Читать книгу Monument Future - Siegfried Siegesmund - Страница 165

The investigated samples are mainly felsic pyroclastic rocks. Cantera Verde Oaxaca (CV_O), Cantera Amarilla Oaxaca (CA_O) and Cantera Rosa Oaxaca (CR_O) are tuff rocks from the city of Oaxaca de Juárez. Cantera Verde Etla (CV_E), Cantera Amarilla Etla (CA_E) and Cantera Rosa Etla (CR_E) 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 (CV_O) 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 (CV_E) 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. CV_E 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 (CA_O) 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) CV_O, b) CV_E, c) CA_O, d) CA_E, e) CR_O, f) CR_E, g) MG, h) MR, i) QB, j) QM, k) QA, l) QN.

Cantera Amarilla Etla (CA_E) 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 (CR_O) 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 (CR_E) is a rhyolitic, vitric ash tuff and very similar to CR_O in terms of texture, grain size and color (Fig. 1f). The thin section shows the similarity of CR_O and CR_E (Fig. 2e–f). CR_E contains smaller grains with smaller pores and the color is slightly brighter than CR_O.

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 and glass within the matrix. Lithic fragments of more than 1 cm size are macroscopically visible in the lithoscan (Fig. 1i). The thin section of QB shows a vitrophyric texture with very few crystals and lithoclasts embedded in a glassy matrix (Fig. 2i).

Figure 2: Thin sections of he investigated tuffs. a) CV_O, b) CV_E, c) CA_O, d) CA_E, e) CR_O, f) CR_E, g) MG, h) MR, i) QB, j) QM, k) QA, l) QN with parallel nicols except g and h with crossed nicols.

Querétaro Melon (QM) is a rhyolithic, vitric lapilli tuff of orange-melon color with lithic fragments, as well as small crystals and big clasts of pumice in a glassy matrix (Fig. 1j). QM shows an eutaxitic texture with firmly welded glassy matrix and few smaller crystals under the microscope (Fig. 2j).

Querétaro Amarillo (QA) is a dacitic, heterogenous tuff breccia with clasts of several centimeters in size. The matrix is yellowish-brown and contains clasts that are often whitish-gray. The clasts are partly pumice, mostly xenoliths and a small amount of crystals (Fig. 1k). The matrix of QA is 122strongly altered to clay minerals. The pumice are mostly unwelded and show little signs of elongation or compression (Fig. 2k).

Querétaro Naranja (QN) is a vitric lapilli tuff of trachydacitic composition with lithic fragments in the centimeter scale. Small fragments of orange pumice and strongly altered crystals in a devitrified matrix of orange color are visible in Figure 1l. The thin section shows round and angular xenocrysts and altered feldspar in the fine-grained matrix (Fig. 2l).

In sum, all samples have an acidic composition. Particularly glass-rich samples are CR_O, CR_E, QM, QA and QN. Feldspar could be detected in all samples, while quartz was not detected by XRD in CV_E, CR_E, QB and QM. CV_E and QB instead contain the high temperature modifications of SiO₂ (tridymite and cristobalite). The cation exchange capacity (CEC) and the specific surface area (BET), as well as the content of clay minerals and zeolites in all samples are presented in Table 1.

Samples with significant amounts of zeolites are CV_O, CV_E and CA_E. The specific surface area (BET) is high for these samples. The main zeolites are heulandite and clinoptilolite, minor amounts of mordenite were detected in CV_E. Clay minerals were detected in all samples. The content of swellable clay minerals indicated by a high CEC value is especially high in CA_O, MG and CV_O. Smectite and muscovite-illite make up the main content of the clay fraction. Chlorite was only detected in CV_E and kaolinite was found in MG and could be present in small amounts in CV_O, CV_E, CA_O and QA.