Introduction

Mallestigite was reported as a new mineral (IMA 1996-043) from the ore dump of a former copper-lead-zinc mine, which is located on Mallestiger Mittagskogel mountain, Carinthia, Austria. Brief reports about the mineral and its properties were subsequently published by Sima et al. (1996) and Sima (1998). But, due to issues regarding crystallography a full report of the mineral was postponed. Since then, the mineral was reported from further occurrences in Tyrol and Styria, Austria (Poeverlein and Gröbner 2007; Walter et al. 2016), Northeastern Italy (Giarduz et al. 2015), Spain (Rewitzer et al. 2020) and as a weathering product from a slag dump near Goslar, Germany (Schnorrer 2003). The present paper aims to give an evaluation of the mineral from the type locality including a new structure determination, and to lay out structural relationships to related minerals and synthetic material investigated in recent years.

Occurrence and paragenesis

The type locality of mallestigite is an ore dump of a small abandoned copper-lead-zinc mine, located ⁓1195 m above sea level, ⁓1 km NW of the Mallestiger Mittagskogel peak, Karawanken range, in Carinthia, Austria (46°31’56.7’’ N, 13°52’21.7’’ E). This mine belongs to the polymetallic sulfide ore district of western South-Karawanken (Weber 1997; IRIS 2023) comprising small former sediment-hosted copper mines and prospects that operated mainly in the 15th–16th and in the 19th century in an approximately 10 km East-West extending area south of Finkenstein near Villach, Carinthia, Austria (Puttner 1994). They all seem to be bound to strips of E-W extending limestones in schist or sandstone strata of Palaeozoic age, but geologic and genetic details are still unclear (Weber 1997; IRIS 2023). The small orebodies of interest were stratiform or impregnations and carried fahlore, galena, sphalerite and secondary minerals like malachite and azurite. One of these mining places is the “Neufinkenstein-Grabanz” district and to this district does the type locality of mallestigite belong, known under the historic names “Segen Gottes” mining claim and “Glückauf” mining tunnel (Puttner 1994). Here fahlore, chalcopyrite, galena and sphalerite, with baryte as gangue, and secondary malachite were mined from a zone near the tectonic contact between Palaeozoic and lower Triassic formations (Canaval 1926; Puttner 1994, 1995; Weber 1997; IRIS 2023). The raw ore mined in the 1880s was collected on an ore dump of ~ 60 t that was not recovered because of non-profitability. This ore dump was since exposed to weathering in a humid continental climate with snow in winter and warm wet summers.

The mineral is found in narrow fractures and cavities of gangue and ore together with primary sulfides and secondary minerals. Fahlore – zoned, chemically inhomogeneous crystals of tennantite/tetrahedrite-(Zn) solid solutions with Sb/As ratios varying from 0.07 to 4.7 (for details see Taucher 1998) – is typically nearby. Mallestigite is found as idiomorphic to hypidiomorphic crystals up to 2 mm in length and 0.4 mm in thickness (Fig. 1). It may be confused with quartz or mimetite in its common hexagonal prismatic habit. It is also forming radial aggregates with diameters up to 3 mm. The mineral may have formed during weathering of the primary ore minerals galena and fahlore in the over 100 years old raw ore dump though oxidized Cu ore was once mined, too (Puttner 1994). It is associated with the sulfates anglesite, PbSO4, brochantite, Cu4(SO4)(OH)6, langite, Cu4(SO4)(OH)6·2H2O, linarite, PbCu(SO4)(OH)2 and the rare arsenate schultenite, Pb(HAsO4). More than 60 further primary and secondary minerals have been reported from this site in the course of time, mainly sulfates, arsenates and carbonates of Cu, Pb, Zn, and among them theisite, Cu5Zn5(AsO4,SbO4)2(OH)14, and camérolaite, Cu6Al3(OH)18(H2O)2[Sb(OH)6](SO4), as two further Sb-bearing species apart from mallestigite. A list of the currently known minerals from this location has been presented in the supplementary materials together with corresponding references. Type specimens of mallestigite are deposited in the collections of the Centre of Natural History, Universalmuseum Joanneum, Weinzöttlstraße 16, A-8045 Graz, Austria, catalogue numbers 80.147 (holotype) and 86.348–86.350 (cotypes). The present investigation was based on holotype material except for Fig. 1.

Fig. 1
figure 1

Typical crystals of mallestigite (cotype 86.350) on corroded quartz and weathered ore. Photograph Hans-Peter Bojar

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Analytical characterization

Physical properties

Crystals of mallestigite from the type locality are transparent to translucent. They are colourless with adamantine lustre and give a white streak. The mineral is brittle with splintery fracture, no cleavage was observed. It does not fluoresce in ultraviolet radiation. Mallestigite has a simple morphology: hexagonal prism {100} elongated parallel [001] and terminated by the hexagonal dipyramid {101} (Fig. 1). Morphological twinning was not detected, but twinning by merohedry with (001) and (110) as twin planes was observed via X-ray investigations.

Optically, mallestigite is uniaxial positive, non-pleochroic and indices of refraction (λ = 589 nm) are nω = 1.760(4) and nε = 1.801(4) according to measurements on a spindle stage using the λ-T-variation method. Mass density was not measured because it exceeded that of the Clerici solution and the sample amount was too small for pyknometry. The calculated mass density is 4.91 g cm−3 for the chemical formula Pb3Sb(SO4)(AsO4)(OH)6·3H2O and the unit cell dimensions given below. Micro-indentation hardness measurements, using a microhardness tester, gave VHN10 = 176. This corresponds approximately to Mohs hardness of 4.

FTIR examinations were performed on a PerkinElmer Spectrum 100 instrument which was equipped with a diamond universal attenuated total reflectance (ATR) sampling accessory. The same powder of mallestigite was used for PXRD and FTIR analytics. The spectrum represents the sum of 20 scans from 4000 to 500 cm−1 with a resolution of 2 cm−1. No ATR correction was performed. Absorption bands in the IR spectrum of mallestigite (Fig. 2) and their tentative assignments are (cm−1; s – strong band, sh – shoulder): ⁓3400sh, 3201, 2812 (O-H stretching vibrations); 2317 (overtone or combination modes); 1630 (H-O-H bending vibrations); 1216, 1117s, 1067sh, 1037s (asymmetric stretching vibrations of SO42–); 964 (symmetric stretching vibrations of SO42–); 798s (stretching vibrations of AsO43–); 669sh, 616, 585s (unassigned). The spectrum and all bands are in good accordance with the two mallestigite spectra reported by Chukanov (2014).

Fig. 2
figure 2

IR spectrum of mallestigite

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Chemical composition

A mallestigite crystal was placed in a cylindrical brass sample holder and polished to a surface quality of 1 μm and coated with carbon. Chemical analyses were undertaken on a Jeol 6610 LV scanning electron microscope equipped with Oxford wavelength-dispersive spectrometers and an Oxford energy-dispersive detector. An accelerating voltage of 20 kV and a beam current of only 2 nA was used because mallestigite is very unstable at vacuum and under the electron microbeam. Moreover, the beam was rasterized on an area of 8 to 12 μm to reduce sample damage. Beside Pb, Sb, S and As, no other elements with an atomic number higher than 11 were detected in the energy-dispersive spectra. Following reference materials were used for quantification of the wavelength-dispersive analyses: PbS (galena) for Pb and S, GaAs for As, and Sb2Te3 for Sb. Data reduction was performed with the PAP routine (Pouchou and Pichoir 1991) as implemented in the INCA software of Oxford Instruments and element counts were normalized to 11 O atoms giving element proportions Pb:Sb:As:S of 3.03:1.08:0.99:0.93 (spot 1) and 3.02:1.04:0.93:1.02 (spot 2) with mean values of 3.02:1.06:0.96:0.97 and a sum of 6.01 atoms. The empirical formula of mallestigite is Pb3.02Sb1.06(SO4)0.96(AsO4)0.97(OH)6·3H2O, where the OH and H2O content is based on the crystal structure determination, giving the idealized chemical formula Pb3Sb(SO4)(AsO4)(OH)6·3H2O subsequently applied. Chukanov (2014; pp 1174–1175) reported for a sample of mallestigite from the type locality a small amount of 0.1 SiO4 replacing AsO4. Giarduz et al. (2015) gave for mallestigite from Monte Avanza, Italy, the approximate chemical formula Pb3.3Sb1.2(S0.9O4)(As1.2O4)(OH)6·3H2O.

Powder X-ray diffraction

Powder X-ray diffraction data for mallestigite from the type locality were obtained at 293 K using a Bruker D8 diffractometer with CuKα radiation, an auto-divergence slit and a Lynxeye detector. The finely ground sample was placed on a low-background silicon specimen support, homogeneously distributed with a drop of ethanol, and was spun during measurement. Unit cell parameters were derived by Rietveld refinement with fundamental parameters using TOPAS 4.2 software (Bruker 2009) and the structural data of the single-crystal structure determination (see below; atomic coordinates fixed, Uiso of Pb, Sb, As, and S optimized). The dimensions of the hexagonal unit cell were found as a = 8.9390(2) Å, c = 11.1007(3) Å and V = 768.18(3) Å3, in good agreement with those obtained from the single-crystal study. When the sample was pressed with a glass slide, intensities of h00 reflections became texture enhanced. This appears to be in agreement with the observed morphology of mallestigite crystals and suggests that (100) is a cleavage plane. A comparison of the measured and calculated X-ray powder patterns is presented in Fig. 3. The measured pattern includes background as measured, the calculated pattern is background-free, and a difference curve measured minus Rietveld-fit intensities is shown below. Rwp = 0.029 and RBragg = 0.024 was obtained in the Rietveld refinement with a reflection half-width of 0.11° for the strongest peak. A minute peak of a small impurity of anglesite at 2θ = 20.6° was neglected in the Rietveld refinement. It should be noted that the powder diffraction intensities are heavily dominated by Pb, Sb, As, and S due to their pseudosymmetric spatial arrangement, whereas O and H atoms play only a small role. Intensities and d values of the mallestigite X-ray powder pattern are presented in Table 1. Figure 3 includes also an X-ray powder pattern of fleischerite, Pb3Ge(SO4)2(OH)6·3H2O, hexagonal, a = 8.867 Å, c = 10.875 Å, calculated with the structural data of Otto (1975). A pronounced similarity to the pattern of mallestigite can be seen in the 2θ range 10 to 42°. However, the Bragg angles of fleischerite peaks are unmistakably shifted to higher values with increasing 2θ. It is worth mentioning that the similarity of mallestigite and fleischerite X-ray powder patterns did lead to misidentification, as happened for the type locality (Puttner 1995), and very likely for two reported fleischerite occurrences from deposits with Pb, Sb, As in sufficient amount but without Ge on the record (Dill et al. 1997; Navarro and Cardellach 2009).

Fig. 3
figure 3

X-ray powder pattern of mallestigite, Pb3Sb(SO4)(AsO4)(OH)6·3H2O, for CuKα radiation: a as measured; b Rietveld fit to measured pattern without background and with the 8 strongest reflections indexed; c difference between measured and calculated pattern, Rwp = 0.029; d calculated X-ray powder pattern of fleischerite, Pb3Ge(SO4)2(OH)6·3H2O (Otto 1975), shown for comparison

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Table 1 X-ray powder diffraction data for mallestigite
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Structure determination

Previous investigations by rotation and Weissenberg photographs of several crystals as well as a structure determination based on diffractometer data of a severely twinned crystal had shown that mallestigite is hexagonal and that it belongs to the fleischerite group (Sima et al. 1996; Sima 1998). However, the space group symmetry of the mineral, finally given as P63, and reliability of structural parameters remained open to question due to pseudosymmetry, twinning, and severe absorption.

For the present investigation a well-shaped hexagonal prism of nominal dimensions of 0.030 × 0.036 × 0.25 mm3 was investigated on a Bruker Smart APEX CCD diffractometer with graphite-monochromatized MoKα radiation. After ascertaining that the crystal showed no superstructure or other detectable order-disorder effects, a total of 15,183 reflections covering a complete Ewald sphere up to θmax = 32° was collected by fine-sliced ω scans (0.3°, 30 s) in five hemi-circle swings. After data integration with program SAINT (Bruker 2007), the reflection intensities were corrected for absorption by a combination of optimized analytic and multi-scan method using program SADABS (Bruker 2007). The hexagonal unit cell, a = 8.9326(4) and c = 11.1044(5) Å, determined by least-squares refinement of 7676 reflections in the θ range 2.6 ‒ 31.7°, is in good agreement with the values extracted from X-ray powder diffraction. The analysis of the intensity data proved that the Laue symmetry is 6/m (Fig. 4) with 00 l = 2n + 1 as the only systematic extinction and a satisfactory Rint on merging symmetry equivalent reflections. This result corresponds with space groups P63 (non-centrosymmetric polar) and P63/m (centrosymmetric) exclusively. Knowing from earlier investigations that mallestigite is closely related to fleischerite group minerals and their symmetry issues (see below), the crystal structure of mallestigite was assumed to be unknown and redetermined ab-initio using program SHELXT (Sheldrick 2015a) in multi-solution mode with all trigonal and hexagonal space groups considered. Space group P63 fared best among the 45 space groups evaluated followed by the hexagonal space groups P\(\overline6\) and P63/m. Refinement of the fully ordered structure model in space group P63 with anisotropic displacement parameters for all atoms, a small inversion twinning parameter, and a correction for extinction led rapidly to R1 = 0.0196 (Sheldrick 2015b). At this stage a small orientation disorder of a sulfate oxygen atom was recognized that lowered R1 to 0.0186. Then the hydrogen atoms of the OH groups and the H2O molecule were inferred from stereochemistry and were included in the refinement using the restraints O‒H = 0.80 Å and H‒O‒H = 106° while their x,y,z were allowed to vary. In the final refinement R1 = 0.0184 for 1632 merged Fo > 4σ(Fo) was obtained with a satisfactory agreement between Fo and Fc in all classes of reflections subdivided by Fo/Fmax and resolution. The parameter for merohedral twinning on (001) refined to 0.07(1). A check of displacement ellipsoids, geometric data, and specific structural features was also satisfactory. The resulting crystallographic data are summarized in Table 2, and atomic parameters are given in Tables 3 and 4. A CIF with comments and unmerged reflection data is included in the supplementary materials. Structure graphics was generated with programs Diamond (Brandenburg 2016) and Mercury (Macrae et al. 2006).

Fig. 4
figure 4

hk0 layer of the weighted reciprocal lattice of mallestigite demonstrating the 6/m Laue symmetry. The largest spots ‒hk0 = 220, 600, 440, 820, 660 ‒ reflect the two-dimensional pseudosymmetric arrangement of the heavy atoms Pb, Sb, and As in the unit cell shown on the lower right

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Table 2 Crystal data and structure refinement for mallestigite
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Table 3 Atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) for mallestigite
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Table 4 Atomic displacement parameters (Å2) for mallestigite
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Discussion

The crystal structure of mallestigite

A general view of the crystal structure of mallestigite is shown in Fig. 5, closer details of the structure including bond lengths are outlined in Fig. 6 and Tables 5 and 6. The structure of mallestigite is built up from [Sb5+(OH)6]1– octahedra that are linked by triplets of edge-sharing Pb2+ to form infinite chains {Pb3[Sb(OH)6]}5+ extending along 63 axes at 0,0,z parallel to the c axis. Each three of these chains are mutually linked via Pb-bonded SO42– and AsO43– anions located on 3-fold axes and three H2O per formula unit around them. The SO42– and AsO43– anions alternate along the 3-fold axes and point with their apex atoms O1 and O3 all in the same direction making the structure clearly polar.

Fig. 5
figure 5

The crystal structure of mallestigite, Pb3Sb(SO4)(AsO4)(OH)6·3H2O, in a general view (a), and in a projection along the c axis (b). 63 axes run along 0,0,z and 3-fold axes along 1/3,2/3,z and 2/3,1/3,z. There are no other symmetry elements and the choice of origin with Sb in z = 0 is arbitrary. H atoms of OH groups omitted. The structure is acentric polar with the upper vertices of the SO4 and AsO4 tetrahedra all pointing in the direction of the positive c axis. The red line in (b) represents the (110) plane to emphasize a pseudosymmetry that is outlined in the text

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Fig. 6
figure 6

Details of the crystal structure of mallestigite, Pb3Sb(SO4)(AsO4)(OH)6·3H2O, in ball-and-stick presentation with bond lengths in Å and H atoms in blue. Numbers in italics give O···O distances for hydrogen bonds

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Table 5 Selected bond lengths and angles for mallestigite
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Table 6 Hydrogen bonds for mallestigite [Å and °]
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The pentavalent Sb atom is located on the 63 axis at 0,0,z and 0,0,z+½ with z = 0 fixed arbitrarily but not by symmetry requirements. It is in a distorted octahedral coordination by each three hydroxyl oxygen atoms O5h and O6h with Sb–O bond lengths of 1.964(6) and 1.988(6) Å, giving a mean value < Sb–O > = 1.976 Å, that is identical with the grand mean value reported for 158 Sb5+O6 octahedra (Gagné and Hawthorne 2018a). The edge lengths of this octahedron – O5h–O5h = 2.74 Å, O6h–O6h = 2.67 Å, O5h–O6h = 2.87 and 2.90 Å – show that it is distorted and stretched along the c axis. The distances between Sb and the two triangular faces of O5h and O6h are 1.16 and 1.26 Å, respectively. They and the Sb–O bond distances point to an asymmetry that is rooted in the Pb2+ atoms.

Pb2+ is located by 0.069 Å above z = ¼ and 0.085 Å off the (110) plane. It has a distinctly asymmetric coordination figure by four OH oxygen atoms, one sulfate oxygen O1, one arsenate oxygen O4 and two H2O oxygen atoms O7w, which can be described to define a very distorted square antiprism with 2 O5h + 2 O6h and O1 + O4 + 2 O7w as the two square faces. This coordination figure is supplemented by O2 at a distinctly longer Pb–O distance (3.117 Å) to give a [9]-coordination with a Pb–O mean bond length of 2.743 Å. By far the strongest bond is to O4 of the AsO43– group, Pb–O4 = 2.363 Å. It corresponds to a bond valence of 0.47 vu (vu are valence units calculated according to Gagné and Hawthorne 2015). Five Pb–O bonds with bond distances of 2.66–2.80 Å and bond valences of 0.24 to 0.18 vu (O6h, O7w, O1, O6h, O7w) subtend with the short bond Pb–O4 angles below 90° (O4–Pb–O = 76–84°) to give an umbrella-like arrangement. The remaining three oxygen ligands of Pb, two O5h and one O2, have longer Pb–O bonds (2.82, 2.84 and 3.12 Å, respectively) and subtend with the Pb–O4 bond angles between 130 and 145°. This demonstrates clearly that Pb is in a one-sided coordination figure and that the formal electron lone pair of Pb2+ is stereochemically active (Gagné and Hawthorne 2018a). In correspondence with this one-sidedness are the bonds Sb–O5h in the Sb(OH)6 octahedron by 0.024 Å shorter than Sb–O6h.

The tetrahedral anions SO42– and AsO43– adopt distinctly different functions in the structure with Pb–O and hydrogen bonds selective for each group leading to their alternating arrangement along the 3-fold axis. The AsO43– group is anchored by the three strong Pb–O4 bonds (2.363 Å), each supplemented by the short hydrogen bond O5h–H5···O4 (O5h···O4 = 2.64 Å) providing O4 with a bond strength of 0.47 + 0.26 = 0.73 vu that combines well with the 1.25 vu for the As–O bond of an ideal AsO43– tetrahedron. The oxygen O4 lies nearly exactly in the plane Pb–As–O3 (deviation 0.011 Å) and keeps thereby the bond distances Pb–O4 and O5h···O4 to a minimum. The apical oxygen O3 is not bonded to Pb but accepts three short hydrogen bonds from H2O7w, O7w···O3 = 2.69 Å. The As–O mean bond length, 1.663 Å, may be compared with grand mean distance of 1.687(27)Å for 508 AsO4 tetrahedra (Gagné and Hawthorne 2018a).

The SO42– group has a mean bond length of 1.470 Å in good agreement with < S–O > = 1.473 Å (Gagné and Hawthorne 2018b) for many sulfates. It is anchored in the structure by the apical oxygen O1 which is bonded to three Pb at Pb–O1 = 2.69 Å, corresponding to a bond strength of 3 × 0.23 = 0.69 vu that fits to the long bond S–O1 = 1.503 Å. Its basal oxygen O2 is weakly bonded to Pb and accepts a short hydrogen bond from O6h and a longer one from water O7w (Fig. 6). At variance with O4 of AsO43– the sulfate oxygen O2 does not lie on the plane Pb–S–O1 in order to minimize Pb–O2, but is off from this plane by 0.52 Å [0.56 Å from (110)]. The cause for this position must sought in the hydrogen bond O7w···O2 = 2.903 Å, which would become overly long otherwise. The same feature can be seen for the Ba-Sn-Se compound addressed below. According to the refinement, about 13(2)% of oxygen O2 adopt a mirror-related position O2a (not shown in the Figures for clearness). This has similar bond distances to S, Pb, O6h, and O7w (1.45(5), 3.14, 2.72, 2.93 Å, respectively), very much like the main site but still probably requiring only a minute adjustment in the orientation of H2O7w.

The hydroxyl groups O5h–H5h and O6h–H6h adopt similar triangular pyramidal coordination figures by Sb and two Pb atoms. Their approximate hydrogen atom positions are, therefore, geometrically predetermined. However, they could also be refined with the X-ray data using O–H distance restraints. Both groups form essentially straight hydrogen bonds of average strength (0.26 and 0.21 vu according to Ferraris and Ivaldi 1988), to O4 and O2 as H-bond acceptors (Table 6). Water H7wa–O7w–H7wb is in a distorted tetrahedral environment by two Pb and the H-bond acceptors O3 and O2 with tetrahedral angles of 95–122° about O7w. Interestingly, the two Pb–O7w bonds differ by 0.13 Å. Forcing the two Pb–O7w bonds in a trial least-squares refinement to become equal in length led to a significant increase in R values and, thus, was not supported by the diffraction data. Three O7w and three Pb form a 6-membered puckered ring (Fig. 6) with O7w by 0.22 Å above the mean plane of the three Pb atoms. With this arrangement the contact distances of O7w(x,y,z) to O1(x,y,z), O4(x,y,z), and O4(1-y,1+x-y,z) in the Pb3(O1)(O4)3As(O7w)3 cage (Fig. 6, right side) are 3.06, 3.20, and 3.11 Å, respectively.

Pseudosymmetry

In the projection of the structure down [001] (Fig. 5b) one can easily note a significant pseudosymmetry with a near mirror symmetry on the plane (110). The atoms Sb, S, As, O1 and O3 are in special positions and located exactly on this pseudo-mirror plane. The remaining atoms – Pb, O5h, O6h, O2, O4, O7w and their H atoms – are in general positions. However, all of them – except the sulfate oxygen atom O2 – are very close to this pseudo-mirror plane. The specific deviations from plane (110) are in Å: Pb 0.085, O5h 0.037, O6h 0.005, O2 0.557, O4 0.064, O7w 0.017. Small as they are, these deviations are significant already in the (001) projection, but more so in 3 dimensions. Particularly the 0.085 Å deviation of Pb is highly significant and find its expression in the low Laue symmetry of the hk0 layer of the weighted reciprocal lattice shown in Fig. 4. Forcing Pb to lie exactly on the (110) plane would increase the R1 value from 0.0184 (Table 2) to ⁓0.15. However, it is not surprising that this pseudosymmetry facilitates merohedral twinning on (110) and this was a problem in earlier work (Sima et al. 1996; Sima 1998). Within the limits of detection, the high-quality crystal used in the present investigation was free of it. However, the crystal showed a modest 7(1)% merohedral twinning on (001). This reverses the polar axis of the structure and reverts the orientation of the SO4 and AsO4 tetrahedra upside down in parts of the crystal. A thorough check for diffuse or superstructure reflections of the investigated crystal showed no effects. Likewise, the X-ray powder patterns appeared clean and sharp. This indicates that the reversal is based on twinning and not on disorder.

Mallestigite and the fleischerite group

Mallestigite belongs to the group of minerals and synthetic compounds, which was named after fleischerite, Pb3Ge4+(SO4)2(OH)6·3H2O, as its first known representative (Frondel and Strunz 1960). Based on their crystal structures this growing group can now be subdivided into three subgroups:

  1. 1.

    The minerals schaurteite, Ca3Ge4+(SO4)2(OH)6·3H2O (Origlieri and Downs 2013), genplesite, Ca3Sn4+(SO4)2(OH)6·3H2O (Pekov et al. 2018), and despujolsite, Ca3Mn4+(SO4)2(OH)6·3H2O (Barkley et al. 2011).

  2. 2.

    The synthetic compound Ba3Sn4+(SeO4)2(OH)6·3H2O (Reuter and Kamaha 2022), mallestigite, Pb3Sb5+(SO4)(AsO4)(OH)6·3H2O (this work), and, probably, fleischerite Pb3Ge4+(SO4)2(OH)6·3H2O (Otto 1975).

  3. 3.

    Compounds with presently unknown status, namely synthetic Sr3Ge4+(SO4)2(OH)6·3H2O (Otto 1975) and a slag mineral with the tentative formula Ba3Sb5+[(Si,S)O3(OH)]2(OH,O)6·3H2O (Kolitsch et al. 2013).

All members of the fleischerite group crystallize with hexagonal unit cells of a ≈ 9 Å and c ≈ 11 Å. They are built up from M(OH)6 octahedra – M = Mn4+, Ge4+, Sn4+, Sb5+ – linked by large divalent cations A (Ca, Sr, Ba, Pb) into infinite chains A3M(OH)6 which in turn are crosslinked by two tetrahedral anions (SO4, SeO4, AsO4) and three H2O per formula unit. A compilation of these compounds is given in Table 7.

Table 7 Comparison of fleischerite type crystal structures, A[8–9]3M[6](T[4]O4)2(OH)6·3H2O, A2+ = Ca,Ba,Pb; M = Mn4+,Ge4+,Sn4+,Sb5+; T = S6+,Se6+,As5+
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Members of subgroup (1) crystallize in the centrosymmetric space group P63/mmc. They are at present exclusively based on Ca. They can be derived from mallestigite (Fig. 6) by inserting a center of inversion at x,y,z = 0, by adding a mirror plane parallel to (001) at z = ¼ and straightening out the Pb and H2O layer, and by replacing the SO4 tetrahedron with the (001) mirror image of the AsO4 tetrahedron. By these means Ca becomes [8]-coordinated with a very regular square antiprism as coordination polyhedron; the OH groups of O5h and O6h become symmetry equivalent and donate hydrogen bonds to equivalents of O4 while the H2O donate all hydrogen bonds to equivalents of O3 (Fig. 5a). Moreover, the (110) plane drawn in Fig. 5b becomes a crystallographic mirror plane leading ultimately to the space group symmetry P63/mmc with all atoms of the structure lying on (110). A view of the corresponding structure, here schaurteite, is shown in Fig. 7a. Despujolsite, Ca3Mn4+(SO4)2(OH)6·3H2O (Barkley et al. 2011), as the only member of this subgroup reported to crystallize with a different space group, namely P\(\overline6\)2c, is so close to P63/mmc symmetry, that it was assumed to adopt this space group. This assumption could be verified by a re-refinement with the published reflection data (Barkley et al. 2011) in space group P63/mmc that improved previous R1 from 0.0235 (P\(\overline6\)2c) to 0.0196 (P63/mmc). For further details see supplementary materials.

Fig. 7
figure 7

Comparison of the crystal structures of a schaurteite, b mallestigite, and c Ba3Sn4+(SeO4)2(OH)6·3H2O (atoms relabeled and repositioned) using displacement ellipsoid presentation for oxygen atoms and the tetrahedral centers. Pb–O2 bond (3.12 Å) in (b) not shown. H bonds are thin blue lines

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Two members of subgroup (2) can be compared in Fig. 7b, c. Apart from mallestigite (Fig. 7b), what appears to be most interesting is the recently published and very well refined crystal structure of Ba3Sn4+(SeO4)2(OH)6·3H2O, a = 9.2550(6), c = 11.4441(8) Å, T = 100 K, R1 = 0.008 for 1653 reflections (Reuter and Kamaha 2022) presented in Fig. 7c. Like mallestigite, it adopts space group P63 and has the same architectural features, particularly the unidirectionally oriented two independent SeO42– groups. With Ba2+ as a closed shell cation, a comparison with Pb2+ in mallestigite is of particular interest. For this purpose, the published structure (Reuter and Kamaha 2022) was transformed to mallestigite setting and its atoms relabeled (Fig. 7c). A list with crystallographic data of the transformed structure is provided in the supplementary materials. The SnO6 octahedron, which has a mean bond length of 2.053 Å, is more regular than the SbO6 octahedron in mallestigite, but shares with it an elongation along c axis. The SeO4 tetrahedra (mean bond lengths 1.637 and 1.639 Å) are similar in size like the AsO4 group in mallestigite and bigger than SO4. Ba–O mean bond length is 2.837 Å, compared to Pb–O in mallestigite 2.743 Å. While the ionic radius of Ba2+ is only by about 0.1 Å bigger than that of Pb2+, it is most significant that Ba–O4 = 2.714 Å is by ⁓0.35 Å longer than the corresponding short bond Pb–O4 = 2.363 Å in mallestigite. The two Ba–O6h bonds are ~ 0.06 Å longer and the two Ba–O5h bonds ~ 0.04 Å shorter than their counterparts in mallestigite (range for Ba 2.74–2.78 Å, for Pb 2.66–2.84 Å). Both Ba–O7w bonds are notably longer (on average by ~ 0.17 Å) than the two Pb–O7w bonds in mallestigite but differ less in length. The only bond that remains nearly unchanged is Ba–O2 = 3.11 Å compared with Pb–O2 = 3.12 Å. These features attest clearly that the observed asymmetry of the Pb2+ coordination in mallestigite is due to the lone pair effect.

The crystal structure of fleischerite, Pb3Ge4+(SO4)2(OH)6·3H2O (Otto 1975), the namesake mineral of the group was investigated by photographic X-ray methods using CuKα radiation. The quality of natural material was so poor that a thin synthetic crystal was used for structure determination with multi-layer Weissenberg photographs. The Laue symmetry of this crystal was claimed as 6/mmm and a nearly centrosymmetric structure model in the acentric space group P\(\overline6\)2c was worked out and refined to R1 = 0.039 for 319 reflections (Otto 1975). This structure model included an upside-down orientation disorder for the SO4 groups which represents in principle a mallestigite type structure with a 1:1 twinning on (001) at z = ¼. Having experienced comparable features in earlier work on mallestigite (Sima et al. 1996; Sima 1998) and under consideration that Pb2+ should show a likewise coordination asymmetry, it is plausible to assume that fleischerite crystallizes in a mallestigite type structure with space group P63 and a corresponding ordered arrangement of the SO4 tetrahedra, a possibility that was already considered by Otto (1975) albeit invoking space group P31c as an alternative. Thus, Fig. 7b can be considered as an image of the fleischerite structure, when AsO4 is replaced by a smaller SO4 group. In order to test this hypothesis, the published diffraction data of Otto (1975) for fleischerite were used in a least-squares refinement with a mallestigite type structure. Starting with R1 = 0.042 (structure model of Otto in space group P\(\overline6\)2c) this refinement in space group P63 improved the reliability index to R1 = 0.036. It generated a notably asymmetric coordination for Pb (Pb–O4 = 2.59(2) Å, Pb–O2 = 2.99(2) Å; for atom designations see Fig. 6), and showed that the H2O oxygen atom O7w is by 0.32 Å above the plane of the adjacent three Pb atoms. These results provide substantial support for a P63 symmetry of fleischerite. For further details see the supplementary materials.

Fleischerite subgroup (3) of unknown status comprises at present two compounds: For synthetic Sr3Ge4+(SO4)2(OH)6·3H2O only unit cell dimensions are known (Otto 1975). They are a = 8.837(4), c = 10.863(7) Å, similar to fleischerite and notably bigger than for schaurteite. As Sr2+ is frequently found in well-balanced [8]-coordination, a P63/mmc type structure like schaurteite seems more likely than a P63 structure like the Ba-Sn selenate. Of the slag mineral Ba3Sb5+[(Si,S)O3(OH)]2(OH,O)6·3H2O (Kolitsch et al. 2013) only unit cell dimensions, a = 8.810(1), c = 11.776(2) Å, and the assigned space group P\(\overline3\) were reported. Comparing the unit cell data with those of mallestigite and the synthetic Ba-Sn selenate, then a axis is shorter and c axis longer than one would expect for the given chemistry.

Further structural relationships

A short review on secondary antimony minerals was given by Roper et al. (2012). They typically form by oxidation of primary sulfides in the supergene zones of Sb-bearing ores and their mine wastes and they comprise a considerable variety of Sb5+ as well as Sb3+ oxygen compounds. One intermediate in their formation is the [Sb(OH)6]1– complex. This is stable in near neutral to alkalic waters from which simple salt-like compounds crystallize, e.g. mopungite, Na[Sb(OH)6], a hydroxide-perovskite of tetragonal symmetry belonging to the stottite family (Bittarello et al. 2015), or brandholzite, Mg[Sb(OH)6]2·6H2O (Friedrich et al. 2000), the representative of a family with a layered arrangement of M2+(H2O)6, M = Mg, Ni, Co, and Sb(OH)6 octahedra crosslinked by hydrogen bonds (Bonazzi and Mazzi 1996; Friedrich et al. 2003).

A large group of minerals with close structural relationships to the fleischerite family is represented by thaumasite, Ca3Si4+(CO3)(SO4)(OH)6·12H2O, and ettringite, Ca3Al3+(SO4)1.5(OH)6·⁓13H2O, as prototypes (Taylor 1973). These minerals are based on the same A3M(OH)6 chains like the fleischerite group. One particular example is that with schaurteite Ca3Ge4+(OH)6 chains. On virtual transition from schaurteite to thaumasite, Ge4+ in octahedral coordination is replaced by Si4+ while the bridging water molecules and the sulfate O atoms bonded to Ca are substituted by four terminal H2O. This leads to completely hydrated cationic columns [Ca3M4+(OH)6(H2O)12]4+ with 30 active H atoms per formula unit, of which 6 hydroxyl and 3 aquo H atoms are strengthening the columns by internal hydrogen bonds, whereas 21 H atoms are available for external hydrogen bonds to suitable anions. These columns have inherent 63 symmetry and adopt hexagonal rod-packing with interstitial space of trigonal symmetry. They, therefore, tend to adopt anions with suitable symmetry, size, charge and H-bonding propensity, SO42– and CO32– in case of thaumasite and P63 space group symmetry with a ≈ 11.0, c ≈ 10.4 Å (Edge and Taylor 1971; Gatta et al. 2012). In the mineral realm, examples with tetravalent M = Si4+, Ge4+, Mn4+ and anions like SO42–, CrO42–, CO32–, SO32– or combinations thereof are known. If M is trivalent, like Al3+, Fe3+, Cr3+, then the polyhedral columns have a lower net charge [Ca3M3+(OH)6(H2O)12]3+. This seems to lead to complications with anion incorporation, additional water molecules, disorder, and a doubling of the c axis, as revealed in a recent neutron diffraction study of an ettringite with the chemical formula Ca6Al2(SO4)3(OH)12·27H2O (Gatta et al. 2019). The formal hydration of schaurteite [Ca3Ge(OH)6]4+ chains is possible, as demonstrated by the existence of carraraite, Ca3Ge4+(CO3)(SO4)(OH)6·12H2O (Merlino and Orlandi 2001). However, it is unlikely that the higher charged [Pb3Sb(OH)6]5+ chain in mallestigite with Pb2+ as a more polarizable cation might form a stable thaumasite/ettringite type chain with Pb(OH)4(H2O)4 polyhedra.

Concluding remarks

Mallestigite is a secondary mineral with an interesting crystal chemistry. The modes of its formation under laboratory conditions, its solubility and its thermodynamic stability will have to be explored in the future. Present knowledge shows that the crystal structure of mallestigite is embedded in two related structural families of which one, the thaumasite/ettringite family, is widespread and abundant. This points to a notable stability of these minerals and their backbone, in case of mallestigite the [Pb3Sb5+(OH)6]5+ chains, and consequently to a certain propensity of their formation. When, under humid oxidizing conditions, galena and fahlores of the tetrahedrite-tennantite series or complex Pb-As-Sb-sulfosalts are present as primary minerals and sources for Pb2+, Sb5+, AsO43–, and SO42–, then good chances for the formation of mallestigite as supergene mineral are given, as exemplified by the type locality on Mallestiger Mittagskogel and the four presently known further occurences (see introduction). Stockpiled wastes from metal mining and metal extraction with Pb, Sb and As in significant concentrations are widespread (Wilson et al. 2010; Okkenhaug et al. 2011 and references therein). Hence, it is expected that the number of mallestigite occurrences will significantly grow in number if appropriate methods in the identification of the mineral are at hand. Two prospective locations have been highlighted in the chapter X-ray powder diffraction. As a long time sink for the toxic heavy metals Pb, Sb, As the mineral mallestigite is likely not sufficiently stable. Here phases of the alunite/jarosite supergroup like beudantite, PbFe3(SO4)(AsO4)(OH)6, and relatives are certainly more stable and have been shown that they may incorporate significant amounts of Sb5+ in octahedral coordination as replacement for Fe3+ (Kolitsch et al. 1999; Mills et al. 2014; Pekov et al. 2016). In case of the mallestigite type locality, iron does not play a major role and jarosite was there found only as a very rare species.