in the data used for modeling the regression line are shown in light blue. Plots on the right show the corrected data, including also the previously discarded values which were corrected according to the regression line.
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Figure 8. Box plot for GRS data calculated for a moisture content of 0.0%. Outcrop and North Temple calcareous breccia data are shown in orange and dark orange, respectively. Outcrop and North Temple limestone data are shown in light blue and dark blue, respectively. Circles and × indicate all GRS data and mean values, respectively.
Figure 8. Box plot for GRS data calculated for a moisture content of 0.0%. Outcrop and North Temple calcareous breccia data are shown in orange and dark orange, respectively. Outcrop and North Temple limestone data are shown in light blue and dark blue, respectively. Circles and × indicate all GRS data and mean values, respectively.
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Figure 9. Lithological mapping for the west internal wall of the North Temple. Calcareous breccias are represented in orange, and limestones in light blue.
Figure 9. Lithological mapping for the west internal wall of the North Temple. Calcareous breccias are represented in orange, and limestones in light blue.
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Figure 10. Photomicrographs of the most important petrographic analysis results. All were taken at 10× magnification and in uncrossed nicols, unless otherwise indicated. (a) Rounded paleosoil particle; (b) quartz crystal with fractures and reddish alteration (crossed nicols); (c) boundary between thick algal lamination and micrite matrix (5× magnification); (d) example of a thinner algal lamination; (e) microfossils in a micritic matrix. Note the fenestral and intraparticle porosity (crossed nicols); (f) peloids with terrigenous input in sparitic cement. Note the fenestral and interparticle porosity (crossed nicols).
Figure 10. Photomicrographs of the most important petrographic analysis results. All were taken at 10× magnification and in uncrossed nicols, unless otherwise indicated. (a) Rounded paleosoil particle; (b) quartz crystal with fractures and reddish alteration (crossed nicols); (c) boundary between thick algal lamination and micrite matrix (5× magnification); (d) example of a thinner algal lamination; (e) microfossils in a micritic matrix. Note the fenestral and intraparticle porosity (crossed nicols); (f) peloids with terrigenous input in sparitic cement. Note the fenestral and interparticle porosity (crossed nicols).
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Figure 11. Diffractogram for the red soil sample. Halloysite (H), quartz (Q), goethite (G).
Figure 11. Diffractogram for the red soil sample. Halloysite (H), quartz (Q), goethite (G).
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Figure 12. (a) Features induced by material loss; (b) millimetric cavities denominated pitting.
Figure 12. (a) Features induced by material loss; (b) millimetric cavities denominated pitting.
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Table 1. Results from the petrographic analysis and rock classification.
Table 1. Results from the petrographic analysis and rock classification.
SampleMatrixAllochemsMain ClassificationSecondary Classification
B1MicriteClay intraclasts, quartzFloatstoneMudstone–wackestone
DolosparitePeloidsGrainstone
B2MicriteCalcareous and clay intraclastsFloatstoneMudstone
DolosparitePeloidsGrainstone
C1MicriteCoralline and green algae, benthic foraminifera, gastropodsBindstoneMudstone–packstone
C2MicriteCoralline and green algae, benthic foraminifera, gastropodsWackestone-packstoneMudstone
C3MicriteCoralline algaeBindstoneMudstone
CL1MicriteCoralline algaeBindstoneMudstone
SparitePeloids, benthic foraminiferaGrainstone
CL2MicriteCoralline and green algae, benthic foraminifera, gastropods, peloidsBindstoneMudstone–packstone
Micrite-spariteGrainstone
L1SpariteCalcareous intraclasts, peloidsGrainstoneWackestone
MicriteCoralline algae, benthic foraminiferaMudstone
L2SpariteOncoids, peloidsGrainstoneMudstone
MicriteCoralline algae
CIACSpariteBenthic foraminifera, peloidsGrainstoneMudstone
Micrite
Table 2. Major elements (wt. %) performance with XRF method, MFW index, and MAW.
Table 2. Major elements (wt. %) performance with XRF method, MFW index, and MAW.
SampleB1B2C1C2C3CL1CL2L1L2CIACCISASCISUE
SiO25.283.93<0.05<0.05<0.05<0.05<0.050.181.081.150.6243.02
TiO20.2020.1720.005<0.004<0.0040.005<0.0040.0100.0280.0360.0051.029
Al2O34.012.820.130.020.190.100.020.410.991.160.5029.07
Fe2O31.3071.111<0.006<0.006<0.006<0.006<0.006<0.0060.2180.132<0.0067.943
MgO0.0700.1720.0040.0040.0030.0030.0040.0050.0050.0110.0040.086
MnO0.5770.2950.3630.1850.5880.3220.2280.4020.9550.2210.2440.817
CaO48.77651.45657.12758.13156.39356.99857.85256.75254.80555.67956.8421.512
Na2O<0.003<0.0030.0050.004<0.0030.0180.014<0.003<0.003<0.003<0.0030.024
K2O0.2840.2530.0490.0380.0600.0350.0330.0760.1120.1030.1081.527
P2O50.0330.0260.0200.0140.0200.0220.0160.0180.0260.0140.0120.086
LOI39.1939.5442.1541.3242.1742.3641.8341.5541.8241.3541.7514.61
Total99.72999.77599.85399.71699.42499.86399.99799.403100.03999.856100.08599.727
MAW 126.4426.8927.9427.9927.8827.9527.9827.8627.5027.5927.7921.22
MFW 299.6399.5132.0718.9752.526.914.0477.7896.4797.2579.4299.66
1 Mean atomic weight calculated according to [59]. 2 MFW index calculated according to [60].
Table 3. Petrophysical properties determined in laboratory.
Table 3. Petrophysical properties determined in laboratory.
SampleMatrix Density (g/cm3)Bulk Density (g/cm3)Porosity (%)W Value X Axis (kg/m2t−1)W Value Y Axis (kg/m2t−1)Anisotropy (%)DWL (%)
C12.672.3910.481.211.0215.70−0.78
C22.712.3313.930.831.3136.64−1.40
C32.662.0124.412.704.6241.56−2.15
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Méndez-Gaona, A.; Yutsis, V.; López-Doncel, R.A.; García-Solís, C.A.; Aguillón-Robles, A. Insights into Gamma-Ray Spectrometry of Building Stones in the North Temple of the Great Ball Court, Archaeological Zone of Chichen Itza, Mexico. Buildings 2025, 15, 2949. https://doi.org/10.3390/buildings15162949

AMA Style

Méndez-Gaona A, Yutsis V, López-Doncel RA, García-Solís CA, Aguillón-Robles A. Insights into Gamma-Ray Spectrometry of Building Stones in the North Temple of the Great Ball Court, Archaeological Zone of Chichen Itza, Mexico. Buildings. 2025; 15(16):2949. https://doi.org/10.3390/buildings15162949

Chicago/Turabian Style

Méndez-Gaona, Alejandro, Vsevolod Yutsis, Rubén Alfonso López-Doncel, Claudia Araceli García-Solís, and Alfredo Aguillón-Robles. 2025. "Insights into Gamma-Ray Spectrometry of Building Stones in the North Temple of the Great Ball Court, Archaeological Zone of Chichen Itza, Mexico" Buildings 15, no. 16: 2949. https://doi.org/10.3390/buildings15162949

APA Style

Méndez-Gaona, A., Yutsis, V., López-Doncel, R. A., García-Solís, C. A., & Aguillón-Robles, A. (2025). Insights into Gamma-Ray Spectrometry of Building Stones in the North Temple of the Great Ball Court, Archaeological Zone of Chichen Itza, Mexico. Buildings, 15(16), 2949. https://doi.org/10.3390/buildings15162949

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