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.

For key see (Fig. 6a).

Figure 7. 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/⁸⁶Sr_{i(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/⁸⁶Sr_{i(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/⁸⁶Sr_{i(120 Ma)} = 0.7045 and 0.7038) and significantly lower ⁸⁷Sr/⁸⁶Sr_{i(120 Ma)} ratios when compared to Upper Jurassic-Lower Cretaceous sediments of the Silesian Unit (⁸⁷Sr/⁸⁶Sr_{i(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 (An_36–52) 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 (Ab_20–40). 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:

(1)\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:

(2)\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:

(3)\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

${\text{2N}}{{\text{a}}_{\text{3}}}{\text{KA}}{{\text{l}}_{\text{4}}}{\text{S}}{{\text{i}}_{\text{4}}}{{\text{O}}_{{\text{16}}}}{\text{ + SrA}}{{\text{l}}_{\text{2}}}{\text{S}}{{\text{i}}_{\text{2}}}{{\text{O}}_{\text{8}}}{\text{ + 2}}{{\text{H}}_{\text{2}}}{\text{O}}$

(4)\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:

(5)\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.