Monument Future. Siegfried Siegesmund

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      Figure 1: Examples of the historical use of the studied tuffs and relevant geographic locations: a) Hovhannavank Monastery, Ohanavan (Armenia), 5th c.; b) Templo del Carmen, San Luis Potosí (Mexico), 17th–18th c.; c) Eger Castle, Eger (Hungary), 13th–18th c.; d) Porphyry house, Chemnitz (Germany), 1868.

      Miocene volcanic activity produced a large amount of welded and unwelded pumiceous tuffs in Hungary. Besides the prevailing rhyolitic composition, dacitic and andesitic tuffs also formed in Eastern Hungary. Rhyolitic tuffs are particularly common in a 50 × 10 km area known as Bükkalja Volcanic Field in Northern Hungary.

      Emblematic monuments such as the castle or the minaret of Eger were constructed from this material (Figure 1c).

      Significant volcanic tuff deposits in Germany are mainly found in the eastern and western Central part of the country. Permian volcanism led to the deposition of the Hilbersdorf tuffs in eastern Central Germany, near the city of Chemnitz.

      The Quaternary Weibern tuff is located in western Central Germany, in the municipality of Weibern.

      Prominent examples of its application as building stone are, e. g. the Kaiser Wilhelm Memorial Church in Berlin, the Castle Church of Chemnitz or the porphyry house of master stonemason Findewirth in Chemnitz (Figure 1d).

       Methods

      The tuffs were described using polarized-light microscopy. Bulk and material density, porosity (EN 1936:2006), and water absorption at atmospheric pressure (EN 13755:2008) were measured. Pore-size distribution was analyzed by mercury intrusion porosimetry (MIP) (ASTM D4404-84). Ultrasonic 133p-wave velocity was recorded by direct transmission method (EN 14579:2004). Strength parameters such as uniaxial compressive strength (UCS), tensile strength (ASTM D3967-16) were measured, and Young’s modulus of elasticity was also recorded (ASTM D7012-14). Durability was assessed by resistance to freeze-thaw (EN 12371:2001 with modifications) and resistance to salt attack (EN 12370:1999).

       Petrology and Mineralogy

      The creamy rhyolitic tuffs from Northern Hungary are characterized by a porphyritic texture with plagioclase phenocrysts. The pumice content is high, and the groundmass is mostly glassy (Figure 2). Critical parameters associated with high textural variability are crystal-groundmass-pumice ratio; grain size; welding degree. These features may vary in outcrops a few kilometres away, and even within the same quarry, to a lesser extent (Germinario & Török 2019).

      The German Hilbersdorf Regular tuff, appears in irregularly speckled and marbled in pale pink to dark purple and bright beige to greenish colours. A variety of elongated lapilli inclusions are embedded in the groundmass and millimetre to centimetre-large cavities are partly filled with loose, clayey material. It has a porphyritic texture with mono- and polycrystalline quartz, muscovite, hematite, calcite, feldspar relics and lithic clasts in the glassy matrix (Figure 2). The porphyritic Weibern tuff consists of a fine-grained yellowish-brownish matrix in which pumice lapilli of yellow colour and partly elongated clasts of different but mostly grey colour are embedded. The rock fragments are mostly sandstone and shale as well as basalt and volcanic glass fragments. The matrix mainly consists of analcime, muscovite/illite, quartz and calcite (Wedekind et al. 2013).

      The Armenian Hoktemberyan tuffs are trachydacites, while the Golden Armenia varieties have rhyolitic composition. The most popular Hoktemberyan tuff is of characteristic brick-red color, but transitions to orange-brownish and blackish also exist, often given different local trade names. In its fine-grained brick-red groundmass (~75 %), small elongated pumice clasts of red and black colour are embedded. Grey to black rock fragments, as well as huge amounts of white, elongated feldspars and glass particles in the millimetre range give a slightly speckled appearance. Thin-section analyses show feldspar and amphibole phenocrysts as well as volcanic lithoclasts and hematite located in a cryptocrystalline matrix (Figure 2). The yellowish-golden groundmass of Golden Armenia tuff embeds beige, grey and slightly reddish clasts. In thin section, volcanic lithoclasts, quarz phenocrysts, feldspar relics and vitric fragments are oberserved in a glass-rich matrix. Considerable amounts of swellable clay minerals (corrensite) are verified in Pötzl et al. (2018b).

      All the Mexican volcanic tuffs of SLP have rhyolitic composition. The percentage of crystals and matrix vary of 30 %–70 %. The rocks contain mainly 134quartz, with an average abundance of 45 %, alkali feldspar (mostly sanidine and orthoclase) with 35 %, and plagioclase (oligoclase to anorthite) not exceeding 30 %. The texture of these rhyolitic ignimbritic tuffs varies from porphyritic hypocrystalline to vitrophyric (Figure 2).