target="_blank" rel="nofollow" href="#ulink_99923d60-0f51-540e-9122-b6da36619e57">1 Geoscience Centre of the Georg August University Göttingen, Germany
2 Geological Institute of the Autonomous University of San Luis Potosí, Mexico
3 Federal Institute for Geosciences and Natural Resources (BGR), Hanover, Germany
Abstract
The weathering of natural building stones in the South of Mexico is mostly controlled by the influence of atmospheric and meteoric water, thermal stress and the input of salts from the environment. Twelve varieties of volcanic tuff rocks from South and Central Mexico were analyzed regarding their petrographical and petrophysical properties, as well as their weathering behavior. The tuffs show a broad range of properties and differential weathering behavior. Moisture properties like water uptake, water vapor diffusion and hygroscopic water sorption as well as hydric expansion and salt crystallization show great dependence on the pore space properties and the content of swellable clay minerals or zeolites. The deterioration during salt bursting tests is controlled by both salt crystallization pressure and hydric expansion. Especially zeolite-rich samples show intense water-related weathering. Their sorption values and hydric expansion are high, while capillary water uptake is comparably low. The clay and zeolite-rich tuffs furthermore suffer from intense shrinking and fracturing during drying. The very high hydric expansion superimposes the salt crystallization pressure and the effect of thermal dilatation.
Introduction
Zeolites have an important influence on the weathering behaviour of natural building stones. Korkuna et al. (2006) found very small pore sizes for zeolite-rich samples (< 2 nm) resulting in a high specific surface area. The specific surface area, along with the pore size and connectivity of the pores is a controlling factor for water transport and retention in porous rocks and has a great influence on values like capillary water uptake, water vapor diffusion and hygroscopic water sorption (Siegesmund and Dürrast 2011).
Hydric dilatation can cause a significant volume change in tuff rocks and is therefore an important weathering factor. Reasons for hydric expansion may be the presence of swellable clay minerals or a large percentage of micropores (e. g. Gonzales and Scherer 2004). The swelling of natural building stones with a high percentage of micropores may be explained by the process of disjoining pressure, but the process is still under discussion (e.g. Ruedrich et al. 2011; Wedekind et al. 2013; Pötzl et al. 2018a; Pötzl et al. 2018b).
Zeolites adsorb and desorb water molecules reversibly (Di Tchernev 1978). They are therefore able to store heat during desorption and give it back to the environment during adsorption. Between 0 °C and 90 °C zeolites counter the expected thermal 120expansion because of shrinkage due to reversible water loss (Colella et al. 2001). Zeolites accordingly have an influence on the thermal expansion behavior of tuff rocks.
In this study zeolite and clay-rich tuff rocks from the states of Oaxaca in Southern Mexico and Queretaro in Central Mexico were investigated and compared with zeolite-free samples.
Petrography
The investigated samples are mainly felsic pyroclastic rocks. Cantera Verde Oaxaca (CVO), Cantera Amarilla Oaxaca (CAO) and Cantera Rosa Oaxaca (CRO) are tuff rocks from the city of Oaxaca de Juárez. Cantera Verde Etla (CVE), Cantera Amarilla Etla (CAE) and Cantera Rosa Etla (CRE) are from the city of Etla (20 km to the northwest of Oaxaca de Juárez). Mitla Gris (MG) and Mitla Rosa (MR) are from the village Mitla 45 km southeast of Oaxaca. Querétaro Blanco (QB), Querétaro Melon (QM), Querétaro Amarilla (QA) and Querétaro Naranja (QN) are from the state of Querétaro in Central Mexico. Lithoscans and thin section images of all samples are presented in figures 1 and 2.
Cantera Verde Oaxaca (CVO) is a homogenous, dark-green vitric, rhyolitic ash tuff with massive to dense texture and weak orientation of pumice clasts (Fig. 1a). The glassy matrix is strongly altered to zeolite and clay minerals and is partly devitrified. Cantera Verde Etla (CVE) is a homogenous, light-pistaccio green vitrophyric ash tuff of rhyolitic composition. Noticable macrosopically are white clay lenses (Fig. 1b), which disintegrate when in contact with water. CVE has a hypocrystalline-cryptocrystalline matrix with vitrophyric texture. The crystals are often strongly weathered (Fig. 2b). The matrix is rich in zeolites (clinoptilolite and mordenite). SiO2 is present in the form of crystobalite. Pablo-Galán (1986) describes the formation of zeolites in the Etla tuff by alkaline diagenesis of rhyolitic glass. He distinguishes between two varieties of tuff, a clinoptilolite-rich variety and a mordenite-rich variety.
Cantera Amarilla Oaxaca (CAO) is a homogenous, yellow to white vitric ash tuff of rhyolitic composition (Fig. 1c). The rock is very soft and contains a high amount of swellable clay minerals (Tab. 1). Sometimes foliation can be observed in the ash layers. Lithoclasts of up to 0.5 cm are present in the cryptocrystalline to glassy matrix (Fig. 2c).
Figure 1: Lithoscans of the investigated tuffs. a) CVO, b) CVE, c) CAO, d) CAE, e) CRO, f) CRE, g) MG, h) MR, i) QB, j) QM, k) QA, l) QN.
Cantera Amarilla Etla (CAE) is a yellow to orange rhyolitic, vitric ash tuff with characteristic layering (Fig. 1d). The orange layers or rings are colored by iron oxides and make the orientation of the sample clearly visible. The grain size is small (up to 3 mm) and the glassy matrix is homogenous (Fig. 2d). The tuff contains zeolites and swellable smectite (Tab. 1).
Cantera Rosa Oaxaca (CRO) is a homogenous, fine- to medium-grained rhyolitic ash tuff of red to pink color with a glassy matrix and small crystals < 1 cm (Fig. 1e). The rock easily loses sand-sized particles on the surface. The thin section reveals an eutaxitic texture (Fig. 2e) with high amounts of welded pumice and glass. The sample is strongly devitrified in parts and contains clay lenses where the glass is altered.
121Table 1: CEC, BET as well as clay mineral content and zeolites of the investigated tuff rocks.
Cantera Rosa Etla (CRE) is a rhyolitic, vitric ash tuff and very similar to CRO in terms of texture, grain size and color (Fig. 1f). The thin section shows the similarity of CRO and CRE (Fig. 2e–f). CRE contains smaller grains with smaller pores and the color is slightly brighter than CRO.
The Mitla Tuff Gris (MG) is a rhyolic to dacitic crystal tuff of gray to white color (Fig. 1g). It has very variable porosities depending on the origin within the quarry. MG contains pumice clasts and angular crystals in a glassy matrix (Fig. 2g).
Mitla Tuff Rosa (MR) is a dense, rhyolitic tuff with small pores and crystals of up to 3 mm size. It has a pink matrix, which is in some parts discolored to white (Fig. 1h). The thin section (Fig. 2h) shows an overall small proportion (10 %) of cryptocrystalline to microcrystalline matrix with seriate texture. The crystals are hypidiomorphic and poorly altered.
Querétaro Blanco (QB) is a white to gray rhyolitic lapilli tuff with a high amount of pumice
