Acta Musei Moraviae

Calcite pseudomorphs after aragonite from Teschenite Association Rocks in the Beskydy Piedmont area

The occurrence of prismatic hexagonal calcite crystals is known from the Beskydy Piedmont area since 1852. Radial aggregates of such material, enclosed in the green clay mineral, reached the diameter ca. 60 cm. We revised three historical occurrences (Janovice-Baranec: GPS between N 49° 33.284′ E 017° 57.494′ and N 49° 33.182′ E 017° 57.722′, Jičina-Grasmanka: N 49° 33.555′ E 017° 58.200′, and Žilina-Hýlovec: between N 49° 34.390′ E 018° 02.060′ and N 49° 34.327′ E 018° 02.124′) and described two new ones (Kunín-Čarodějka: N 49° 37.984′ E 018° 00.313′, Jičina-Požaha: N 49° 33.460′ E 017° 58.745′). All mentioned localities show similar geological features – they are bound to effusive volcanism of the Early Cretaceous Teschenite Rock Association. Typically to strongly altered (carbonatized, silicified) amygdaloid rocks such as picrites, porphyric olivine nephelinites, and also their tuffs and tuffites, near their contact with sediments. As expected in the past without the serious evidence, the hexagonal calcite crystals are in fact pseudomorphs after aragonite. We managed to find incomplete pseudomorphs, with cores still composed of predominant aragonite. Powder-XRD analyses also revealed that some pseudomorphs contain relatively pure calcite, while others mixture of pure calcite and Mg-Fe-rich calcite. Scanning electron microscopy and microanalysis proved that during the transformation of Sr-rich aragonite (up to 5 wt.% SrO) to calcite (max. 1 wt.% SrO) the excess of strontium was fixed in newly formed grains of strontianite. They are locally abundant, but less than 10 μm in size. Pseudomorphs also contain quartz, pyrite, Mg-Ca-rich siderite, and iron oxide. Green clay mineral is related to glauconite polytype 1M, but was not studied in detail. Stable oxygen isotope ratio δ¹⁸O in the calcite pseudomorphs range between -20.53 and 26.96 ‰ SMOW, while stable carbon isotope ratio δ¹³C varies between -4.05 and -8.03 ‰ PDB. Intersection of these results, with respect to local geological setting, points out the hydrothermal origin of aragonite/calcite. Supporting evidence is the presence of the aragonite at nearby Petřkovická hora site, with the similar macroscopic appearance and mineral association. The easiest method to prove presence of aragonite in the described pseudomorphs is pale yellow to pale brownish-yellow luminescence under longwave ultraviolet light, which can be probably ascribed to some of REE + Sr acting as activators.