Crystal chemistry of the high hydrous analogue of gismondine-Sr and the role of water in the extra-framework cations ordering

1 Introduction

Zeolites are the most common microporous framework minerals. Aluminosilicate framework of the recently discovered zeolite mineral gismondine-Sr, ideally Sr₄(Al₈Si₈O₃₂)·9H₂O, ¹ is topologically identical to the frameworks of other zeolites of the gismondine (GIS) family, namely gismondine-Ca, Ca₄(Al₈Si₈O₃₂)·18H₂O, ^{2 , 3} garronite-Ca, NaCa_2.5(Al₆Si₁₀O₃₂)·14H₂O, ⁴ garronite-Na, Na₆(Al₆Si₁₀O₃₂)·7–10H₂O, ⁵ gobbinsite, Na₅(Al₅Si₁₁O₃₂)·12H₂O, ⁶ and amicite, K₄Na₄(Al₈Si₈O₃₂)·10H₂O. ⁷ Six aforementioned zeolites reveal different extraframework cations and different Si/Al ratios. Gismondine-Ca, amicite and gismondine-Sr are distinguished from this group by the Si/Al ratio equal to one. ¹

A GIS-type structure topology that characterizes these minerals contains double wave-shaped bent chains of alternating SiO₄ and AlO₄ tetrahedra (a crankshaft configuration) with 4-membered rings. These chains are linked in the framework with a system of channels confined by eight-membered rings. The listed zeolites differ in the order of framework cations and vary in content and distribution of extra-framework components, which may lead to symmetry lowering from tetragonal (garronite-Ca) to orthorhombic (gismondine-Sr, gobbinsite) and monoclinic (gismondine-Ca, amicite, garronite-Na). These zeolites, as well as their synthetic pure sodium analogue, are active ionites under mild conditions, i.e. at low temperatures and low concentrations of exchangeable cations (Ag, Cs, Ba, Li, Na, K, Rb, and Pb) in aqueous solutions. ^{8 , 9 , 10 , 11 , 12}

Gismondine-Sr has been described as a new mineral species from the Halamish locality, Hatrurim Complex, Negev Desert, Israel. ¹ The empirical formula of the holotype (below, Sample 1) is (Sr_2.02Ca_1.09Ba_0.02K_0.72Na_0.62)(Al_7.91Si_8.09O_31.85)·9H₂O and structural investigation revealed orthorhombic symmetry (B22₁2). Subsequently, potassium-rich gismondine-Sr with the empirical formula (Sr_1.74Ca_1.05Ba_0.09K_1.56Na_0.49)(Al_7.98Si_8.06O₃₂)·9.62H₂O and the same space group has been described as a component of a xenolith from the Bellerberg paleovolcano in Germany (below, Sample 2). ¹³ In the present work, the crystal structure of an unusually highly hydrous and partially cation-ordered, Sr-dominant and K-rich gismondine with the simplified formula Sr₂CaK₂⎕(Al₈Si₈O₃₂)·15–16H₂O from the Bellerberg paleovolcano (Sample 3) was studied. Our results point out the relationship between hydration degree and extra-framework cation ordering in a zeolite structure.

2 Experimental

2.1 Occurrence and general appearance of the mineral

A highly hydrous analogue of gismondine-Sr studied in this work has been discovered in a calcic xenolith from the southern lava flow of the Bellerberg paleovolcano, East Eifel Mountains, Rhineland-Palatinate, Germany. Earlier, low-hydrous gismondine-Sr from the same xenolith was described. ¹³ The main primary, high-temperature minerals of the xenolith are wollastonite, åkermanite, gehlenite, larnite, combeite, bredigite, nepheline and leucite. Crystals of low-temperature minerals formed at the hydrothermal stage (the zeolites gismondine-Sr and flörkeite, as well as afwillite, ettringite, members of the tobermorite group, etc.) occur in cavities of the xenolith. The crystals of gismondine-Sr have a pseudo-tetragonal bipyramidal habit (Figure 1a). Some of these crystals might be evidence of dehydration cracks (Figure 1b).

Figure 1:

Gismondine-Sr (nearly dipyramidal) and flörkeite (tabular) crystals from the Bellerberg paleovolcano. Scanning electron microscopic images, back-scattered electron mode. The field of view widths are 350 μm (a) and 220 μm (b). Photo – Rafał Juroszek.

3 Results

The crystal structure of the highly hydrous analogue of gismondine-Sr is characterized by the GIS type framework, which is built by doubly connected chains of four-membered rings [1 and references therein]. It contains two secondary building units (SBU), namely four- and eight-membered rings of tetrahedra. ¹⁶ The four-membered rings are connected by sharing oxygen atoms, forming a double crankshaft chain. There are two systems of double crankshaft chains that create eight-membered rings forming channels. Cages hosting extra-framework cations and water molecules are formed at the intersections of the mutually perpendicular double crankshaft chains. A review of crystallographic features of the zeolites with GIS structure type is extensively reported in ref. 1]. In the structure of the studied mineral, Al and Si atoms are highly ordered at T sites, which is unambiguously proved by T–O distances (the averaged T(Si)–O distances vary from 1.610 to 1.623 Å and the T(Al)–O distances are in the range from 1.739 to 1.743 Å). Four extra-framework cation sites were found. The Sr1 site was refined as Sr versus Ca giving the composition Sr_0.964Ca_0.036. The Ca2 site is split over a two-fold axis and occupied by Ca in 48 %, whereas two K sites, K1 and K2, showed the site occupancy factors of 0.687(7) and 0.378(9) for the K scattering curve, respectively, and were assumed to be partly filled in the following way: K_0.45Ca_0.05(H₂O)_0.35⎕_0.15 (K1) [e_ref 13.05, e_calc 13.05] and Na_0.27K_0.22⎕_0.51 (K2) [e_ref 14.36, e_calc 14.3], where ⎕ is vacancy.

Minor Ba impurities may occur at the Sr, K1, and K2 sites. The remaining electron density on the difference Fourier synthesis was assigned to fully occupied (Ow1–Ow4) and partially occupied (Ow5a, Ow5b, Ow6a, Ow6b, Ow6c, Ow7, Ow8a, and Ow8b) sites of water molecules. The refinement resulted in the following formula obtained from the structure data: Sr_1.92Ca_1.14K_1.34Na_0.54(Al₈Si₈O₃₂)·15.7H₂O (Z = 2) that is in good agreement with chemical composition data. The crystal chemical formula is: (Sr_0.96Ca_0.04)₂(Ca_0.48⎕_0.02)₂(K_0.45Ca_0.05(H₂O)_0.35⎕_0.15)₂ (⎕_0.51Na_0.27K_0.22)₂(Al₈Si₈O₃₂)·15H₂O.

4 Discussion

Chemically and structurally, the highly hydrous analogue of gismondine-Sr is close to the recently reported holotype gismondine-Sr from Hatrurim Complex, ¹ and gismondine-Sr from the Bellerberg paleovolcano studied in ref. 13] (Table 4). A significant difference between these samples is in water content, which is considerably higher in the mineral studied in the present work. This results in the larger unit cell of the highly hydrous analogue of the gismondine-Sr in comparison with gismondine-Sr (V = 2090.19(6) ų) for the highly hydrous analogue of the gismondine-Sr from Bellerberg paleovolcano (Sample 3), 2023.44(4) ų for gismondine-Sr from Israel (Sample 1) and 2013.73(5) ų for gismondine-Sr from Bellerberg paleovolcano (Sample 2). Structural differences between gismondine-Sr samples studied earlier and their highly hydrous analogue studied in this work are related to the tilting of the framework tetrahedra as well as the distribution and ordering of extra-framework cations. This leads to different space groups (B22₁2 for the former and P2₁2₁2 for the latter). The comparison of tetrahedral frameworks of gismondine-Sr and its highly hydrous analogue is shown in Figure 2.

Figure 2:

Tetrahedral framework (in two projections) in the structure of the highly hydrous analogue of gismondine-Sr (Sample 3) (left part) and gismondine-Sr (Sample 2) (right part) drawn after ref. 13]. Si-centred tetrahedra are red, Al-centred tetrahedra are lilac. The unit cells are outlined.

In the structure of low-hydrous gismondine-Sr (Samples 1 and 2), two topologically identical cages are filled by cations and water molecules in a different way whereas in the mineral studied in this work (Sample 3) three types of topologically identical cages can be distinguished (Figure 3). One of the cages is filled by Sr²⁺ (with minor Ca²⁺) and water molecules. The next cage is occupied by K1 [K⁺ + Ca²⁺ + water molecules] and water molecules, whereas the last contains the Ca site, K2 site [K⁺ + Na⁺ + water molecules], and water molecules (Figure 4).

Figure 3:

Cages filled by extra-framework cations and water molecules projected along [101] in the structures of the highly hydrous analogue of gismondine-Sr studied in this work (Sample 3) (a) and gismondine-Sr (Sample 2) drawn after ref. 13] (b). Sr sites are purple spheres, K sites – yellow spheres, Ca sites – dark green spheres, water molecules – blue spheres, Na cations – green spheres.

Figure 4:

Cages present in the highly hydrous analogue of gismondine-Sr (Sample 3); Sr sites are green spheres, K sites – purple spheres, Ca sites – dark blue spheres, water molecules – red spheres.

A comparison of crystal structures of Samples 1, 2, and 3 suggests that a high degree of hydration prevents the disordering of extra-framework cations over numerous sites with low populations, which may lead to differences in symmetry between the distinct crystals. Interestingly, an analogous observation has been made for flörkeite and minerals of the phillipsite series. These minerals have frameworks of the PHI-type topology, which is built up by wave-shaped bent chains (crankshaft configuration), similar to structures with GIS-type topology. The topological symmetry in PHI-type structures is orthorhombic, space group Cmcm. However, the symmetry is lowered to monoclinic and triclinic in minerals of the phillipsite series, (K,Na,Ca_0.5,Ba_0.5)(Al _x Si_16–xO₃₂)·12H₂O, and flörkeite, (K₃Ca₂Na)(Al₈Si₈O₃₂)·12H₂O, respectively. ¹⁷ In minerals of the phillipsite series, symmetry is reduced to P2₁/m due to the distortion induced by the presence of large cations such as K and Ba in the channels. ^{18 , 19} These minerals exhibit a disordered arrangement of Al and Si cations over tetrahedral sites and a disordered or partially ordered distribution of extra-framework cations. In flörkeite, the symmetry is firstly lowered to B2/b owing to the ordered framework cations. ^{18 , 20} The reduction of symmetry to a triclinic crystal system occurs due to the ordered distribution of extra-framework cations and water molecules, leading to the doubling of the unit-cell parameter a. ²⁰ According to the ideal formulas, both flörkeite and minerals of the phillipsite series may host 12 H₂O molecules per formula unit. ²¹ However, most data on phillipsite-type minerals indicate significantly lower hydration degree. ^{22 , 23 , 24 , 25} On the other hand, flörkeite with strictly ordered extra-framework cations accommodates 12 H₂O molecules. ^{17 , 20} The higher degree of hydration seems to favor the partially ordered or ordered distribution of extra-framework cations in zeolites, leading to symmetry variation.

Moreover, extra-framework cation ordering may be triggered, not only by hydration but also by other causes that lower free volume in the pores. The ion exchange reaction of amicite with the empirical formula H_20.20Na_3.62K_4.06(Al_7.71Si_8.29O₃₂)O_10.09 with Cs, Rb, Pb, and Ag ⁹ has shown that the sizes of ions are relevant to the degree of hydration and arrangement in the extra-framework space. The empirical formulae of the ion-exchanged samples are H_16.45Na_3.445Cs_4.19(Al_8.10Si_7.90O₃₂)O_8.00, H_8.64Na_0.77K_0.44Rb_6.55(Al_7.86Si_8.14O₃₂)O_4.27, H_30.95Pb_3.98(Al_7.89Si_8.11O₃₂)O_15.51 and – H_27.90Ag_8.00(Al_7.96Si_8.04O₃₂)O_13.97, which corresponds to 8.2, 4.3, 15.5 and 14.0 H₂O molecules per formula unit, respectively.

Products of that ion exchange are monoclinic, pseudo-orthorhombic, and the replacement of Na and K with Pb is accompanied by a threefold increase in the c parameter (from ∼10 Å in the original sample to ∼29 Å). Simultanious incorporation of large Pb²⁺ cations and 15.5 H₂O molecules per formula unit into the GIS-type structure resulted in a mechanical stress and destruction of crystals of Pb-exchanged gismondine.

Incorporating the large Rb⁺ cation in the amicite structure is accompanied by significant dehydration, and extra-framework cations are ordered at three Rb-dominant sites and one K-dominant site. On the contrary, in Ag-exchanged amicite, small Ag⁺ ions are disordered over 11 sites with low occupancy factors. In the ion-exchange reaction of amicite with cesium, only K⁺ is substitured with Cs⁺ whereas the content of smal Na⁺ cations remains unchanged. As a result, the content of H₂O molecules decreases insignificantly and is accompanied by the transformation of two sites of extra-framework cations to four sites, Na1, Na2, Cs1 and Cs2. ⁹

As a result of the ion exchange of gismondine-related zeolites with other cations, the volume of the unit cell decreases significantly (from 1,047.6 ų for the initial amicite to 962–1,015 ų in the ion exchange products), ^{9 , 10} which indicates a high degree of elasticity of the framework.

It was also found that on heating above 75°С, amicite partially dehydrates and degrades into K- and Na-dominant phases. ²⁶ It is important to notice that hydration level, and consequently arrangement of extra-framework cations, might influence the ion-exchangeable capacity of zeolites. Therefore, insight into the microstructure of zeolites is indispensable for the development of zeolite-based ion exchangers. ²⁷

Synthetic zeolites occurring in hydrothermal sodium-aluminosilicate systems with GIS topology are well-known family called P zeolites. ²⁸ These compounds attract attention due to their ion-exchange, water-softening properties, they could be used for in environmental-friendly detergents, gas separation and removal of toxic and radioactive wastes [29] and references therein].

The GIS-type zeolite MAP, Na₈(Al₈Si₈O₃₂)·nH₂O, exhibits a high framework flexibility and a high intrinsic thermodynamic selectivity for calcium over sodium. ¹¹ The exchanges of Na by Li, K, Rb, Cs, Mg, Ca, Sr, and Ba in MAP have been examined. ¹² Structural data showed that cation exchange often caused changes in symmetry.

5 Conclusions

Taking into account a partial ordering of extra-framework cations in the crystal structure of the highly hydrous analogue of gismondine-Sr, its simplified formula can be written as Sr₂CaK₂⎕(Al₈Si₈O₃₂)·15H₂O. One can suppose that in low-hydrous varieties of gismondine-Sr, enhanced free volume of zeolite channels promotes the multiplication of the number of sites of extra-framework cations, which results in their disordering. On the other hand, the large extra-framework cations may prevent the disordering in extra-framework space, as has been shown in amicite. The high content of large K⁺ cations in flörkeite may be the main cause of the lowering of the free volume, which could promote the ordering of extra-framework cations and is favorable for the ordered arrangement of water molecules.