Записки Российского минералогического общества, 2020, T. 149, № 6, стр. 110-121

1. INTRODUCTION

Lead aluminosilicates represent technologically and mineralogically important group of mineral phases that occur in complex sinters and slugs produced during lead and zinc smelting and are considered as possible matrices for immobilization of Pb (Lu, Shih, 2011, 2012, 2015; Lu et al., 2013; Yang et al., 2020). The “Pb₄Al₄Si₃O₁₆” phase was found on Roman coins of the III–IVth centuries, where it most probably formed as a constituent of the patinas the patinas (Mata et al., 2010). Only one Pb aluminosilicate mineral without additional cations (except for H) or anions is known to date, rongibbsite, Pb₂(Si₄Al)O₁₁(OH), that is based upon a zeolite-like microporous interrupted tetrahedral framework (Yang et al., 2013).

The PbO–Al₂O₃–SiO₂ system was first investigated by Geller and Bunting (1943), who reported formation in this system of three ternary compounds: Pb₈Al₂Si₄O₁₉, Pb₄Al₂Si₂O₁₁, and Pb₆Al₂Si₆O₂₁. The existence of these compounds was confirmed by Mylyanych et al. (1999) during their studies of crystallization of Pb-containing aluminosilicate glasses. Haensel et al. (1976) described another ternary compound, Pb₄Al₄Si₃O₁₆, and reported its X-ray diffraction pattern (ICDD PDF #32-0505). In the detailed study of phase equilibria in the PbO–Al₂O₃–SiO₂ system, Chen et al. (2001) found eleven different ternary crystalline phases: Pb₈Al₂Si₄O₁₉, Pb₄Al₂Si₂O₁₁, Pb₆Al₂Si₆O₂₁, Pb₄Al₄Si₃O₁₆, PbAl₂Si₂O₈, Pb₃Al₁₀SiO₂₀, Pb₄Al₄SiO₁₂, Pb₄Al₄Si₅O₂₀, Pb₅Al₂Si₁₀O₂₈, Pb₁₂Al₂Si₁₇O₄₉, and Pb₁₂Al₂Si₂₀O₅₅. Dörsam et al. (2008) reported the high-pressure and high-temperature (2 GPa, 650 °C) hydrothermal synthesis of Pb₂Al₂Si₂O₉ that was described as an Al analogue of kentrolite, Pb₂Mn₂Si₂O₉, and melanotekite, Pb₂Fe₂Si₂O₉. The high-pressure phase Pb_0.8Al_1.6Si_2.4O₈ with a hollandite-type structure was studied by Downs et al. (1995).

However, despite such a rich and outstanding chemical diversity of Pb aluminosilicates, the crystal-structures are known for four of them only: PbAl₂Si₂O₈ (the Pb analogue of feldspar; Benna et al., 1996, 1999; Tribaudino et al., 1998; Curetti et al., 2015), Pb₂Al₂Si₂O₉ (Dörsam et al., 2008), Pb₆Al₂Si₆O₂₁ (Siidra et al., 2009), and Pb_0.8Al_1.6Si_2.4O₈ (Downs et al., 1995).

Herein we report on the crystal structure of Pb₁₀Al₁₀Si₈O₄₁, a lead aluminosilicate, which is most probably identical to the phase previously reported as Pb₄Al₄Si₃O₁₆.

2. EXPERIMENTAL

Several tiny colorless transparent crystals of Pb₁₀Al₁₀Si₈O₄₁ were obtained as a by-product during our experiments in the PbO–SiO₂ system (Siidra et al., 2009). The mixture of PbO and SiO₂ in the 1 : 1 ratio was ground, mixed in an agate mortar and placed into an alumina crucible that was heated to 800 °C and kept at that temperature for 1 hour. The crucible was then cooled to room temperature at the rate of 2.5 °C/min. The main product was a vitreous matrix with few tiny crystals of the title compound grown on the border between the matrix and the crucible walls.

One of the crystals was placed onto a Bruker SMART three-circle diffractometer equipped with an APEX CCD area detector. More than a hemisphere of the diffraction data was collected by using the exposure time of 45 s per frame. The absorption correction was done using the SADABS program. The crystal structure was solved and refined by means of the SHELX program package (Sheldrick et al., 2015). The crystal data and experimental parameters of the X-ray diffraction experiment are given in Table 1, atom coordinates, site-occupancies and displacement parameters are provided in Table 2. Table 3 contains selected interatomic distances for the crystal structure.

3. RESULTS

The crystal structure of Pb₁₀Al₁₀Si₈O₄₁ contains three symmetrically independent Pb atoms, each having distorted coordination environment due to the stereoactivity of the 6s² lone electron pair. The Pb1 and Pb2 atoms form four Pb–O bonds shorter than 2.5 Å, arranged at the corners of the base of PbO₄ square pyramid with Pb atom at the apical corner (Fig. 1). The similar coordination is observed in the crystal structure of litharge, PbO. However, in the title compound the PbO₄ square pyramid is complemented by four and six long Pb–O bonds (Pb–O > 3.0 Å) for the Pb1 and Pb2 atoms, respectively. The Pb3 atoms is coordinated by three O atoms with the Pb–O bonds shorter than 2.6 Å and seven Pb–O bonds with the bond lengths in the range 2.8–3.2 Å.

Fig. 1.

Coordination of Pb²⁺ cations in the crystal structure of Pb₁₀Al₁₀Si₈O₄₁. Pb and O atoms are shown as black and red, respectively. The Pb–O bonds longer than 2.6 Å are shown as black lines. Рис. 1. Координация катионов Pb²⁺ в кристаллической структуре Pb₁₀Al₁₀Si₈O₄₁. Атомы Pb и O изображены черным и красным, соответственно. Связи Pb–O с длиной выше 2.6 Å показаны черными линиями.

There are four tetrahedrally coordinated sites in the structure, T1–T4, with the average 〈T–O〉 bond lengths varying from 1.66 to 1.69 Å. These bonds are longer than the ideal Si–O bonds and shorter than the ideal Al–O bonds for Si and Al in tetrahedral coordinations, which points out that the T1–T4 sites are statistically occupied by Si and Al. In order to fulfill the stoichiometry requirements, the site occupancies for these sites were fixed at Si_0.5Al_0.5. There is one additional Al site that has a fivefold trigonal bipyramidal (TBP) coordination with the O9 site half-occupied and split over two symmetrically equivalent positions located at 1.56 Å from one another (see below). The TBP coordination is typical for Al (Gorelova et al., 2019).

The crystal structure is based upon tubular tetrahedral chains, [T₄O₁₀], running parallel to the a axis and formed by T1–T4 atoms coordinated by O (Fig. 2a). The chains have a square section and belong to the family of tubular silicate anions reviewed by Rozhdestvesnkaya and Krivovichev (2011). The chains are linked into framework via Pb atoms and dimers of corner-sharing AlO₅ trigonal bipyramids located in between the chains.

Fig. 2.

The crystal structure of Pb₁₀Al₁₀Si₈O₄₁ projected along [100] (a) and its slice along the (001) plane showing linkage of aluminosilicate tetrahedral chains via Pb²⁺ cations and dimers of (AlO₅) trigonal bipyramids (b). Рис. 2. Кристаллическая структура Pb₁₀Al₁₀Si₈O₄₁ в проекции вдоль [100] (a) и ее срез по плоскости (001), показывающий связь алюмосиликатных тетраэдрических цепочек через катионы Pb²⁺ и димеры тригональных бипирамид (AlO₅) (b).

4. DISCUSSION

The Pb₁₀Al₁₀Si₈O₄₁ stoichiometry has not been observed previously for the PbO–Al₂O₃–SiO₂ system (see Introduction), but possesses some similarity to the Pb₄Al₄Si₃O₁₆ stoichiometry first reported by Haensel et al. (1976). The Pb : Al : Si ratios for the Pb₁₀Al₁₀Si₈O₄₁ and Pb₄Al₄Si₃O₁₆ compositions are equal to 5 : 5 : 4 and 4 : 4 : 3, or 1 : 1 : 0.80 and 1 : 1 : 0.75, respectively, which are rather close to each other and can potentially be mixed up in the chemical analyses. The comparison between the experimental X-ray powder diffraction pattern for Pb₄Al₄Si₃O₁₆ (ICDD PDF #32-0505; Haensel et al., 1976) and theoretical one calculated on the basis of crystal-structure data reported herein show remarkable similarities, except for the reflection with d = 4.36 Å versus the calculated value of 4.27 Å (in our opinion, a misprint cannot be excluded). The closeness of the two diffraction patterns provides strong evidence in favor of the identity of Pb₁₀Al₁₀Si₈O₄₁ studied by us with the Pb₄Al₄Si₃O₁₆ compound reported previously.

According to the crystal-structure study, the crystal-chemical formula of Pb₁₀Al₁₀Si₈O₄₁ can be written as Pb₁₀Al₂O[(Si₂Al₂)O₁₀]₄ that emphasizes the composition of tetrahedral chains with disordered distribution of Si and Al over tetrahedral sites. The crystal structure of the title compound is related to that of Pb₆O[(Si₆Al₂)O₂₀] (Siidra et al., 2009), which is based upon the tubular [(Si₆Al₂)O₂₀]^10– chains with the same topology as observed in Pb₁₀Al₁₀Si₈O₄₁ (Fig. 3a). However, the linkage of the chains is different and is provided by the oxocentered (Pb₆O)¹⁰⁺ units located in between the chains (Fig. 3b). In fact, each of these units forms a core of the larger {(Pb₆${\text{O}}_{8}^{T}$)O_a} units, where O^T and O_a notations correspond to the O atoms bonded to T atoms and additional O atoms, respectively. The (Pb₆${\text{O}}_{8}^{{{\text{Si}}}}$) configuration has the shape of a rhombododecahedron with Pb atoms at the tetravalent corners and O atoms at the trivalent corners. The similar units are present in the title compound as well. Figure 4 compares geometries of the (Pb₆${\text{O}}_{8}^{{{\text{Si}}}}$) units in Pb₁₀Al₂O[(Si₂Al₂)O₁₀]₄ and Pb₆O[(Si₆Al₂)O₂₀] (Figs. 4a and b, respectively). It can be seen that the units in the latter compound are more compressed, most likely due to the presence of the additional O_a atom at the center of the (Pb₆) octahedron. The Pb^…Pb distances in the octahedron are in the range 3.89–3.99 Å in the title compound (with “empty” (Pb₆) octahedron) and 3.55–3.79 Å in Pb₆O[(Si₆Al₂)O₂₀] (with the (Pb₆) octahedron centered by the O_a atom). The occurrence of the same structural units in similar structure types with occupied and non-occupied polyhedral cavities has been observed previously in inorganic structures with lone-electron-pair cations [e.g., the “empty” (Pb₆) octahedron in Pb₂₁[Si₇O₂₂]₂[Si₄O₁₃] (Siidra et al., 2014) versus (ClPb₆) octahedron in hyttsjöite, Pb₁₈Ba₂Ca₅${\text{Mn}}_{2}^{{2 + }}{\text{Fe}}_{2}^{{3 + }}$Si₃₀O₉₀Cl · 6H₂O (Grew et al., 1996)]. It should be noted that, in the title compound, only Pb1 and Pb2 atoms participate in the formation of (Pb₆) octahedra, whereas the Pb3 atom is associated with the dimers of (AlO₅) trigonal bipyramids.

Fig. 3.

The crystal structure of Pb₆O[(Si₆Al₂)O₂₀] projected along [001] (a) and the linkage of aluminosilicate tetrahedral chains to the (OPb₆) units (b). Рис. 3. Кристаллическая структура Pb₆O[(Si₆Al₂)O₂₀] в проекции вдоль [001] (a) и объединение алюмосиликатных тетраэдрических цепочек с группами (OPb₆) (b).

Fig. 4.

Rhombododecahedral Pb–O units in the crystal structures of Pb₁₀Al₁₀Si₈O₄₁ (a) and Pb₆O[(Si₆Al₂)O₂₀] (b) and the geometrical parameters of Pb₆ octahedral motifs. See text for details. Рис. 4. Ромбододекаэдрические Pb–O единицы в кристаллических структурах Pb₁₀Al₁₀Si₈O₄₁ (a) и Pb₆O[(Si₆Al₂)O₂₀] (b) и геометрические параметры октаэдров Pb₆. См. пояснения в тексте.

The crystal structure of Pb₁₀Al₁₀Si₈O₄₁ is also similar to that of narsarsukite, Na₂TiO[Si₄O₁₀] (Pyatenko, Pudovkina, 1960; Peacor, Buerger, 1962), and isotypic compound K₂ScF[Si₄O₁₀] (Kolitsch, Tillmanns, 2004). Both structures are based upon tubular chains of four-membered rings, which are interlinked by the chains of octahedra running parallel to the silicate chains. Figure 5 shows general features of the crystal structure of K₂ScF[Si₄O₁₀].

Fig. 5.

The crystal structure of K₂ScF[Si₄O₁₀] projected along [001] (a) and the linkage of octahedral and tetrahedral chains through sharing common corners (b). Рис. 5. Кристаллическая структура K₂ScF[Si₄O₁₀] в проекции вдоль [001] (a) и объединение тетраэдрических и октаэдрических цепочек через общие вершины (b).

The structure type of the title compound is new for inorganic compounds and may be viewed as a hybrid of the structure types of Pb₆O[(Si₆Al₂)O₂₀] and narsarsukite. On one hand, it can be obtained from narsarsukite by replacing each second pair of corner-sharing (TiO₆) octahedra by (Pb₆) octahedron (as a consequence, the sixfold coordination of Ti transforms into fivefold coordination of Al). On the other hand, the crystal structure of Pb₁₀Al₁₀Si₈O₄₁ can be obtained from that of Pb₆O[(Si₆Al₂)O₂₀] by the replacement of each second (Pb₆O) octahedron by the TBP dimer surrounded by four Pb3 atoms. The disordered and ordered configurations of the dimer are shown in Fig. 6. The O9 site that is shared between two adjacent (AlO₅) units is half-occupied so that the averaged configuration (Fig. 6a) is the overlap of two ordered configurations (Fig. 6b). As a result, the atoms involved in the disordered moiety (especially Al, O9 and O10) possess strongly elongated displacement parameters, indicating that the observed Al–O bond lengths are strongly distorted due to the libration effects.

Fig. 6.

The disordered (a) and ordered (b) configurations of trigonal bipyramidal dimers in the crystal structure of Pb₁₀Al₁₀Si₈O₄₁. Рис. 6. Разупорядоченная (a) и упорядоченная (b) конфигурации тригонально-бипирамидальных димеров в кристаллической структуре Pb₁₀Al₁₀Si₈O₄₁.

The structure type of Pb₁₀Al₁₀Si₈O₄₁, being the ordered hybrid version of the narsarsukite and Pb₆O[(Si₆Al₂)O₂₀] structure types, can be considered as a superstructure of both of them. The analysis of the unit-cell parameters for the three structure types (Table 5) shows that the unit cell of the title compound corresponds to the 2 × 2 × 2 (eightfold) supercell relative to both narsarsukite and Pb₆O[(Si₆Al₂)O₂₀]. The information-based structural complexity parameters (Krivovichev, 2013) of Pb₁₀Al₁₀Si₈O₄₁ (Table 5) are therefore higher than those of the parent structure types with the total structural information per cell (590.100 bits) about 6.2–6.5 times larger. This agrees well to the previous observations on structural complexity for the minerals with the “structure-superstructure” relations (Krivovichev et al., 2019).

The author is grateful to Igor Pekov for the useful remarks on the manuscript. The reported study was funded by the Russian Science Foundation, project number 19-17-00038.