Introduction
Stronalsite [SrNa2Al4Si4O16] is a rare orthorhombic tectosilicate from the feldspar group. The ideal formula can be derived from the simple addition of one molecule of slawsonite [SrAl2Si2O8] and two molecules of K-free nepheline [NaAlSiO4] (Hori et al., Reference Hori, Nakai, Nagashima, Matsubara and Kato1987). Two other tectosilicates have similar stoichiometry of ANa2Al4Si4O16: banalsite [BaNa2Al4Si4O16] (Campbell Smith et al., Reference Campbell Smith, Bannister and Hey1944a, Reference Campbell Smith, Bannister and Hey1944b) and lisetite [CaNa2Al4Si4O16] (Rossi et al., Reference Rossi, Oberti and Smith1986). Stronalsite forms a solid-solution series with banalsite (Koneva, Reference Koneva1996; Liferovich et al., Reference Liferovich, Mitchell, Locock and Shpachenko2006a, Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b). In contrast, the miscibility with lisetite should be restricted due to differences in their crystal structures (Liferovich et al., Reference Liferovich, Mitchell, Locock and Shpachenko2006a, Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b). Stronalsite and banalsite crystallise in space group Iba2 (Matsubara, Reference Matsubara1985; Hori et al., Reference Hori, Nakai, Nagashima, Matsubara and Kato1987; Liferovich et al., Reference Liferovich, Mitchell, Locock and Shpachenko2006a) and lisetite in space group Pbc21 (Rossi et al., Reference Rossi, Oberti and Smith1986). The basic framework of both Iba2- and Pbc21-structured tectosilicates is topologically similar (Si and Al cations are fully ordered in tetrahedral positions and build up four-fold and eight-fold rings consisting of vertex-sharing tetrahedrons arranged in a –UDUD– sequence; Liferovich et al., Reference Liferovich, Mitchell, Locock and Shpachenko2006a), however, the location of large intra-framework cations differ in both structures. In stronalsite and banalsite, ordered XBa or XSr and VINa cations form alternating layers parallel to (001) and shifted by ¼ c (Liferovich et al., Reference Liferovich, Mitchell, Locock and Shpachenko2006a; Fig. 1a), whereas VIICa and VINa cations in lisetite are distributed throughout common Ca + 2Na layers (Rossi et al., Reference Rossi, Oberti and Smith1986; Fig. 1b).
Structures of ANa2Al4Si4O16 tectosilicates, a perspective view of the unit cell along the a axis: (a) Iba2 structure of stronalsite [SrNa2Al4Si4O16] and banalsite [BaNa2Al4Si4O16] (modified after Liferovich et al., Reference Liferovich, Mitchell, Locock and Shpachenko2006a and https://www.mindat.org/min-3804.html); (b) Pbc21 structure of lisetite [CaNa2Al4Si4O16] (after Rossi et al., Reference Rossi, Oberti and Smith1986; Liferovich et al., Reference Liferovich, Mitchell, Locock and Shpachenko2006a and https://www.mindat.org/min-2414.html).
The occurrence of tectosilicates with stoichiometry ANa2Al4Si4O16 is related to high-grade metamorphic and metasomatic rocks (Campbell Smith et al., Reference Campbell Smith, Bannister and Hey1944a, Reference Campbell Smith, Bannister and Hey1944b; Smith et al., Reference Smith, Kechid and Rossi1986; Hori et al., Reference Hori, Nakai, Nagashima, Matsubara and Kato1987), altered Si-poor alkaline igneous rocks (predominantly nepheline syenites), ultramafic xenoliths in alkaline rocks, and alkaline ultramafic rocks, commonly associated with carbonatites (Khomyakov et al., Reference Khomyakov, Shpachenko, Polezhaeva, Ivanova, Dudkin and Arzamastsev1990; Koneva, Reference Koneva1996; Dunworth and Bell, Reference Dunworth and Bell2003; Liferovich et al., Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b; Dahlgren and Larsen, Reference Dahlgren and Larsen2012). Banalsite was identified for the first time by Campbell Smith et al. (Reference Campbell Smith, Bannister and Hey1944a, Reference Campbell Smith, Bannister and Hey1944b) from the Benallt manganese mine in the Llŷn Peninsula (Wales), whereas lisetite was described by Smith et al. (Reference Smith, Kechid and Rossi1986) from the Liset eclogite pod in Selje (Norway). The first occurrence of stronalsite was described by Hori et al. (Reference Hori, Nakai, Nagashima, Matsubara and Kato1983, Reference Hori, Nakai, Nagashima, Matsubara and Kato1987) and Matsubara (Reference Matsubara1985) in a pectolite–slawsonite veinlet intersecting mafic metatuff xenolith in serpentinite at Rendai, Kochi Prefecture, Japan. Hori et al. (Reference Hori, Nakai, Nagashima, Matsubara and Kato1987) also validated the second occurrence of stronalsite in a jadeite–serpentine rock at Mt. Ohsa (Okayama, Japan). The existence of a complete solid solution between stronalsite and banalsite was confirmed by Koneva (Reference Koneva1996) in samples from the Zhidoy massif, Eastern Sayan, SE Russia. Tectosilicates of the stronalsite–banalsite series occurred there in feldspar–zeolite veins penetrating alkali pyroxenites at the contact with nepheline syenite. Most of the known stronalsite occurrences are related to nepheline syenites of the Kola Alkaline Province in NW Russia. In this region Khomyakov et al. (Reference Khomyakov, Shpachenko, Polezhaeva, Ivanova, Dudkin and Arzamastsev1990) investigated botryoidal aggregates of stronalsite enclosed by a Na–Sr-rich melilite in a strongly altered xenolith of a cuspidine–melilite rock hosted by nepheline syenite of the Khibina peralkaline complex. Similarly, Dunworth and Bell (Reference Dunworth and Bell2003) identified stronalsite in the cuspidine-bearing nepheline melilitolite at the Turiy Mys complex of ultramafic–alkaline rocks and carbonatites. Liferovich et al. (Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b) re-examined xenolith samples from Khibina and interpreted stronalsite from both occurrences to be a metasomatic phase formed by a relatively high-temperature alteration of Na-Sr-rich melilite and/or nepheline. Liferovich et al. (Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b) also described three new occurrences from the Kola Alkaline Province: in leucocratic ijolite cut by calciocarbonatite and banded phoscorite at the Turiy Mys ultramafic–alkaline complex, in coarse-grained urtite at the Gremyakha-Vyrmes complex of mafic–ultramafic rock, quartz syenites and feldspathoid rocks, and in medium-grained essexite xenolith in a nepheline syenite at the Sakharjok alkaline massif. In addition, Liferovich et al. (Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b) described paragenesis and composition of stronalsite, banalsite and their intermediate compositional members from feldspathoid syenites of the Pilansberg peralkaline complex in South Africa, and from xenoliths of apatite–calcite-bearing stronalsite clinopyroxenite to serpentine–calcite mylonite and diverse rocks of the ijolite series at the Prairie Lake complex of alkaline rocks and carbonatites in the Superior Alkaline Province, NW Ontario, Canada. Another occurrence of stronalsite was described by Ahijado et al. (Reference Ahijado, Casillas, Nagy and Fernández2005) from metasomatic calc-silicate reaction zones in skarn associated with metamorphosed carbonatites at Punta del Peñón Blanco area, Fuerteventura, Canary Islands, Spain. Sporadic occurrences of stronalsite–banalsite have been reported also from the Late Archean Mikkelvik alkaline stock in the West Troms Basement Complex (Zozulya et al., Reference Zozulya, Kullerud, Ravna, Corfu and Savchenko2009), and from amygdales in alkaline ultramafic rocks of the Brunlanes ultramafic volcanic series (Dahlgren and Larsen, Reference Dahlgren and Larsen2012), both in Norway. Finally, Ferraris et al. (Reference Ferraris, Parodi, Pont, Rondeau and Lorand2014) identified stronalsite and banalsite in mineral association with trinepheline and fabrièsite in nephelinic-albitic jadeitite from the metamorphic veins of the Tawmaw-Hpakant jadeite deposit in Myanmar.
This work deals with the first occurrence of stronalsite in the rocks of the teschenite association in the Silesian Unit (Flysch Belt of the Western Carpathians, Czech Republic). Two compositionally different types of stronalsite were identified in pseudomorphs after an unknown phenocrystic mineral in Sr-enriched hydrothermally altered dykes of leucocratic teschenite, consisting dominantly of alkali feldspar and zeolites. The first type is a common Ca-poor member of the stronalsite–banalsite solid-solution series, whereas the second one has an unusual Ca-enriched composition. We discuss possible substitution mechanisms and try to clarify the origin of the stronalsite investigated on the basis of mineral paragenesis and chemical and Sr-isotopic composition.
Geological background
The teschenite association is represented by mostly alkaline magmatic rocks (alkaline basalts, basanites and nephelinites) and ultrabasic picrites (e.g. Kudělásková, Reference Kudělásková1987; Hovorka and Spišiak, Reference Hovorka and Spišiak1988; Safai, Reference Safai2020). The term teschenite is defined in the sense of the IUGS petrographic classification (Le Maitre et al., Reference Le Maitre2002) as analcime gabbro. However, the classification of rocks of the teschenite association is difficult due to their large variability in mineralogy, different structural and textural features and strong hydrothermal alteration (analcimisation, chloritisation, smectitisation and carbonatisation; Kapusta and Włodyka, Reference Kapusta and Włodyka1997; Pour et al., Reference Pour, Rapprich, Matýsek and Jirásek2022; Rapprich et al., Reference Rapprich, Matýsek, Pour, Jirásek, Míková and Magna2024). For these reasons, the term teschenite was used for various types of hydrothermally altered foid syenite, foid monzosyenite, foid monzogabbro, foid gabbro and alkaline lamprophyres (Pacák, Reference Pacák1926; Šmíd, Reference Šmíd1978; Kudělásková, Reference Kudělásková1987; Hovorka and Spišiak, Reference Hovorka and Spišiak1988; Dostal and Owen, Reference Dostal and Owen1998; Rapprich et al., Reference Rapprich, Matýsek, Pour, Jirásek, Míková and Magna2024), which typically occur in the northern foothills of the Beskydy Mountains between towns of Hranice in Czech Republic and Bielsko-Biała in Poland (Hovorka and Spišiak, Reference Hovorka and Spišiak1988). This area is a part of the Silesian Unit (Flysch Belt of the Outer Western Carpathians), which is a remnant of an external sedimentary basin developed on the southern margin of the European Platform (Nemčok et al., Reference Nemčok, Nemčok, Wojtaszek, Ludhova, Oszczypko, Sercombe, Cieszkowski, Paul, Coward and Ślaczka2001) and later incorporated into the Carpathian accretion wedge during the Alpine orogeny (Plašienka, Reference Plašienka, Grecula, Putiš, Kováč, Hovorka, Grecula, Hovorka and Putiš1997; Froitzheim et al., Reference Froitzheim, Plašienka, Schuster and McCann2008; Stráník et al., Reference Stráník, Stráník, Bubík, Gilíková and Petrová2021). The alkaline magmatism was associated with early rifting (Narebski, Reference Narebski1990; Spišiak and Hovorka, Reference Spišiak and Hovorka1997; Brunarska and Anczkiewicz, Reference Brunarska and Anczkiewicz2019) or with reactivation of deep faults within the basin during the Lower Cretaceous (Dostal and Owen, Reference Dostal and Owen1998). The 40K–40Ar and 39Ar–40Ar whole-rock and in situ mineral U–Pb radiometric age determination of the teschenites revealed an age of ∼138–120 Ma (Lucińska-Anczkiewicz et al., Reference Lucińska-Anczkiewicz, Villa, Anczkiewicz and Ślaczka2002; Grabowski et al., Reference Grabowski, Krzemiński, Nescieruk, Szydlo, Paszkowski, Pecskay and Wójtowicz2003; Szopa et al., Reference Szopa, Włodyka and Chew2014; Matýsek et al., Reference Matýsek, Jirásek, Skupien and Thomson2018; Brunarska and Anczkiewicz, Reference Brunarska and Anczkiewicz2019). Magmatic activity was coeval with deposition of the Hradiště Formation (Valanginian-Aptian; Eliáš et al., Reference Eliáš, Skupien and Vašíček2003; Stráník et al., Reference Stráník, Stráník, Bubík, Gilíková and Petrová2021). The lithology of the Hradiště Formation includes typical flysch sediments (various types of grey calcareous claystones, sandy limestones and sandstones), dark organic silicites, pelocarbonates, and bodies of igneous rocks of the teschenite association (mostly hypabyssal sills, submarine extrusions, pillow lavas and volcanoclastics; Stráník et al., Reference Stráník, Menčík, Eliáš, Adámek, Přichystal, Obstová and Suk1993, Reference Stráník, Stráník, Bubík, Gilíková and Petrová2021). The intrusions also penetrated the underlying calcareous sediments, represented by deep-water dark brown-grey calcareous claystones of the Vendryně Formation (Oxfordian-Tithonian; Eliáš, Reference Eliáš1970; Menčík et al., Reference Menčík, Adamová, Dvořák, Dudek, Jetel, Jurková, Hanzlíková, Houša, Peslová, Rybářová, Šmíd, Šebesta, Tyráček and Vašíček1983) and micritic or biodetritic Těšín limestone (Tithonian-Valanginian; Eliáš, Reference Eliáš1970; Stráník et al., Reference Stráník, Stráník, Bubík, Gilíková and Petrová2021). Trace-element contents and the Nd, Sr and Hf isotopic composition indicate that the magma was probably a product of ∼2–6% partial melting of upper mantle garnet peridotite at ∼60–80 km depth. Geochemical studies also suggest compositional similarities to ocean island basalts (OIB) with HIMU affinities and possible mixing with a more depleted, MORB-type component (Dostal and Owen, Reference Dostal and Owen1998; Harangi et al., Reference Harangi, Tonarini, Vaselli and Manetti2003; Brunarska and Anczkiewicz, Reference Brunarska and Anczkiewicz2019).
Occurrence and paragenesis
The Čerťák (or ‘Čertův mlýn’) occurrence (49°33ʹ58”N, 17°59ʹ54”E) represents a well-known surface exposure of a teschenite sill ca. 2 km south from the town of Nový Jičín, Western Carpathians, Czech Republic. This sill runs in a SW–NE direction for over 2 km (Fig. 2) and is composed of various petrographic types of teschenitic rocks. The dominant type is hydrothermally altered mesocratic teschenite, which is coarse grained to porphyritic and consists of phenocrysts of black prismatic clinopyroxene and amphibole (up to 3.5 cm long) and a grey-pinkish groundmass composed of feldspars, zeolites, biotite, apatite and other accessory or secondary minerals (Pacák, Reference Pacák1926; Šmíd, Reference Šmíd1978; Kudělásková, Reference Kudělásková1987; Hovorka and Spišiak, Reference Hovorka and Spišiak1988; Matýsek and Jirásek, Reference Matýsek and Jirásek2016; Kropáč et al., Reference Kropáč, Dolníček, Uher, Buriánek, Safai and Urubek2020, Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024). Less abundant are melanocratic pyroxene-rich varieties or, conversely, leucocratic varieties that, before being altered by hydrothermal fluids, typically had a nepheline-rich character (Pacák, Reference Pacák1926). They form fine- to medium-grained dykes, streaks or nests, up to several cm in thickness, distributed randomly in the mesocratic teschenite. The mineral association of leucocratic teschenites from the Čerťák occurrence was recently studied in detail by Matýsek and Jirásek (Reference Matýsek and Jirásek2016) and Kropáč et al. (Reference Kropáč, Dolníček, Uher, Buriánek, Safai and Urubek2020, Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024). Accurate classification of leucocratic teschenites is problematic due to a substantial hydrothermal alteration. Matýsek and Jirásek (Reference Matýsek and Jirásek2016) compare these rocks to metasomatic rodingites, but this interpretation is considered to be incorrect due to a lack of association with serpentinite bodies. The rock consists of subhedral lamellae or anhedral irregular crystals of alkali feldspar (Na-rich microcline with ∼0.30 apfu Na), celsian (≤0.21 apfu Sr) and slawsonite (≤0.91 apfu Sr; Matýsek and Jirásek, Reference Matýsek and Jirásek2016). Primary calcic plagioclase is not preserved due to hydrothermal alteration, which began immediately after solidification (Kropáč et al., Reference Kropáč, Dolníček, Uher, Buriánek, Safai and Urubek2020). Plagioclase was probably replaced by analcime, natrolite and Sr-rich thomsonite-Ca, or, alternatively, by albite, K-feldspar and epidote-(Sr) to Sr-rich epidote (Kropáč et al., Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024). Slawsonite is intergrown with mica of the muscovite (illite)–paragonite series (Matýsek and Jirásek, Reference Matýsek and Jirásek2016). Mafic components are represented by sporadic euhedral prismatic phenocrysts of clinopyroxene (Ti-rich diopside rimmed by hedenbergite or aegirine–augite to aegirine) and amphibole (kaersutite or ferro-kaersutite with a hastingsite or ferro-pargasite rim) and leaflets of biotite (annite). The accessory minerals are: fluorapatite; REE-rich fluorapatite; Ti-rich magnetite; (OH, F)-rich grossular; epidote-(Sr); Sr-REE-rich epidote; Sr-rich allanite-(Ce); Zr–Nb-rich titanite; pyrochlore; zircon; and vesuvianite together with prehnite; chlorite (chamosite); pyrite; calcite; and baryte (Kropáč et al., Reference Kropáč, Dolníček, Uher, Buriánek, Safai and Urubek2020, Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024).
Geological position of the Čerťák teschenite sill in the Western Carpathians, Czech Republic (modified according to Cháb et al., Reference Cháb, Stráník, Eliáš, Adamovič, Aichler, Babůrek, Breiter, Cajz, Domečka, Fišera, Hanžl, Holub, Hradecký, Chlupáč, Klomínský, Krejčí, Lexa, Mašek, Mlčoch, Opletal, Otava, Pálenský, Potfaj, Prouza, Roetzel, Růžička, Schovánek, Slabý, Valečka and Žáček2007). Inset (a) details the location of the region in the box.
Methods
We re-examined 11 samples of leucocratic dykes, streaks or nests from the Čerťák locality, which were investigated recently by Kropáč et al. (Reference Kropáč, Dolníček, Uher, Buriánek, Safai and Urubek2020, Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024). Only two samples (Č7 and Č10), which were found to contain stronalsite, were selected for detailed investigation. Electron microprobe analyses were performed using Cameca SX-100 apparatus at the National Museum in Prague, Czech Republic (Z. Dolníček analyst). The measurements were carried out on carbon-coated polished thin sections in wavelength-dispersive mode. The detection limits, acceleration voltage, beam current and diameter, analytical lines, standards and diffracting crystals are specified in Supplementary Tables S1–S5, which include all analyses of the tectosilicates investigated. Non-stoichiometric analyses of the feldspar–muscovite or the Na–Ca zeolite mixtures were omitted. The raw counts were converted to wt.% using the automatic PAP procedure (Pouchou and Pichoir, Reference Pouchou, Pichoir and Armstrong1985). Mineral abbreviations are according to Warr (Reference Warr2021).
In situ micro-Raman analyses of minerals were performed using a DXR dispersive Raman Spectrometer mounted on a confocal Olympus microscope housed in the National Museum in Prague, Czech Republic. The Raman signal was excited by an unpolarised 633 nm laser and detected by a CCD detector. The spectrometer was calibrated using a software-driven procedure based on emission lines of neon (calibration of Raman shift), Raman bands of polystyrene (calibration of laser frequency) and a standardised source of white light (calibration of intensity). The parameters of measurement were 100× objective, 5 s exposure time, 100 exposures, 50 μm pinhole spectrograph aperture, 8 mW laser power, and 40–3700 cm–1 spectral range. Spectral manipulations were performed using Omnic 9 software.
Results
Stronalsite bearing mineral association
Stronalsite occurs as a constituent of columnar, rectangular or hexagonal skeletal pseudomorphs in hydrothermally altered leucocratic dykes or streaks. These pseudomorphs are randomly distributed in medium- to coarse-grained areas close the boundary with the host mesocratic amphibole–pyroxene teschenite (Figs 3a,b). The mineral association in the vicinity of pseudomorphs (Figs 3c,d) consists mostly of euhedral-to-subhedral laths of alkali feldspars, anhedral natrolite, thomsonite-Ca and rarely also analcime, columns of amphibole, leaflets of chloritised biotite, needles or hexagonal skeletal crystals of fluorapatite and other accessory and/or secondary minerals (epidote-group minerals, titanite, pyrochlore, Ti-rich magnetite, chlorite, prehnite, pyrite, calcite and baryte). Stronalsite forms anhedral colourless grains (Fig. 3e) with low birefringence. Some grains are corroded by zeolites and/or muscovite. In rare cases, where both Ba- and Ca-rich stronalsite occur together within a single pseudomorph, the Ba-rich stronalsite is clouded by products of hydrothermal alteration, whereas Ca-rich stronalsite is relatively well preserved (Fig. 3f). The paragenetic relationships are more obvious in the back-scattered electron (BSE) images (Figs 4a–c and Figs 5a,b). On the basis of results of wavelength dispersive spectroscopy (WDS) analysis and Raman spectroscopy, it can be determined that both Ca- and Ba-rich stronalsite are replaced along rims and subparallel cracks by natrolite, thomsonite-Ca, muscovite and their mixture, similar to slawsonite. In addition, a small amount of anhedral K-feldspar and tiny euhedral-to-anhedral grains of celsian may occur in this hydrothermal association (Fig. 4a). If they occur in a single pseudomorph, the Ba-rich stronalsite overgrows slawsonite, which must have crystallised earlier (Fig. 4b). The Ca-rich stronalsite appears relatively more homogenous in a BSE image. Rarely it also forms a marginal zone on hydrothermally altered Ba-rich stronalsite inside the hexagonal skeletal pseudomorph (Fig. 4c) and is thus younger. The ‘atoll’ itself consists only of slawsonite and a Na–Ca zeolite–muscovite mixture (Fig. 4c). Some ‘atolls’ have been completely replaced by prehnite, which also fills cracks and cavities in rock and is therefore younger. Most hexagonal skeletal pseudomorphs, although rather frequent in the rock, do not contain stronalsite. Their central part is usually filled with subhedral-to-anhedral titanite, epidote, hedenbergite, alkali feldspars and Na–Ca zeolites (Fig. 4d).
(a, b) Macroscopic appearance of the teschenites (samples Č7 and Č10). Pseudomorphs with stronalsite (marked with red arrows) occur mostly in the marginal parts of leucocratic dykes or streaks. (c, d) Mineral association in the vicinity of a rectangular stronalsite-bearing pseudomorph in sample Č10: (c) plane-polarised light (PPL), (d) crossed polars (XPL). (e) The stronalsite-bearing rectangular pseudomorph (red box in c and d) at high magnification (PPL). (f) The inner part of a hexagonal skeletal pseudomorph in sample Č10. The Ba-rich stronalsite is turbid, whereas Ca-rich stronalsite is very transparent (PPL).
BSE images of pseudomorphs from samples Č7 (a) and Č10 (b–d). (a) The Ba-rich stronalsite replaced along cracks and margins by natrolite, thomsonite-Ca, K-feldspar, celsian and muscovite. (b) A pseudomorph consisting of slawsonite (brighter in BSE) and Ba-rich stronalsite (slightly darker rim in BSE). Both Sr feldspars share subparallel cracks (or traces of cleavage inherited from mineral precursor) and are altered to a mixture of Na-Ca zeolites and muscovite. (c, d) Hexagonal skeletal pseudomorphs. The ‘atoll’ consists of slawsonite, Na-Ca zeolites and muscovite, the ‘inner lagoon’ is filled by: (c) zeolites and Ca-rich stronalsite rimming porous brighter Ba-rich stronalsite, or by (d) hedenbergite, alkali feldspar and Na-Ca-zeolites.
Raman spectra of the Ba-rich stronalsite from Čerťák, Western Carpathians, Czech Republic with ≤0.02 apfu Ca (a) and Ca-rich stronalsite with up to 0.23 apfu Ca (b) in sample Č10. The BSE images show the context of the sites from which Raman spectra were collected.
Raman spectroscopy
Raman spectroscopy confirmed the presence of natrolite, thomsonite-Ca, muscovite, slawsonite and stronalsite in the pseudomorphs. Raman spectra of compositionally different types of stronalsite were also measured in rectangular pseudomorphs (sample Č10). The peak position of both Ba- and Ca-rich stronalsite varieties are practically identical (Figs 5a,b).
Composition
Representative compositions of stronalsite are shown in Table 1 (all data are available in Table S1). Most of the analysed stronalsite represents typical members of the stronalsite–banalsite solid-solution series. The Ba-rich stronalsite is characterised by high contents of Sr (0.55–0.83 apfu) and subordinate contents of Ba (0.12–0.37 apfu) and/or Ca (0.00–0.08 apfu) (Tables 1 and S1, Figs 6a–c). Only sample Č10 also included stronalsite rich in Ca, which contains 0.65–0.82 apfu Sr, 0.10–0.23 apfu Ca and only 0.01–0.06 apfu Ba (Tables 1 and S1, Figs 6a–c). The Na contents vary in a wide range 1.82–2.42 apfu, whereas the K contents are mostly insignificant (0.00–0.09 apfu). The poor stoichiometry of the Na site in some analyses can be explained mainly by problems with the determination of Na by electron microprobe. Similar alkali-loss problems with the estimation of Na in stronalsite have been noted in other investigations (Matsubara, Reference Matsubara1985; Khomyakov et al., Reference Khomyakov, Shpachenko, Polezhaeva, Ivanova, Dudkin and Arzamastsev1990; Koneva, Reference Koneva1996; Liferovich et al., Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b). The T site contains 3.91–4.26 apfu Si, 3.76–4.00 apfu Al and 0.00–0.11 apfu Fe3+ (Table S1).
Representative compositions (wt.%) of Ba- and Ca-rich stronalsite from Čerťák, Western Carpathians, Czech Republic (apfu values are based on 16 oxygen atoms).
Abbreviations: n.d. = not detected; ‘Lst’ – compositional analogue of lisetite component; Sns–stronalsite component; Bns – banalsite component.
The composition of slawsonite is variable (Sws49–83Cls4–19Or1–12Ab2–22An1–8; Fig. 6d, Table S2). The Sr and Ba contents range is 0.51–0.93 and 0.04–0.24 apfu, respectively. Most celsian are close to its near-ideal composition (Cls93–99; Fig. 6d, Table S3). Alkali feldspars correspond to K-feldspar (Or88–98Ab1–7An0–2Sws0–2Cls0–3) or to a K-feldspar with a significant portion of the albite component (Or64–73Ab26–32An1–2Sws0–1) (Fig. 6e, Table S4). Both types form mostly irregular zones in feldspar grains, or in some cases, the Na-poor K-feldspar overgrows the Na-rich core. In addition, some K-feldspars show growth zonation due to enrichment in Ba (Cls7–10; brighter in BSE in Fig. 5a,b). The composition of zeolites is illustrated in Table S5. Only one analysis was performed in analcime due to its rare occurrence in the samples and its composition is close to ideal stoichiometry. This also applies to the more commonly present natrolite. The contents of Fe, Mg, Ca and K in natrolite are close to or below the detection limit. In contrast, thomsonite-Ca has a variable contents of Ca (1.22–1.90 apfu) and Sr (0.14–0.73 apfu) (Table S5). This variability is not dependent on position within the sample (pseudomorphs or groundmass).
Compositional variations of the stronalsite investigated (a–c), slawsonite and celsian (d) and alkali feldspars (e). The data set is supplemented with stronalsite compositions from: Rendai (Kochi Prefecture, Japan, Matsubara, Reference Matsubara1985; Hori et al., Reference Hori, Nakai, Nagashima, Matsubara and Kato1987); Punta del Peñón Blanco (Fuerteventura, Canary Islands, Ahijado et al., Reference Ahijado, Casillas, Nagy and Fernández2005); Khibina, Sakharjok, Gremyakha-Vyrmes and Turiy Mys (Kola Alkaline Province in NW Russia, Liferovich et al., Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b); Prairie Lake (Superior Alkaline Province, NW Ontario, Canada, Liferovich et al., Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b); Pilansberg (peralkaline complex in South Africa) (Liferovich et al., Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b); and Mikkelvik (West Troms Basement Complex in Northern Norway, Zozulya et al., Reference Zozulya, Kullerud, Ravna, Corfu and Savchenko2009). Slawsonite, celsian and Na-rich microcline data are from the Čerťák site (Western Carpathians, Czech Republic, Matýsek and Jirásek, Reference Matýsek and Jirásek2016).
Discussion
Crystal chemistry of stronalsite
The composition of Ba-rich stronalsite from the Čerťák locality generally matches the composition of minerals of the stronalsite–banalsite solid-solution series described by Liferovich et al. (Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b). A similar Ba-rich composition has also been found in samples from the localities at Prairie Lake, Sakharjok and Gremyakha-Vyrmes (see Liferovich et al., Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b). However, our analyses do not approximate the end-member composition represented by stronalsite from the Khibina occurrence (Fig. 6a–c). Ca-rich stronalsite is extremely rare worldwide. To date, the highest content of Ca in stronalsite reported is from the Gremyakha-Vyrmes occurrence (1.4 wt.% CaO, i.e. 0.15 apfu Ca; Liferovich et al. Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b). Stronalsite from sample Č10, however, contains up to 2.2 wt.% CaO (i.e. 0.23 apfu Ca; Tables 1 and S1; Figs 6a–c).
Three substitution mechanisms explaining the incorporation of Ca into the stronalsite structure are proposed. The first involves the entrance of Ca to the A position of the ideal formula, i.e. substituting divalent cations (Sr and Ba). This approach suggests the presence of up to ∼25 mol.% of the lisetite component [CaNa2Al4Si4O16], if miscibility with banalsite–stronalsite exists (Tables 1 and S1; Figs. 6a–c). Stoichiometric criteria of our WDS data strongly support this possibility: the sum of cations in A position (i.e., Sr + Ba + Ca) are then close to the ideal value of 1 apfu (average 0.96, range 0.89–1.04; Table S1) for Ca-rich compositions. However, the formation of a solid solution between banalsite–stronalsite and lisetite would be hampered by differences in their crystal structures (Liferovich et al., Reference Liferovich, Mitchell, Locock and Shpachenko2006a, Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b). In the first case, the Ba or Sr and Na cations are ordered, and populate alternating layers parallel to (001), separated by ¼ c, whereas in lisetite Ca and Na cations are distributed throughout common Ca + 2Na layers (Figs 1a,b; Rossi et al., Reference Rossi, Oberti and Smith1986; Liferovich et al., Reference Liferovich, Mitchell, Locock and Shpachenko2006a). Although data plotted in Fig. 7a suggest that there is no evidence of a (Sr,Ba)2+ ↔ Ca2+ substitution (R 2 = 0.50), a relatively narrow range of compositions (Ca contents cover range over 0.23 apfu for the whole dataset) in combination with a comparably wide scatter of the total A-site occupancy (within 0.19 apfu) probably influenced the scattering of data points.
Substitution diagrams showing the relationships among contents (apfu) of Ca and Sr, Ba, Na, Si, Al and Fe3+ in the stronalsite investigated with calculated R 2 values: (a) Ca vs. Sr + Ba; (b) Al + Ca vs. Si + Na; (c) Ca vs. Si; and (d) Ca vs. Al + Fe3+.
In a second scheme, the incorporation of Ca could also occur in the Na position of the ideal formula. Replacing Na for Ca would solve the above-mentioned crystal-structure limitations. However, such an approach results in a strong deficit in the A position (average 0.77, range 0.70–0.87 apfu Sr + Ba) and generally a significant excess of atoms in the Na position (average 2.24, range 2.01–2.45 apfu Na + K + Ca) for Ca-rich compositions. The latter, however, could be possibly due to problems with Na loss during determinations using an electron microprobe. Moreover, the heterovalent Na+ ↔ Ca2+ substitution would be coupled with Si4+ ↔ (Al, Fe)3+ substitution in order to obtain an electroneutral substitution vector: Si4+ + Na+ ↔ Al3+ + Ca2+ (Fig. 7b). However, no correlations are obtained for this coupled substitution (R 2 = 0.44) as well as for simple pairs Ca–Si or Ca–(Al, Fe)3+ (R 2 ≤0.04; Figs 7c,d).
The third possibility is a modification of the second scheme. In the absence of substitutions involving the tetrahedral site, the electroneutrality can also be achieved by a coupled heterovalent substitution comprising A and Na positions: NaNa+ + ASr2+ ↔ ANa+ + NaCa2+. Regarding the difficulties with Na analysis, this mechanism could be easily verified due to the existence of a correlation between the ‘vacancy’ in the A position (expressed as a sum of Sr and Ba) and Ca contents. However, such a correlation is not evident (R 2 = 0.50; Fig. 7a).
It can be concluded that, at the present state of knowledge, the role of Ca in the stronalsite formula cannot be specified unambiguously. This is partly due to common analytical limitations, but also due to the material investigated, which only covers a narrow range of compositions (within ca. 0.15 apfu in case of Ca-rich stronalsite). Future studies are necessary to constrain this task adequately. Nevertheless, the entry of an elevated amount of Ca into the structure of stronalsite did not apparently cause its significant deformation, which is documented by the identical Raman spectra of both Ca-rich and Ca-poor varieties (Figs 5a,b).
The source of strontium
The source of Sr was ascertained on the basis of comparison of the 87Sr/86Sri(120 Ma) isotopic ratio of the Sr-rich leucocratic teschenites (samples Č7 and Č10) with different rock types of the teschenite association, Upper Jurassic-Lower Cretaceous sediments of the Silesian Unit and Lower Cretaceous seawater (Kropáč et al., Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024). This isotopic study confirmed earlier evidence (Dolníček et al. Reference Dolníček, Kropáč, Uher and Polách2010a, Reference Dolníček, Urubek and Kropáč2010b; Kropáč et al., Reference Kropáč, Dolníček, Uher and Urubek2017), which clearly showed that Sr isotope composition must have been modified during post-magmatic interaction with fluids of an external origin. The leucocratic dykes Č7 and Č10 have slightly higher 87Sr/86Sri(120 Ma) ratios (0.7047 and 0.7046, respectively) than other members of the teschenite association including the host mesocratic teschenite from the Čerťák site (87Sr/86Sri(120 Ma) = 0.7045 and 0.7038) and significantly lower 87Sr/86Sri(120 Ma) ratios when compared to Upper Jurassic-Lower Cretaceous sediments of the Silesian Unit (87Sr/86Sri(120 Ma) = 0.7073–0.7083; Kropáč et al., Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024) and Lower Cretaceous seawater (87Sr/86Sr = 0.7071–0.7075; Veizer et al., Reference Veizer, Ala, Azmy, Bruckschen, Bruhl, Bruhn, Carden, Diener, Ebneth, Godderis, Jasper, Korte, Pawellek, Podlaha and Strauss1999). On the basis of the mass balance, Kropáč et al. (Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024) calculated a contribution of at least 6–17% Sr from the surrounding claystones of the Hradiště Formation (if the system claystone–host teschenite is considered) or 8–21% Sr from Lower Cretaceous seawater (if the system seawater–host teschenite was inferred). The primary magmatic intermediate plagioclase was probably the main source of Sr, even though it was not preserved at the Čerťák locality. Tabular relics of andesine–labradorite (An36–52) with up to 0.44–0.57 wt.% SrO have been described from leucocratic dykes at the Řepiště occurrence in the Silesian Unit (Kropáč et al., Reference Kropáč, Dolníček, Uher, Buriánek, Safai and Urubek2020, Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024).
The genesis of stronalsite
Large amounts of alkali- and Sr-Ba-bearing tectosilicates in the pseudomorphs testify to the complex genesis of the mineral association, which involved the participation of both late-magmatic and hydrothermal processes. Reconstruction of the magmatic evolution is difficult due to intense superimposed hydrothermal alteration. The teschenite magma in the late stage already carried sporadic phenocrysts of apatite, pyroxene and amphibole. The Sr-enriched plagioclase, nepheline and alkali feldspar also crystallised from the felsic melt. After the sill intruded the water-saturated unconsolidated seafloor sediments, the residual melt rapidly cooled and solidified, probably in the form of a glassy groundmass (Pour et al., Reference Pour, Rapprich, Matýsek and Jirásek2022; Rapprich et al., Reference Rapprich, Matýsek, Pour, Jirásek, Míková and Magna2024). The end of the late-magmatic phase was associated with autometamorphic (autometasomatic) processes, which were triggered by high-temperature magmatic brines interacting with fluids of an external origin (Dolníček et al., Reference Dolníček, Kropáč, Uher and Polách2010a, Reference Dolníček, Urubek and Kropáč2010b; Kropáč et al., Reference Kropáč, Dolníček, Uher and Urubek2017, Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024; Rapprich et al., Reference Rapprich, Matýsek, Pour, Jirásek, Míková and Magna2024).
Clarifying the relationships between Sr-, Ba- and alkali feldspars is essential for resolving the genesis of stronalsite. According to Matýsek and Jirásek (Reference Matýsek and Jirásek2016), slawsonite crystallised in an autometamorphic phase together with celsian and Na–K feldspar (Ab20–40). However, K-feldspar with ∼30 mol.% of the Ab component crystallises in igneous systems usually at temperatures significantly higher than ∼600°C. The existence of a Na-rich homogeneous crystal below this temperature is limited by the miscibility gap (e.g. Brown and Parsons, Reference Brown and Parsons1989). We therefore consider the Na–K-feldspar associated with pseudomorphs to be a product of crystallisation from a melt that corroded phenocrysts in the late stage of magmatic evolution. Alternatively, an inclusion of melt trapped in a growing phenocryst cannot be excluded, as is evidenced by Kropáč et al. (Reference Kropáč, Dolníček, Buriánek, Urubek and Mašek2015). But could Ba and Sr feldspars have formed from the melt under the same conditions? This is not indicated by textural features that suggest a hydrothermal origin, nor by the fact that apparently primary magmatic minerals, such as apatite or calcic plagioclase, do not show a trend of gradually increasing Sr contents during crystallisation (Kropáč et al., Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024). Therefore, we suggest that the Ba-Sr phases crystallised under subsolidus conditions after the system was opened to seawater, which is also supported by the Sr isotopic composition (Kropáč et al., Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024).
The hydrothermal alteration of Sr-bearing plagioclase had probably already commenced during the high-temperature autometasomatic stage and continued into the hydrothermal stage. The formation of Sr-absent secondary minerals, such as analcime, K-feldspar or albite, during the decomposition of calcic plagioclase, resulted in increased Sr concentrations in the hydrothermal solution (Kropáč et al., Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024). Stronalsite could only crystallise under silica-poor conditions, because a slight increase in Si would instead favour the formation of slawsonite (Matsubara, Reference Matsubara1985; Hori et al., Reference Hori, Nakai, Nagashima, Matsubara and Kato1987). As slawsonite is successively older than stronalsite, we can assume higher Si concentrations at the initial phase of Sr-feldspar crystallisation, reflecting the hydrothermal breakdown of primary silicates and glassy groundmass. The Sr/Ba ratio in the hydrothermal solution was probably influenced by celsian precipitation. The formation of Ca-rich stronalsite may be related to the depletion of Ba ions after the crystallisation of celsian.
Crystallisation mechanisms
The genesis of minerals of stronalsite–banalsite series in altered alkaline rocks was discussed by Liferovich et al. (Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b). On the basis of the textural relationships between primary nepheline and analcime-rich secondary assemblage, these authors interpreted stronalsite to be mostly a product of nepheline replacement during subsolidus reaction with a deuteric alkaline fluid. They schematically expressed this reaction as follows:
\begin{equation}\begin{gathered}
{\text{2N}}{{\text{a}}_{\text{3}}}{\text{KA}}{{\text{l}}_{\text{4}}}{\text{S}}{{\text{i}}_{\text{4}}}{{\text{O}}_{{\text{16}}}}{\text{ + 2SrO + A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{ + 4Si}}{{\text{O}}_{\text{2}}}{\text{ + 2}}{{\text{H}}_{\text{2}}}{\text{O}} \hfill \\
\to {\text{ 2SrN}}{{\text{a}}_{\text{2}}}{\text{A}}{{\text{l}}_{\text{4}}}{\text{S}}{{\text{i}}_{\text{4}}}{{\text{O}}_{{\text{16}}}}{\text{ + 2NaAlS}}{{\text{i}}_{\text{2}}}{{\text{O}}_{\text{6}}}{\cdot}{{\text{H}}_{\text{2}}}{\text{O + }}{{\text{K}}_{\text{2}}}{\text{O}}{\text{.}} \hfill \\
\end{gathered} \end{equation}Nepheline was probably an abundant primary mineral in leucocratic teschenites, as is indicated by numerous hexagonal pseudomorphs (Pacák, Reference Pacák1926). Matýsek and Jirásek (Reference Matýsek and Jirásek2016) also favour nepheline (and also plagioclase, but less likely) as a precursor for slawsonite pseudomorphs. Therefore, stronalsite in the rocks could have crystallised at the expense of nepheline. Unlike the mineral associations investigated by Liferovich et al. (Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b), analcime is relatively rare in leucocratic teschenites with originally nepheline-rich composition (Pacák, Reference Pacák1926). Šmíd (Reference Šmíd1978) noted that secondary analcime replaces alkali feldspars or plagioclase in teschenites, but never nepheline. However, the low amount of analcime can be possibly explained by intense natrolitisation during the late hydrothermal alteration.
Liferovich et al. (Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b) also noted that under specific physico-chemical conditions, hydrothermal alteration of nepheline can also lead to the crystallisation of stronalsite and albite instead of analcime:
\begin{equation}\begin{gathered}
{\text{2N}}{{\text{a}}_{\text{3}}}{\text{KA}}{{\text{l}}_{\text{4}}}{\text{S}}{{\text{i}}_{\text{4}}}{{\text{O}}_{{\text{16}}}}{\text{ + 2SrO + A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{ + 6Si}}{{\text{O}}_{\text{2}}} \hfill \\
\to {\text{ 2SrN}}{{\text{a}}_{\text{2}}}{\text{A}}{{\text{l}}_{\text{4}}}{\text{S}}{{\text{i}}_{\text{4}}}{{\text{O}}_{{\text{16}}}}{\text{ + 2NaAlS}}{{\text{i}}_{\text{3}}}{{\text{O}}_{\text{8}}}{\text{ + }}{{\text{K}}_{\text{2}}}{\text{O}}{\text{.}} \hfill \\
\end{gathered} \end{equation}Although albite was identified together with K-feldspar and epidote-(Sr) in the pseudomorphs, possibly after plagioclase, in the samples investigated (Kropáč et al., Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024), it was never observed in the stronalsite-bearing pseudomorphs, making this explanation improbable.
The K released during hydrothermal reactions could subsequently be incorporated into the structure of late K-feldspar and mica (Liferovich et al., Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b). An alternative possibility may be that the K-feldspar and muscovite were produced together with stronalsite during hydrothermal alteration of nepheline associated with slawsonite:
\begin{equation}\begin{gathered}
{\text{2N}}{{\text{a}}_{\text{3}}}{\text{KA}}{{\text{l}}_{\text{4}}}{\text{S}}{{\text{i}}_{\text{4}}}{{\text{O}}_{{\text{16}}}}{\text{ + 3SrA}}{{\text{l}}_{\text{2}}}{\text{S}}{{\text{i}}_{\text{2}}}{{\text{O}}_{\text{8}}}{\text{ + 4Si}}{{\text{O}}_{\text{2}}} \hfill \\
\to {\text{ 3SrN}}{{\text{a}}_{\text{2}}}{\text{A}}{{\text{l}}_{\text{4}}}{\text{S}}{{\text{i}}_{\text{4}}}{{\text{O}}_{{\text{16}}}}{\text{ + 2KAlS}}{{\text{i}}_{\text{3}}}{{\text{O}}_{\text{8}}}{\text{,}} \hfill \\
\end{gathered} \end{equation}and
${\text{2N}}{{\text{a}}_{\text{3}}}{\text{KA}}{{\text{l}}_{\text{4}}}{\text{S}}{{\text{i}}_{\text{4}}}{{\text{O}}_{{\text{16}}}}{\text{ + SrA}}{{\text{l}}_{\text{2}}}{\text{S}}{{\text{i}}_{\text{2}}}{{\text{O}}_{\text{8}}}{\text{ + 2}}{{\text{H}}_{\text{2}}}{\text{O}}$
\begin{equation} \to {\text{ SrN}}{{\text{a}}_{\text{2}}}{\text{A}}{{\text{l}}_{\text{4}}}{\text{S}}{{\text{i}}_{\text{4}}}{{\text{O}}_{{\text{16}}}}{\text{ + 2KA}}{{\text{l}}_{\text{2}}}\left( {{\text{AlS}}{{\text{i}}_{\text{3}}}{{\text{O}}_{{\text{10}}}}} \right){\left( {{\text{OH}}} \right)_{\text{2}}}{\text{ + 2N}}{{\text{a}}_{\text{2}}}{\text{O}}{\text{.}}\end{equation}The breakdown product of nepheline and associated slawsonite could theoretically also be K-feldspar and natrolite, together with stronalsite:
\begin{equation}\begin{gathered}
{\text{2N}}{{\text{a}}_{\text{3}}}{\text{KA}}{{\text{l}}_{\text{4}}}{\text{S}}{{\text{i}}_{\text{4}}}{{\text{O}}_{{\text{16}}}}{\text{ + SrA}}{{\text{l}}_{\text{2}}}{\text{S}}{{\text{i}}_{\text{2}}}{{\text{O}}_{\text{8}}}{\text{ + 6Si}}{{\text{O}}_{\text{2}}}{\text{ + 4}}{{\text{H}}_{\text{2}}}{\text{O}} \hfill \\
\to {\text{SrN}}{{\text{a}}_{\text{2}}}{\text{A}}{{\text{l}}_{\text{4}}}{\text{S}}{{\text{i}}_{\text{4}}}{{\text{O}}_{{\text{16}}}}{\text{ + 2KAlS}}{{\text{i}}_{\text{3}}}{{\text{O}}_{\text{8}}}{\text{ + 2N}}{{\text{a}}_{\text{2}}}{\text{A}}{{\text{l}}_{\text{2}}}{\text{S}}{{\text{i}}_{\text{3}}}{{\text{O}}_{{\text{10}}}}{\cdot\text{2}}{{\text{H}}_{\text{2}}}{\text{O}}{\text{.}} \hfill \\
\end{gathered} \end{equation}The coeval genesis of stronalsite, K-feldspar and natrolite seems problematic because natrolite postdates the feldspars, as is clearly evidenced by textural features. Natrolite fills fractures in stronalsite and occurs in intimate mixtures with thomsonite-Ca, indicating very low crystallisation temperatures (Kristmanndóttir and Tómasson, Reference Kristmanndóttir, Tómasson, Sand and Mumpton1978). Thomsonite-Ca additionally contains up to 0.73 apfu Sr, which might have been released from hydrothermally altered stronalsite and slawsonite.
Considering that the system was most probably opened during the hydrothermal alteration of the rock, it is necessary to take all suggested mechanisms with caution. Hydrothermal decomposition of the glassy groundmass could enrich the deuteric fluids in Na (Rapprich et al., Reference Rapprich, Matýsek, Pour, Jirásek, Míková and Magna2024) and the contribution of other ions by mixing with external fluids should also be considered (Dolníček et al., Reference Dolníček, Kropáč, Uher and Polách2010a, Reference Dolníček, Urubek and Kropáč2010b; Kropáč et al., Reference Kropáč, Dolníček, Uher, Buriánek, Safai and Urubek2020, Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024).
Crystallisation conditions
As stronalsite is probably younger than slawsonite and crystallised with analcime before the formation of natrolite and thomsonite-Ca, we can at least indirectly estimate the conditions of its crystallisation. Liebscher et al. (Reference Liebscher, Thiele, Franz, Dörsam and Gottschalk2009) synthesised slawsonite and other Ca–Sr-phases from an oxide-hydroxide-fluid mixture at a temperature of 400–500°C and a pressure of 390–500 MPa. However, in natural systems, slawsonite can crystallise under even lower P–T conditions, as evidenced by Tasáryová et al. (Reference Tasáryová, Frýda, Janoušek and Racek2014). These authors investigated slawsonite in association with celsian and hyalophane in picrites from the Upper Ordovician strata of the Prague Basin, Czech Republic. Tasáryová et al. (Reference Tasáryová, Frýda, Janoušek and Racek2014) concluded that Sr- and Ba-feldspars precipitated directly from a fluid phase, which caused decomposition of calcic plagioclase at T ≤350°C and P <500 MPa.
According to Liou (Reference Liou1971), analcimisation of nepheline probably occurred at temperatures below 450°C. On the basis of the model for autometasomatic alteration of sodic peralkaline rocks according to Marks and Markl (Reference Marks and Markl2003), Liferovich et al. (Reference Liferovich, Mitchell, Zotulya and Shpachenko2006b) estimated the conditions for the conversion of nepheline to analcime at temperatures below 300°C, H2O activities between 0.5 and 1.0, and oxygen fugacity above the magnetite–hematite buffer. These conditions can also be applied to our case. A higher oxygen fugacity is evidenced by the presence of aegirine–augite or aegirine and Sr-rich minerals of the epidote group, which crystallised from hydrothermal solutions probably at pressures below 100 MPa and at ∼250–430°C (Kropáč et al., Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024), based on the chlorite thermometry (∼250–310°C) and fluid-inclusion studies from other occurrences of teschenites in the Silesian Unit (Dolníček et al., Reference Dolníček, Kropáč, Uher and Polách2010a). The homogenisation temperatures of primary and pseudosecondary fluid inclusions in analcime from miaroles, amygdales and veins hosted by rocks of the teschenite association range between 100 and 320°C and trapped aqueous fluids have low salinity of 0.5–4.5 wt.% NaCl equiv. (Włodyka and Kozlowski, Reference Włodyka and Kozlowski1997; Urubek et al., Reference Urubek, Dolníček, Kropáč and Lehotský2013). Although these microthermometric data were not obtained from samples under this investigation, they may serve as important clues for approximating the origin of stronalsite. The temperature conditions of analcime crystallisation may overlap to some extent with natrolite crystallisation, which are probably younger than the stronalsite investigated. The genesis of natrolite is associated generally with volatile-rich fluids and lower-temperature environment and its crystallisation occurs usually at T <200°C (Senderov and Khitarov, Reference Senderov, Khitarov, Flanigen and Sand1971). This observation also applies to minerals of the thomsonite subgroup. For instance, Kristmanndóttir and Tómasson (Reference Kristmanndóttir, Tómasson, Sand and Mumpton1978) determined the crystallisation temperature of natrolite and thomsonite from Iceland geothermal fields to be even less than 100°C.
Therefore, stronalsite in the rocks investigated could crystallise in a relatively wider range of temperature conditions. The presence of Sr-enriched high- to medium-temperature hydrothermal fluids is evidenced by the crystallisation of rare epidote-(Sr) at T ≈ 250–430°C (Kropáč et al., Reference Kropáč, Dolníček, Uher, Buriánek and Urubek2024). In contrast, the Sr-rich thomsonite-Ca represents most probably late-hydrothermal redeposition of Sr derived from alteration of Sr feldspars at T <100°C. Together with microthermometric data from analcime (Włodyka and Kozlowski, Reference Włodyka and Kozlowski1997; Urubek et al., Reference Urubek, Dolníček, Kropáč and Lehotský2013), we can estimate the conditions for crystallisation of stronalsite in the pseudomorphs investigated at temperatures ∼250–320°C and pressures not exceeding 100 MPa.
Conclusion
The stronalsite investigated represents the first occurrence of this rare Sr tectosilicate in the teschenite rocks of the in the Silesian Unit, and in the Western Carpathians. Two compositionally distinct types have been described in leucocratic teschenites: Ba-rich stronalsite and Ca-rich stronalsite. Both show identical Raman spectra. The substitution mechanism by which Ca ions enter the stronalsite structure cannot be unambiguously determined from the available data. In common with older slawsonite, stronalsite was formed by hydrothermal alteration of phenocrysts (probably nepheline). In addition to both Sr tectosilicates, pseudomorphs also contain K-feldspar, celsian, natrolite and thomsonite-Ca. The major source of Sr was probably primary magmatic plagioclase, which, like nepheline, was completely altered during hydrothermal alteration. Part of the Sr could also be derived from fluids of external origin. The crystallisation conditions are difficult to determine because both the pseudomorphs after felsic minerals and the groundmass were subject to intense zeolitisation. On the basis of the textural relationships of minerals present in the pseudomorphs investigated and comparison with earlier investigations, we suggest that stronalsite crystallised just between a high-temperature and a low-temperature hydrothermal stage, probably in a temperature range of ∼250–320°C at a pressure of <100 MPa.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1180/mgm.2024.97
Acknowledgements
This work was supported by the Palacký University Olomouc (IGA_PrF_2019_017) to K.K and by the Ministry of Culture of the Czech Republic (long-term project DKRVO 2024–2028/1.II.b; National Museum, 00023272) to Z.D. We also thank to three anonymous reviewers for constructive and helpful comments, which improved the clarity of the manuscript. The Principal and Production Editors are thanked for editing the manuscript.
Competing interests
The authors declare none.
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