Mordenite occurs in hydrothermally altered rhyolitic volcanic rocks at the margin of a Late Miocene lake basin on the island of Samos, Greece. The mordenite was identified by its X-ray powder diffraction pattern and appearance in scanning electron micrographs. Electron microprobe analyses show high Si, Ca, and K, low Na, and high balance errors. The b cell dimension is consistent with a high Si/(Al + Si) ratio, and the balance errors are apparently due to Na deficiency. The missing Na was estimated from the amount required to give (Si + Al) = 48 and yielded analyses comparable with X-ray fluorescence analyses of samples predominantly of mordenite. Even after this correction, the mordenite had a low Na:K ratio compared with most analyses reported in the literature. The peculiar chemistry of the mordenite may have resulted from a high-temperature metasomatic origin as a result of basaltic volcanism at the basin margin and characterized by hydrothermal circulation of alkaline lake water rich in K.

We report on the newly discovered almandine garnet phenocrysts in rhyolitic ignimbrites (Sukhda Tuff) in the Precambrian Churtela Shale Formation of the Chhattisgarh Supergroup in central India. SHRIMP ages of igneous zircon from the ignimbrites range from 990 Ma to 1020 Ma. These ignimbrites exhibit characteristic eutaxitic texture with compacted curvilinear glass shards with triple junctions. Quartz (commonly embayed; bluish cathodoluminescence) and albite (altered but retaining ghosts of twinning) are common phenocrysts; others are apatite, ilmenite, rutile, magnetite, zircon, monazite and garnet. There are no metamorphic or granitic xenoliths in the ignimbrites. Garnet grains occur as isolated broken isotropic crystals with sharp or corroded boundaries in a very fine-grained groundmass of volcanic ash that consists principally of albite, quartz, magnetite and glass. They do not have any systematically distributed inclusions. A few have penetratively intergrown phenocrysts of apatite, ilmenite, rutile and zircon, which we interpret as subophitic texture. Extensive SEM-BSE imaging of more than 100 grains and electron microprobe traverses across about 30 grains showed no zoning or systematic compositional variability. Common (metamorphic) garnets are usually zoned with respect to Fe–Mg–Mn and typically have mineral inclusions. We infer, therefore, that these observed garnets are not metamorphic xenocrysts. The average major oxide composition of analysed garnets from five different horizons within the Sukhda Tuff, spanning approximately 300 m of the stratigraphic section, have very small standard deviation for each element, which is suggestive of a single magmatic source. Phenocrysts of quartz, including those in contact with coexisting garnets, show blue scanning electron CL, indicating rapid cooling from high temperature; this suggests that adjacent coexisting garnets are not slowly cooled restites. We conclude, on the basis of texture, mineral chemistry and absence of any indicative xenoliths or xenocrysts, that these almandine garnets (Al78.7Py12.3Gr7.4Sp1.6) are phenocrysts within the Sukhda Tuff. Almandine of such composition is stable under high pressure. We infer that almandine crystallized at lower crustal depths in a magma that ascended very rapidly and may have erupted explosively.