Stellerit z Košťálova u Semil (Česká republika)

Abstract

Stellerite was discovered in an active quarry at Košt’álov near Semily (Liberec Region, Czech Republic). The mine- ral occurs on fractures of Paleozoic basaltic andesite in association with fluorapophyllite-(K), chabazite-Ca and calcite. Stellerite forms clear columnar striated crystals reaching 1.5 mm in size. Unit cell parameters derived from the powder X-ray data are: a 13.586(6) Å, b 18.204(7) Å, c 17.839(12) Å and V 4412(2) Å3. Chemical analyses gave the empirical formula (Ca3.97Na0.08K0.03)Σ4.08(Si28.22Al7.69)O72·28 H2O. Transparent colourless dipyramidal and prismatic crystals of fluor- apophyllite-(K) are up to 4 mm in size. The chemical analyses correspond to the empirical formula K0.73Na0.10Ca3.98 (Si7.78Al0.22)Σ8.00O20(F0.94OH0.06)Σ1.00·8 H2O. Chabazite-Ca with an empirical formula (Ca1.34K1.02Na0.67Ba0.10Sr0.01)[Al3.22 Si8.44O24]·12 H2O forms intergrowths of colourless rhombohedrons up to 6 mm in size. Chabazite-Ca contains very small, elongated inclusions of baryte. Calcite in groups of colourless to pale cream scalenohedral crystals up to 2 mm in size is the youngest mineral of the mineral assemblage. In the case of stellerite this is the first description within the Czech Republic, in the case of fluorapophyllite-(K) it is the first detailed characterization from the locality of Košt’álov.