Paragenesis, composition and origin of Ba- and Ca-rich stronalsite, a rare strontium tectosilicate, in rocks of the teschenite association, Silesian Unit, Western Carpathians, Czech Republic

Abstract Barium- and Ca-rich stronalsite [ideally SrNa 2 Al 4 Si 4 O 16 ] occurs rarely as pseudomorphs, which are probably after nepheline, and is found in hydrothermally altered Sr-enriched leucocratic dykes or streaks hosted by mesocratic amphibole–pyroxene teschenite in the Silesian Unit (Flysch Belt of the Western Carpathians, Czech Republic). In addition to stronalsite, the pseudomorphs consist of slawsonite, celsian, biotite, muscovite, alkali feldspar, natrolite and thomsonite-Ca. The surrounding groundmass is rich in alkali feldspars and zeolites and sporadically also contains amphibole phenocrysts, chloritised biotite, fluorapatite and other accessory and/or secondary minerals. Both compositional types of stronalsite have identical Raman spectra. The Ba-rich stronalsite contains 0.55–0.83 apfu Sr, 0.12–0.37 apfu Ba, and <0.08 apfu Ca. In contrast, Ca-rich stronalsite contains 0.65–0.82 apfu Sr, 0.10–0.23 apfu Ca, and <0.06 apfu Ba. The substitution mechanisms by which Ca enters the structure of stronalsite could not be satisfactorily clarified from the available data; the best stoichiometric fit suggests for substitution of Sr, which should not be possible due to the different crystal structure of the Ca-analogue of stronalsite, lisetite [ideally CaNa 2 Al 4 Si 4 O 16 ]. The Na contents range is 1.82–2.42 apfu and the K contents are consistently low (<0.09 apfu). The T site contains 3.91–4.26 apfu Si, 3.76–4.00 apfu Al and 0.00–0.11 apfu Fe 3+ . The main source of Sr was probably primary magmatic plagioclase that underwent hydrothermal alteration by post-magmatic high-temperature brines mixed with fluids of external origin. On the basis of previous research and paragenetic relationships, we estimate that stronalsite crystallised at T ∼250–320°C and P <100 MPa.