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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).

Figure 2: Thin-section photomicrographs in plane- and cross-polarized light from a selection of the studied tuffs (see detailed explanations in the text).