Pseudomorphs of fibrous gypsum on gypsum roses, with minor amounts of inclusions of anhydrite and bassanite

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Exceptionally well-formed pseudomorphs, consisting of large groups of gypsum crystals arranged in "rose" formations, were discovered in the lignite mine at Pöttsching and have been documented as notable mineralogical specimens. These occurrences were initially described by Tauber (1953) as paramorphs. The pseudomorphs developed naturally via dehydration of primary gypsum crystals, attributed to the spontaneous combustion of lignite exposed to air.

The chemical transformation mechanism, as originally detailed by Tauber (1953) and subsequently refined through X-ray diffraction studies by Koritnig (1967), involves a sequence whereby primary gypsum is first converted into pseudomorphic anhydrite through complete dehydration. This is followed by the formation of fibrous gypsum pseudomorphs as anhydrite undergoes natural rehydration. Koritnig’s research demonstrated that the extent of rehydration influences the composition of these pseudomorphs; in the specific case examined, fibrous gypsum pseudomorphs contained up to 35% unrehydrated anhydrite and 8% bassanite, the latter serving as an intermediate product in the rehydration process. Furthermore, Koritnig observed that the proportion of bassanite declines as the concentration of anhydrite decreases.

Based on this reaction pathway, Koritnig concluded that the fibrous gypsum roses from the Pöttsching mine, containing both anhydrite and bassanite, should be classified as pseudomorphs rather than paramorphs. Additionally, it was noted that the minor phases identified are not necessarily present in every specimen.
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The open-pit mine, now inundated and forming Lake Pöttschinger (Pöttschinger See), was initially abandoned in 1934, reopened in 1948, and finally ceased operations in 1956. The provided coordinates refer to the locations of the former mine dumps, which are currently overgrown.

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