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An exceptional new occurrence of the mineral tamarugite, NaAl(SO 4 ) 2 · 6H 2 O, from a short karst cavity (Diana Cave, Baile Herculane; SW Romania) is described. It was formed by corrosion of the bedrock (limestone and marls) by a SO 4 2− -rich steam condensate resulting from oxidized S 2− ions escaping from the thermo-mineral water emerging from depth in the cave. Tamarugite forms dull white earthy aggregates. Scanning-electron microscope (SEM) observations reveal tabular subhedral crystals never exceeding 15 μm across. The cell parameters refined from the powder data for the monoclinic space group P 2 1 / a are: a = 7.358(6), b = 25.23(2), c = 6.093(5) A, β = 95.16(5) °, V = 1126.98(1) A 3 . The δ 34 S values of the cave sulphates and the thermal water confirm marine evaporites as the source of sulphur. The sulphate-acid alteration of limestone with contribution of Al 3+ and Na + from the marls and the thermal water is responsible for the formation of tamarugite. The steam-condensate alteration paragenesis includes native sulphur, bassanite, anhydrite, epsomite, pickeringite, halotrichite, apjohnite and alunite, as well as quartz and halite, all primary and secondary speleogenetic by-products.

Eighteen minerals belonging to eight chemical groups were identified from three caves within Salitrari Mountain, in the upper Cerna River basin (Romania) by means of scanning electron microscopy, electron microprobe analysis, and X-ray powder diffraction. One passage in the Great Cave from Salitrari Mountain, the largest cave investigated, exhibits abnormal relative humidity and temperature ranges, allowing for a particular depositional environment. The cave floor is covered by alluvial sediments (ranging from cobble, sand, and clay to silt-sized material), bear bones, bat guano, and rubble. These materials reacted with percolating meteoric water and hydrogen sulfide-rich hypogene hot solutions, precipitating a variety of secondary minerals. Most of these minerals are common in caves (e.g. calcite, gypsum, brushite), however, some of them (alunite, aluminite, and darapskite) require very particular environments in order to form and persist. Cave passage morphologies suggest a complex speleogenetic history that includes changes from phreatic to vadose conditions. The latter was punctuated by a sulfuric acid dissolution/precipitation phase, partly overprinted by present-day vadose processes. The cave morphology and the secondary minerals associated with the alluvial sediments in these caves are used to unravel the region’s speleogenetic history.