Lazerckerite, Ag3.75Pb4.5(Sb7.75Bi4)S24, from Kutná Hora, Czech Republic: a new Sb–Bi member of the andorite branch of the lillianite hom*ologous series (2024)

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Pažout, R., Dušek, M., Sejkora, J., Plášil, J., Ilinca, G., and Dolníček, Z.: Lazerckerite, Ag_3.75Pb_4.5(Sb_7.75Bi₄)S₂₄, from Kutná Hora, Czech Republic: a new Sb–Bi member of the andorite branch of the lillianite hom*ologous series, Eur. J. Mineral., 36, 641–656, https://doi.org/10.5194/ejm-36-641-2024, 2024.

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A new sulfosalt mineral species, lazerckerite, ideally Ag_3.75Pb_4.5(Sb_7.75Bi₄)S₂₄, has been found in medieval mine dumps of the Staročeské pásmo Lode of the historic Kutná Hora Ag–Pb–Zn ore district, Central Bohemia, Czech Republic (Fig.1). The mineral is named after Lazarus Ercker (1528–1594), the supreme Royal Bergmeister of the Kingdom of Bohemia (Minister of Mining) and the Master of Prague Mint, a renowned metallurgist, chemist, and mining specialist and the author of the Books on Assaying (original title Beschreibung Allerfürnemisten Mineralischen Ertzt und Berckwercksarten). This work – published in 1574 – was reissued 15 times, translated into four languages, and until the end of the 18th century was used as a principal textbook by all mining universities and Bergakademie in Europe. The pronunciation of the mineral name is “lazerke'rait”. The new mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA2022-113) (Pažout et al., 2023b). Holotype material of lazerckerite (polished section) is deposited in the collections of the Department of Mineralogy and Petrology, National Museum in Prague, Cirkusová1740, 19300 Praha9, Czech Republic, under the catalogue number P1P 11/2022.

Lazerckerite is a new member of the lillianite group of natural sulfosalts of Ag–Pb–(Sb,Bi,As). The last comprehensive review of both lillianite (Bi-rich) and andorite (Sb-rich) branches of the lillianite hom*ologous series was conducted by Makovicky and Topa(2014). A general overview of Sb–Bi mixed members was given in Topa at al.(2016). Lazerckerite belongs to the andorite branch of the lillianite hom*ologous series, and it is a new addition to the fairly new group of Sb–Bi mixed members of the series, which up to date is comprised of five minerals: terrywallaceite (lillianite branch; Yang et al., 2013), oscarkempffite (andorite branch; Topa et al., 2016), clino-oscarkempffite (andorite branch; Makovicky et al., 2017), staročeskéite (lillianite branch; Pažout and Sejkora, 2018), and holubite (andorite branch; Pažout et al., 2023a).

This paper describes the sixth Sb–Bi member of the series, lazerckerite; its physical and chemical properties; its crystal structure; and its relation to other lillianite hom*ologues.

Lazerckerite was found in 21 different samples in gangue material of the medieval mine dumps of the Staročeské pásmo Lode (the Old Bohemian Lode in English) of the historic Kutná Hora Ag–Pb–Zn ore district. Kutná Hora is a hydrothermal-vein-type deposit of Variscan age (Holub et al., 1982), mined mainly between the14th and17th centuries with 10 major lodes (zones) (Fig.1), with each lode consisting of several principal veins (Pažout et al., 2017). From the point of view of geology and mineralogy, two mineral assemblages are distinguished in this ore district, one “silver-rich” in the southern part of the ore district and one “pyrite-rich” in the northern part (Malec and Pauliš, 1997). The Staročeské pásmo Lode belongs to the northern pyrite-rich lodes, and it is the biggest lode of the Kutná Hora ore district, both in terms of the amount of extracted ore and the amount of extracted silver (estimated 300 t of Ag). The full list of minerals (over 150 species) recorded from this area is given at http://www.mindat.org/loc-18419.html, last access: 30July2024.

In the holotype sample (ST48), lazerckerite forms grains in association with terrywallaceite, Bi-rich jamesonite, and remnants of Ag,Bi-bearing galena (Fig.2). The hand specimen is formed by limonite-coloured quartz with silvery-grey metallic grains of the above Bi-bearing minerals up to 3×3 mm. The quartz gangue of the holotype sample contains grains and lenses of pyrrhotite several millimetres in size, with no other base sulfides present.

In most other samples, the mineral occurs frequently as replacement rims of earlier Ag–Pb–Bi–Sb minerals and grain aggregates, growing together in aggregates up to 0.6×0.3 mm. It often occurs in close association with Ag,Bi-bearing galena, terrywallaceite, and holubite, occasionally accompanied by other lillianite hom*ologues. In Fig.3 Ag,Bi-bearing galena is being replaced by terrywallaceite, and the replacement rims are formed by holubite, which is subsequently replaced by lazerckerite (darker than holubite in backscatter electron (BSE) images due to significantly lower Pb content).

The origin of the mineral is related to the later Bimineralization, and we assume the mineral is a replacement product of earlier, Bi-richer lillianite hom*ologues and galena that crystallized from originally low-temperature (ca.100–250 °C) hydrothermal fluids in the tectonically opened fractures in yet earlier hydrothermal Fe–Cu–Zn–Sn–As base sulfide ore vein fillings (Pažout and Sejkora, 2018). Lazerckerite is one of the latest in the Bi sulfosalt mineralization stage, following the usual sequence of crystallization in the Kutná Hora ore district: native Bi → galena ss → matildite → izoklakeite/cosalite → eskimoite/treasurite/vikingite → gustavite → Sb-rich gustavite → staročeskéite/terrywallaceite → holubite → lazerckerite → Bi-rich jamesonite (Pažout and Sejkora, 2018). A general succession trend observed in the Ag–Pb–Bi–Sb mineralization in Kutná Hora ore district is from Bi-dominant to Sb-dominant mineral phases.

Chemical analyses of the holotype sample were performed using a JEOL JXA-8600 electron probe microanalyser of the University of Salzburg in WDS mode (25 kV, 35 nA) and with a beam diameter of 5 µm. The following standards and X-ray lines were used: CuFeS₂ (CuKα, FeKα), Ag (AgLα), PbS (PbLα), Bi₂S₃ (BiLα, SKα), Sb₂S₃ (SbLα), CdTe (CdLβ, TeLα), Bi₂Te₂S (BiLα, TeLα), and Bi₂Se₃ (SeKα). Raw data were corrected with an online ZAF-4 procedure. A second set of polished sections with lazerckerite was measured on a CAMECA SX100 electron probe microanalyser at the National Museum, Prague. The analytical conditions were as follows: WDS mode; accelerating voltage of 25 kV; beam current of 20 nA; electron-beam diameter of 2 µm; and the standards chalcopyrite (SKα), Bi₂Se₃ (BiMβ), PbS (PbMα), Ag (AgLα), halite (ClKα), Sb₂S₃ (SbLα), CdTe (CdLα), HgTe (HgMα), pyrite (FeKα), Cu (CuKα), ZnS (ZnKα), NiAs (AsLβ), and PbSe (SeLβ). Peak counting time is 20 s for each element and 10 s for each background, and the detection limits for the four trace elements (Cu, Fe, Cd, and Se) were in the range 0.03 wt %–0.06 wt %. Measured data were corrected using PAP software (Pouchou and Pichoir, 1985). Analytical data for the holotype sample are given in Table2. Its empirical formula (calculated on the basis of 44 apfu) is represented by grouping the constituents according to valence state: (Ag_3.61Cu_0.04)_Σ3.65(Pb_4.55Fe_0.01Cd_0.01)_Σ4.57 (Sb_7.87Bi_3.75)_Σ11.62(S_24.15Se_0.01)_Σ24.16.

The simplified formula is (Ag,Cu)_3.7(Pb,Fe,Cd)_4.6 (Sb_7.9Bi_3.8)_Σ11.7S₂₄, and the ideal formula is Ag_3.75Pb_4.50(Sb_7.75Bi₄)S₂₄, which requires Ag10.47, Pb24.13, Sb25.21, Bi20.28, and S19.92 weight percentages, totalling 100.00 wt %, corresponding to N_chem=4.00, L% = 93.75, and Bi$/$(Bi+Sb) = 0.34. Lazerckerite can be differentiated from chemical results from similar Sb–Bi mixed members of the lillianite hom*ologous series on the basis of L% (the percentage of the (Ag + (Bi,Sb) = 2Pb) substitution and the Bi : Sb ratio expressed as Bi$/$(Bi+Sb) (both calculated from electron probe microanalyser (EPMA) results).

A tiny fragment of lazerckerite with dimensions of $\mathrm{0.040}\times \mathrm{0.040}\times \mathrm{0.035}$ mm was extracted from a polished section of the holotype sample and separated under the optical stereomicroscope. The intensity data were collected at ambient temperature using an Oxford Diffraction Gemini single-crystal diffractometer equipped with SuperNova CCD detector and using monochromated MoKα radiation from a conventional sealed X-ray tube (55 kV, 30 mA), collimated with a fibre-optic Mo-enhanced collimator. A total of 25 883 reflections were measured. After averaging, 5189 reflections were independent and 2750 classified as observed [I_obs>3σ(I)]. Data were corrected for background, Lorentz, and polarization effects, and a multi-scan correction for absorption was applied, resulting in R_int of the merged data equal to 0.0852. The twinning tool incorporated in the Jana2020 program (Petříček et al., 2023) revealed the crystal is twinned on (001) in this unit cell orientation and the volume fractions of each twin is 0.548(5) and 0.452(5). The final refinement of occupancies returned values close to those obtained from the microprobe study. Refinement for 146 parameters converged to the final R=0.1063 and wR = 0.1876 for 2750 observed reflections with goodness of fit (GOF) = 2.10 for all reflections.

In the process of indexing, a monoclinic cell was found; images from data collection revealed a primitive unit cell with the presence of reflections doubling the size of the shortest sulfosalt parameter (c in this setting), typical of fizelyite, ramdohrite, and oscakempffite. The unit cell parameters determined from single-crystal data by a least-squares algorithm using the CrysAlisPro Package (Rigaku, 2019) are as follows: a=13.2083(9), b=19.4595(8), c=8.4048(13), β=90.032(7)°, V=2160.3(4) ų, space group P2${}_{\mathrm{1}}/c$, and Z = 2. The powder X-ray diffraction data could not be collected due to the paucity of material; the calculated powder diffraction data are given in Table3.

The structure of lazerckerite was solved using the charge-flipping algorithm (Palatinus and Chapuis, 2007) implemented in the Jana2020 program (Petříček et al., 2023). Systematic absences and intensity statistics indicated the centrosymmetric space group P2${}_{\mathrm{1}}/c$. All atomic positions were found, which were subsequently refined by full-matrix least squares based on F² using Jana2020 (Petříček et al., 2023). The distance and bond valence calculations indicated that the trigonal prismatic sites Pb1 and Pb2 are occupied solely by Pb, similarly to other known structures of lillianite hom*ologues with N = 4 and L≤100 %. Interatomic distances showed that two central octahedral, as well as two marginal octahedral (i.e. positioned at the margin of the ⁴L octahedral slab bordering on the Pb trigonal prism), are in fact (Sb,Bi) mixed sites, which was confirmed by the difference Fourier and refinement results in Jana2020 and charge density calculations in the program ECON21 (Ilinca, 2022). It also showed that the marginal octahedral M9 site is a mixed position (0.835Ag + 0.125Bi) and that the M3 position is in fact a mixed site with (0.64Sb + 0.36Pb). For all mixed sites, the coordinates and atomic displacement parameters were kept identical. All metal atoms were refined with anisotropic atomic displacement parameters, and isotropic displacement parameters were used for sulfur. Details of data collection, crystallographic data, and refinement are given in Table4. Atom coordinates, occupancies, and isotropic displacement parameters are in Table5; anisotropic atomic displacement parameters in Table6; and interatomic distances in Table7. The crystallographic information files have been deposited with the European Journal Of Mineralogy and are available in the Supplement (see below). The final structural formula is Ag_3.65Pb_4.72Sb_7.42Bi_4.11S₂₄, which is in acceptable agreement with the microprobe-established composition (Ag_3.61Cu_0.04)_Σ3.65(Pb_4.55Fe_0.01Cd_0.01)_Σ4.57 (Sb_7.87Bi_3.75)_Σ11.62(S_24.15Se_0.01)_Σ24.16. The empirical formula showing site populations represented by the results of the structure determination is ^Ag10(Ag₂) ^M9(Ag_1.61Bi_0.22)_Σ1.83 ^Pb(1,2)(Pb₄) ^M3(Pb_0.54Sb_1.46)_Σ2.00
^M4(Sb_1.36Bi_0.64)_Σ2.00^M5(Sb_1.23Bi_0.77)_Σ2.00
^M6(Sb_0.97Bi_1.03)_Σ2.00^M7(Sb_0.85Bi_1.09)_Σ1.95 ^Sb8(Sb₂) S₂₄.

The structure of lazerckerite, a natural Sb–Bi monoclinic ^4,4L hom*ologue of the lillianite hom*ologous series, contains 10 cation (metal) sites and 13 anion (sulfur) sites. The metal sites consist of two Pb sites (Pb1 and Pb2) in bicapped trigonal prismatic coordination and eight independent octahedral sites. Four of the octahedral sites are marginal octahedra – two of them (M6 and M9) flanking the trigonal prism of Pb1 and the other two (M7 and Ag10) flanking the bicapped trigonal prism of Pb2 – and four (M3 and Sb8; M4 and M5) are central octahedra inside the ⁴L slabs. Both types of octahedra (marginal and central) can be viewed as being formed by one row (or column) along c, each row consisting of two alternating metal sites. The structure of lazerckerite projected down [001] is depicted in Fig.6.

With regard to the marginal octahedra, in one row (column) the Ag site Ag10 alternates with the (Sb,Bi) mixed site M7 (0.62Bi + 0.38Sb), and in the other row the (Ag,Bi) mixed site M9 (0.825Ag + 0.125Bi) alternates with the (Bi,Sb) mixed site M6 (0.51Bi + 0.49Sb).

The analysis of site populations within the crystal structure of lazerckerite can be enhanced by examining the distortion of the coordination polyhedra expressed by various parameters such as the state of opposing bond pairs, average bond distances, and the departure (EDEV) of ECoN (effective coordination number) from the constrained coordination number (i.e. determined by the ligands with non-zero weight bonds with respect to the charge distribution; Ilinca, 2022). Opposing “in-plane” and “out-of-plane” bond pairs may be analysed in relation to element-specific bond-length hyperbolae, as detailed by Trömel(1980) and Berlepsch et al.(2001a, b). Each bond pair consists of a shorter and an opposite longer connection between the central atom and its ligands. In coordination octahedra, the in-plane bond distances are situated within the common base of the square pyramids. In contrast, the out-of-plane bond pairs are oriented approximately perpendicular to the in-plane pairs and encompass the distances to the apical ligands. The graphical representation of these categories is provided in Fig.7 for clarity.

In lazerckerite, the orientation of the “planes” typically aligns with [001], with deviations from the zone axis ranging from 0.1 to 5.8°, as illustrated in Fig.8. The pairs of opposing bonds in Sb-bearing coordination polyhedra of lazerckerite are listed in Table8. The graphical representation of the bond pairs, plotted against the element-specific bond-length hyperbolae for Sb (Berlepsch et al., 2001b), Bi (Topa et al., 2003), and Pb (Topa, 2001), is shown in Fig.9. Except for Sb8, which closely aligns with the Sb hyperbola, the remaining Sb sites exhibit mixed positions and predominantly fall between the hyperbolae or in proximity to the hyperbola associated with the dominant element within the site population (e.g. Sb6–Bi6, Sb7–Bi7). The most noteworthy departure from this trend is observed in the bond pair S11-M4-S12 of Sb4–Bi4, likely attributed to its unusual sevenfold coordination caused by the split S9 position.

Armbruster and Hummel(1987) developed a very simple discriminating diagram which uses the average distance calculated for the three shortest Me–S bonds plotted against the average distance of the next two shortest bonds. The diagram for lazerckerite is illustrated in Fig.10 and confirms the site populations suggested in the present work. The utilization of EDEV, representing the deviation of ECoN from the constrained coordination number, serves as a valuable metric for evaluating the internal distortion of coordination polyhedra and verifying the assignment of site populations. This approach is grounded in the observation that distinct EDEV values are characteristic of different central atoms or mixed sites (Ilinca, 2022). Table9 and Fig.11 present a comparison between the general average EDEV values and ranges for Pb, Bi, Sb, and Ag and mixtures within six-, seven-, and eightfold coordinations in over 170 sulfosalt crystal structures (Ilinca, 2022). These comparisons are juxtaposed with the specific EDEV values identified in lazerckerite. Overall, the opposing bond pairs, the ratios of bond-length averages and the state of EDEV confirm the site population assignments refined for the crystal structure of lazerckerite.

Based on the current X-ray study we conclude lazerckerite belongs to the group of lillianite hom*ologues with the unit cell type A (a≈19, b≈13, c≈8.5, β≈90.1°, V ≈ 2200 in various cell settings; Table10), such as fizelyite, ramdohrite, uchucchacuaite, menchettite, holubite, and oscarkempffite. Lazerckerite displays some similarity to oscarkempffite, although with several differences (we discuss only octahedral sites where all substitutions occur in lazerckerite): (1)the marginal octahedral (Sb,Bi) mixed site (called Sb1) in oscarkempffite (Topa et al., 2016) is the (Sb,Bi) mixed sites M6 and M7 in lazerckerite; (2)the marginal Ag sites in oscarkempffite become a pure Ag site (Ag10) and an (Ag,Bi) site M9 with (0.835Ag + 0.125Bi) in lazerckerite; and (3)two central octahedral sites in oscarkempffite – pure Sb site (Sb2) and (Sb,Bi) mixed site (called Sb3) – become a pure Sb site, two (Sb,Bi) mixed sites M4 and M5, and an (Sb,Pb) mixed site M3 with (0.64Sb + 0.36Pb). Bond distances of (Sb,Bi) sites in oscarkempffite are very similar to those in lazerckerite. The referred to oscarkempffite structure is the orthorhombic structure published in Topa et al.(2016).

Lazerckerite is a recent addition to the lillianite group, belonging to Strunz class 02.JB.40a and Dana class 3.04.15. It belongs to the mixed Sb–Bi members of the lillianite hom*ologous series, which represents a fairly new and a not-so-frequent group among either Bi-dominant (lillianite branch) or Sb-dominant (andorite branch) members of the series. To this date, it is comprised of four minerals: terrywallaceite, oscarkempffite, clino-oscarkempffite, and staročeskéite. The recent discovery of lazerckerite and holubite (Pažout et al., 2023a) expands this subgroup.

Comparative parameters of all mixed Sb–Bi members and several Sb members for the characterization of lillianite hom*ologues with N = 4, calculated from results of EPMA, are in Table10. Figure12 can serve as a practical tool for EPMA-equipped mineralogists who encounter Sb–Bi mixed phases. It shows the differences in the chemical composition (Bi$/$(Bi+Sb) vs. L%) of individual mixed Sb–Bi members of the lillianite hom*ologous series with N = 4 from the Staročeské pásmo Lode, Kutná Hora, Czech Republic. This graph includes all mixed Sb–Bi members known to this day with the exception of oscarkempffite and clino-oscarkempffite, which have so far not been identified in the Kutná Hora ore district. The distribution of unit cell types (discussed below) in the space of mixed Sb–Bi members of the lillianite hom*ologous series is shown in Fig.13, and the explanation of unit cell types is given in Table11. A comparison of selected data for lazerckerite, quantrandorite, terrywallaceite, and oscarkempffite is given in Table12.

Table10 shows that for L≈70 % there are four distinct mineral species that differ only in Bi content. Pure antimony members are ramdohrite (Makovicky et al., 2013) and fizelyite (Moëlo et al., 1984). When the Bi content substituting for Sb rises to Bi$/$(Bi+Sb) = 0.26–0.32, Bi prevails in two atomic positions (Bi4 and M5) while preserving the cell and symmetry of ramdohrite (unit cell type A), and the mineral becomes holubite. When the Bi content rises even further to 0.45, the cell and symmetry changes to that of orthorhombic lillianite (unit cell type B), and the mineral becomes staročeskéite (Pažout and Dušek, 2010; Pažout and Sejkora, 2018).

Similarly for L≈90 %, there are four mineral species. The pure antimony member is quatrandorite (unit cell type D) (Nespolo et al., 2012). When the Bi content rises to Bi$/$(Bi+Sb) = 0.30, the cell changes to unit cell type A, and the mineral becomes lazerckerite. When the Bi content rises to Bi$/$(Bi+Sb) = 0.40–0.45, the cell changes to unit cell type C, and the mineral becomes terrywallaceite (Pažout and Dušek, 2009; Yang et al., 2013). A further increase in Bi does not change the cell and symmetry. This holds true up to Bi$/$(Bi+Sb) = 0.75, when Bi becomes prevalent in all three (Sb,Bi) metal positions, and the mineral becomes gustavite while keeping the cell and symmetry of terrywallaceite (unit cell type C). Thus gustavite is defined as a mineral with Bi =(Bi+Sb) from 1 (pure bismuthian gustavite with no antimony) to 0.75 (Sb-rich gustavite).

Now if we have the Bi content constant and change the L%, the situation is as follows: for example for Bi$/$(Bi+Sb) ≈ 0.5, the mineral with L = 100 % is terrywallaceite (unit cell type C) with the cell and symmetry of gustavite. If the L% decreases (i.e. Pb rises and Ag + (Bi,Sb) decreases) to L ≈ 70 %, the monoclinic cell changes to an orthorhombic cell of lillianite (unit cell type B), and the mineral becomes staročeskéite. If on the other hand L rises to 125 % (which is the case of the Peruvian ore district of Julcani but which is not so in Europe – not even in Kutná Hora, certainly due to different formation conditions of Andean deposits and omnipresent Pb in formation fluids in Kutná Hora), the mineral becomes clino-oscarkempffite (unit cell type F) with a uniquely tripled ∼ 13.3 Å parameter (Makovicky et al., 2017).

Staying with the constant Bi content, for Bi$/$(Bi+Sb) ≈ 0.3 there are three distinct mineral species, differing only in L%, with all three minerals having the same cell type and symmetry. Starting from the Pb-richer end, the mineral with L = 70 % is holubite. If the Ag and (Bi,Sb) contents rise and Pb drops so that L = 90 %, the mineral becomes lazerckerite. While the two minerals have the same cell type and symmetry, the differences in structures are eye-catching. While in holubite there are only pure Sb sites and one pure bismuth site, in lazerckerite there are multiple (Sb,Bi) mixed sites similar to oscakempffite (Topa et al., 2016) or clino-oscarkempffite (Makovicky et al., 2017). If the Pb content drops even further and Ag + (Bi,Sb) rises even more to the over-substitution of L = 120 % (the over-substitution can be achieved only by replacing the trigonal prismatic Pb by Ag and (Bi,Sb)), the mineral becomes oscakempffite with the same cell and symmetry.

Thus, lazerckerite stands as a unique mineral species both structurally and chemically, distinctly differing from quatrandorite, terrywallaceite, oscarkempffite, holubite, and staročeskéite. It is defined as a lillianite hom*ologue with the following three requirements: N = 4, L (Ag${}^{+}+$ (Bi³⁺,Sb³⁺) ↔ 2 Pb²⁺ substitution) ≈ 90 %–95 %, and about one-third of atom percentage of antimony is replaced by bismuth (Bi$/$(Bi+Sb) ≈ 0.30–0.38). The mineral phases with this N and L% but Bi$/$(Bi+Sb) below 0.20 should be in lack of single-crystal data classified as Bi-rich quatrandorite and the compositions with Bi$/$(Bi+Sb) above 0.45 as terrywallaceite.

If we look at how to evaluate samples from Kutná Hora with border values of Bi$/$(Bi+Sb), which for lazerckerite with L% ≈ 90–95 are 0.28 and 0.46 (values encountered by EPMA by the first author of this article), we can conclude that the value of 0.28 still corresponds in all cases to lazerckerite, no matter in which of the five bismuth mixed sites the depletion of Bi occurs. Analytical points with the values between 0.40 and 0.46 are difficult to classify because these overlap with the substitution field of terrywallaceite. In the case of a mineral with multiple bismuth mixed sites (four (Bi,Sb) mixed sites and one (Ag,Bi) mixed site in lazerckerite and three (Bi,Sb) mixed sites in terrywallaceite), it is hard to speculate on how the increase in Bi will show in individual sites.

While the upper limit for terrywallaceite was deduced by Yang et al.(2013) at about Bi$/$(Bi+Sb) = 0.75 (thus the Sb-rich gustavite of Pažout and Dušek, 2009, with Bi$/$(Bi+Sb) = 0.70 is in fact terrywallaceite), the lower limit is less obvious. Terrywallaceite of Yang et al.(2013) has Bi$/$(Bi+Sb) = 0.49, and even if only the central octahedral sites M2 (0.60Bi + 0.40Sb) and M3 (0.95Sb + 0.05Bi) are considered for Sb for Bi substitution, it still has a space for an increase in Sb (0.1 apfu in M2 and 0.05 apfu in the M3). Such a composition (0.50Bi + 0.50Sb in M2 and only Sb in M3) has Bi$/$(Bi+Sb) equal to 0.44. Since some increase in Sb in the marginal octahedral site M1 site (0.82Bi + 0.18Sb) of Yang et al.(2013) cannot be ruled out and also since the antimony content in the M2 site may climb over 50 %, it is reasonable to consider all composition above Bi$/$(Bi+Sb) = 0.40 to belong to terrywallaceite or to constitute a mixture (intergrowth) of terrywallaceite and lazerckerite. A definitive answer of which mineral a particular composition belongs to would be given by single-crystal diffraction, which should show the a≈19, b≈13, c≈8.5, β≈90°, and V≈2200 cell of lazerckerite or the a≈19, b≈7, c≈8.5, β≈107°, and V≈1100 cell of terrywallaceite.

Comparing the data from Kutná Hora with published data from elsewhere in the past, it is possible that a mineral phase corresponding to lazerckerite has been found before as Keutsch and Brodtkorb(2008) described samples from Oruro, Bolivia, with L% = 94.4 and Bi$/$(Bi+Sb) = 0.23, which may correspond to lazerckerite.

The supplement related to this article is available online at:https://doi.org/10.5194/ejm-36-641-2024-supplement.

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