The zeolite ferrierite was found in tectonic zones of andesite in the drillhole KR-1 near Kremnica (Central Slovakia). Ferrierite occurs in paragenesis with opal and chalcedony, occasionally in assemblage with mordenite. Powder diffraction pattern of ferrierite is identical with reference data, its lattice parameters are a = 19.201(4), b = 14.124(4), c = 7.501(2) Å. Its chemical composition is characterised by Si/Al ratio ranging between 4.331 and 4.963 respectively, predominant exchangeable cations are Mg > Ca > K, and unusual high Ba-contents (0.192–0.298 per fromula unit).