Acta Musei Moraviae

Mordenite from Teschenite Association Rocks in Beskydy Piedmont area (Moravia, Czech Republic)

Teschenite Association Rocks, Early Cretaceous igneous rocks bound to the Silesian Unit of the flysch Outer Carpathians, are typical for the zeolites in the fissures, amygdaloid cavities, rock matrix, as well as in their contact metamorphic rocks. Previously described minerals of the zeolite group include analcime, natrolite, phillipsite, harmotome, heulandite-Ca, ferrierite-Ca, and thomsonite-Ca. Two occurrences of the mordenite were newly recognized in Straník (GPS N 49° 33.130′ E 017° 59.330′) and Fryčovice (GPS N 49° 40.810′ E 018° 13.727′ a N 49° 40.738′ E 018° 13.710′). At both sites mordenite forms very thin acicular aggregates in the cavities of undoubtedly effusive rocks, filled by younger calcite. It is ortho r hombic, space group Cmc2₁, with unit-cell parameters refined from powder X-ray diffraction: a = 18.121(1), b = 20.474(2), c = 7.5213(8) Å (Fryčovice) and a = 18.1265(3), b = 20.4527(3), c = 7.5185(2) Å (Straník). The quantitative chemical composition by EDS are given for both sites. However, their reliability is reduced by nature of very thin crystals (2.5 μm), where analyses from polished section probably overestimate calcium content due to enveloping calcite, whereas analyses from crystals etched by the 10% solution of the acetic acid show significant increase the content of Na⁺ and K⁺. The zeolite framework is not affected by etching, with perfect reproducibility giving exactly the same T_Si = 0.83 for both cases.