Introduction

Ruddlesden-Popper (RP) phases are usually represented by perovskite-like layered oxides with the general formula An+1BnX3n+1, or AX(ABX3)n (n = 1, 2, 3, …, ∞), where A is typically an alkali, alkaline-earth or rare-earth metal ion, B is a transition or post-transition metal ion, and X is usually oxygen, which can be partially replaced by other non-metallic elements such as N, S, and Cl (Sharma and Singh 1998; Lacotte et al. 2014; Yu et al. 2017; Ablitt et al. 2020; Nirala et al. 2020; Ding et al. 2021; Marcondes et al. 2021). The crystal structure of RP family phases is characterised by n consecutive perovskite layers (ABX3) alternating with rock-salt layers (AX) along the crystallographic c-axis direction, where n represents the number of connected perovskite layers of corner-sharing BX6 octahedra, and goes from n = 1 in A2BX4 to infinity in the well-known perovskite phase ABX3 (Ruddlesden and Popper 1957; Lee and Lee 2017; Yu et al. 2017; Xu et al. 2020). As n increases from 1 to ∞, the perovskite structure evolves from the two-dimensional (2D) RP layered to the three-dimensional (3D) simple perovskite structure (Yu et al. 2017; Nirala et al. 2020). The RP phases are known to exhibit a range of versatile and functional physical and optical properties such as magnetoresistance, photoluminescence, catalytic activity, superconductivity, as well as wide applications as electronic and magnetic functional materials (e.g., Moritomo et al. 1996; Hirayama et al. 1998; Gebhardt et al. 1999; Bassat et al. 2004; Gao and Wang 2006; Stanulis et al. 2014; Lee and Lee 2017; Sahu et al. 2018; Suter et al. 2018; Ding et al. 2021).

Among the various Ruddlesden-Popper phases, the members with n = 1 (A2BX4), in particular the calcium manganates Ca2MnO4, are the best known. Such phases adopt the K2NiF4 structure, corresponding to the stoichiometric compound crystalized in the primitive I4/mmm space group (Ruddlesden and Popper 1957; Fawcett et al. 1998; Surace et al. 2014). Moreover, the n = 1 RP oxides allow a wide range of oxygen stoichiometries, from oxygen-stoichiometric to oxygen-deficient and oxygen-excess, depending on the oxygen defects (Chihaoui et al. 2011a; Yang et al. 2019). Similar to other members of the RP family, the Ca2MnO4 perovskite exhibits various magnetic, dielectric, and thermoelectric properties (Tezuka et al. 1999; Matar et al. 2005; Chihaoui et al. 2011b, 2013; Baranovskiy and Amouyal 2016). Furthermore, its layered structure and non-toxic and readily available chemical constituents make it a promising material for new battery cathodes (Surace et al. 2014).

Karlleuite is a natural counterpart of the Ca2MnO4 phase and a new accessory mineral, which has been found in the xenolith sample within the basaltic lava from the Caspar quarry locality, Bellerberg volcano lava field, Eastern Eifel region, Germany. It is also the first layered perovskite mineral with a n = 1 Ruddlesden-Popper type structure. The new mineral (IMA2023-102), proposed name and symbol (Kll) have been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA). The holotype specimen is deposited in the Natural History Museum Mainz, State Collection for Natural History Rhineland-Palatinate, Reichklarastrasse 10, 55,116 Mainz, Germany, with the catalogue number NHMMZ M 2023/2-LS. The name karlleuite is given in honour of Karl Leu (b. 1958) from Nachtsheim, Germany, who started collecting minerals in the Eifel volcanic region in 1981. Karl Leu is an amateur mineralogist, collector, and specialist in the mineralogy of the Eifel region in Germany. His contribution is related to the fact that the lists of minerals found at various outcrops in the Eifel area could be extended. Moreover, Karl Leu is the author of many publications and reports on minerals from this region.

The present contribution provides a detailed description of the new mineral karlleuite from the Caspar quarry locality, including chemical, optical, spectroscopic, and structural data, as well as its comparison with related synthetic counterparts. The origin of karlleuite and its relationship to nomenclature of the perovskite supergroup are also discussed.

Occurrence and paragenesis

Karlleuite was identified in the xenolith sample from the active Caspar quarry, Bellerberg volcano, Germany (50°21’6” N, 7°14’2” E). This distinctive geological area is a part of Quaternary volcanic field in the Eastern Eifel, Mayen-Koblenz, Rhineland-Palatinate, Germany (Hentschel 1987; Mihajlovic et al. 2004). The holotype location is characterised by the occurrence of various thermally metamorphosed xenoliths embedded in the basaltic lava of tephrite-leucite composition (Hentschel 1987; Mihajlovic et al. 2004). The diversity of mineral assemblages, usually in the contact zone of xenolith and volcanic rock, is mainly attributed to high-temperature transformations and different protolith compositions (Hentschel 1987; Juroszek et al. 2018, 2022; Juroszek and Ternes 2022). It is proclaimed by the presence of a dozen rare and new minerals, already found and described from this locality (Abraham et al. 1983; Hamm and Hentschel 1983; Effenberger et al. 1998; Juroszek et al. 2024).

The xenolith specimen with karlleuite was collected by Bernd Ternes and Günter Blaβ in the area of the open quarry in 1980. The xenolith zone, in which the new mineral was discovered, is characterised by different colours related to the main rock-forming components: pink for reinhardbraunsite, bluish for minerals of fluorellestadite-fluorapatite series, brownish-grey for larnite and highly altered gehlenite, and white for several hydrated Ca aluminosilicates. The accessory minerals are represented by chlormayenite, srebrodolskite, brownmillerite, sharyginite, perovskite, and lakargiite. Besides Ca aluminosilicates, typical low-temperature minerals include hydrocalumite, calcite, periclase, and ettringite.

Materials and methods of investigation

Petrographic thin sections and polished fragments of studied xenolith specimen with karlleuite and associated minerals were examined using a Leica DM2700P optical microscope under both transmitted and reflected light. Subsequently, studies of rock samples, including semiquantitative chemical compositions of minerals and their textural relationships, were carried out using a Phenom XL scanning electron microscope equipped with an EDS (energy-dispersive X-ray spectroscopy) detector. The semiquantitative EDS analyses were performed using a high vacuum with a beam voltage of 15 kV and a working distance of 6 mm.

Electron probe microanalyses (EPMA) of karlleuite were performed using a Cameca SX100 instrument operated at 15 kV, with the sample current 15 nA, and spot size ~ 1 μm, with the following lines and standards: TiKα – rutile; MnKα – rhodonite; AlKα – orthoclase; FeKα – Fe2O3; CeKα – CeP5O14; CaKα – diopside.

The Raman spectra of karlleuite (Fig. 3 a, b) were recorded using a WITec alpha 300R confocal Raman microscope equipped with an air-cooled 532 nm solid-state laser and a CCD (charge-coupled device) camera operated at -61 °C. The Raman scattered light was focused by an effective pinhole size of about 30 μm and a monochromator with a 600 mm− 1 grating, calibrated by a Raman scattering line of a silicon plate (520.7 cm− 1). The power of the laser at the sample position was 42 mW. Data acquisition was performed with an integration time of 10 s, accumulation of 15 scans and a resolution of 3 cm− 1. Spectra processing, such as baseline correction and smoothing, was performed using the Spectracalc GRAMS software package. The Raman bands were fitted using a Gauss-Lorentz cross-product function with the minimum number of component bands used for the fitting process.

Single-crystal X-ray studies of karlleuite were carried out on a four-circle XtaLAB Synergy R diffractometer equipped with a rotating anode and HyPix-Arc 100 detector. The measurement was performed under ambient conditions at 302 K, and the data were collected using MoKα radiation (λ = 0.71073 Å). Data reduction and determination of unit-cell parameters were processed using CrystAlisPro software (Rigaku Oxford Diffraction, 2022). Crystal structure solution and refinement of karlleuite were performed using Superflip and Jana2020 programs (Petříček et al. 2023). Further details of the data collection and crystal-structure refinement are given in Table 1. In turn, atomic coordinates, occupancies, equivalent and anisotropic displacement parameters are listed in Tables 2 and 3.

Table 1 Crystal data, parameters for X-ray data collection and crystal-structure refinement details for karlleuite
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Table 2 Atom coordinates (x,y,z), equivalent displacement parameters (Ueq, Å2), and site occupancies for karlleuite
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Table 3 Anisotropic displacement parameters (Å2) of karlleuite
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Results

Physical and optical properties

Karlleuite is characterised by a brown colour with varying degrees of saturation from darker to lighter brown (Fig. 1a). The new mineral usually forms single thin tabular/plate crystals, which do not exceed 15 μm thickness, but range from 40 to 100 μm in length (Fig. 1b-e). Only in a few cases, the crystals reach more than 100 μm. Karlleuite is opaque in transmitted light, with a sub-metallic lustre, a light brown streak, and good cleavage parallel to (001). It is non-fluorescent, brittle and shows uneven fracture.

Fig. 1
figure 1

(a) Optical and (b-e) BSE (backscattered electrons) images of karlleuite and associated minerals within the holotype xenolith specimen. Abbreviations: Cmy – chlormayenite; Gh – gehlenite; Hcl – hydrocalumite; Kll – karlleuite; Per – periclase

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The bulk density was not directly measured due to the fracture and small crystal size. The calculated density 3.79 g/cm 3 was obtained from the empirical formula and unit-cell parameters refined from single-crystal data. Micro-hardness indentation was carried out using VHN (Vickers Hardness Number) for a load of 10 g, which gave a mean value of 254.6 kg/mm3 (range from 240 to 266 kg/mm3). This result corresponds to a hardness of 3.5 on the Mohs scale.

In reflected light, karlleuite is light grey and exhibits weak anisotropy under crossed polarisers. Quantitative reflectance measurements were performed in air relative to a SiC standard using a Leica FILMETRICS microscope-spectrophotometer, and the results are given in Table 4. The reflectance percentages for the four Rmax and Rmin COM (Commission on Ore Mineralogy) wavelengths were: 14.7–14.3 (470 nm); 13.5–12.8 (546 nm); 13.2–13.0 (589 nm) and 13.5–13.3 (650 nm).

Table 4 Reflectance values (%) for karlleuite
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Chemical composition

As karlleuite forms thin crystals, the measurement was challenging and some analyses with lower totals were omitted from the final calculation. The results of 10 spot analyses are given in Table 5. The high enrichment of Ce2O3 suggests the possibility of other REE (rare earth elements) in the chemical composition of karlleuite, so these were also measured. However, the obtained values were below the detection limits.

Table 5 Chemical composition (in wt%) for karlleuite
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The empirical formula of karlleuite calculated on the basis of 4 O atoms per formula unit is (Ca1.97Ce3+0.06)2.03(Mn4 + 0.39Ti0.36Fe3+0.19Al0.09)1.03O4, which leads to the simplified formula (Ca, Ce3+)2(Mn4+,Ti, Fe3+,Al)O4. The ideal end-member formula Ca2MnO4 corresponds to 56.33% CaO and 43.67% MnO2.

The holotype karlleuite is a multi-component phase, not an ideal end-member, as none of the distributed elements prevails over the B site. According to the IMA guidelines, the ideal formula was selected based on the dominant constituent rule (Hatert and Burke 2008). The calculated average composition indicated a significant Ti content (0.36 apfu) at the B site (Table 5). Similarly, the amount of trivalent cations Fe + Al (about 27%) present at this site, is relevant as well. The lowest and highest value (in wt%) of MnO2 and TiO2 do not show significant deviation from the average, indicating a relatively narrow compositional range (Table 5; Fig. 2). Nevertheless, some crystals showed an increased content of both MnO2 and TiO2. To confirm this observation, two additional analyses of karlleuite characterised by the highest content of MnO2 (spot 16) and TiO2 (spot 13) have been added to Table 5; Fig. 2. These data, not included in the calculation of the average composition due to the lower total amount, show the relationship between these two dominant constituents. An increase in the content of one leads to a decrease in the content of the other. Furthermore, these measurements confirm the existence of a solid solution between the two end-members Ca2MnO4-Ca2TiO4. The calculated empirical formula (Ca2.03Ce3+0.03)2.06(Ti0.44Mn4+0.31Fe3+0.18Al0.10)1.03O4 from analytical spot 13 shows the dominance of Ti at the B site and the possibility of recognition of a Ti-analogue of karlleuite within the holotype specimen.

Fig. 2
figure 2

Ternary plot of the B site contents (apfu) of karlleuite according to the results obtained from EMP analyses

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Raman spectroscopy

The Raman spectrum of karlleuite (Fig. 3a) is characterised by a strong and broad band at 559 cm− 1 with a shoulder, characterised by the relatively low intensity at 632 cm− 1. These bands are assigned to the symmetric stretching mode (A1g) of the Mn-O-Mn vibration along the c-axis of the [MnO6] octahedra. Two bands at 312 cm− 1 (Eg) and 505 cm− 1 (B2g) are attributed to the sliding- and bending-type motions of the O atoms in the ab plane, respectively. The Raman band at 180 cm− 1 represents the rotational mode (A1g) of [MnO6] along the c-axis, similar to the Raman band at 286 cm− 1, corresponding to the rotation of Mn-octahedra. A band in the lower wavenumber region, ~ 110 cm− 1, is related to Ca-O stretching and lattice vibrations. The substitution by Ti at the octahedral site in the karlleuite structure is also evident from the Raman spectrum. The Raman band at 207 cm− 1 is related to the rotational vibration of [TiO6], whereas that at 765 cm− 1 is assigned to the breathing mode (A1g) of [TiO6] (Fig. 3a). In the Raman spectrum of karlleuite (Fig. 3b) performed in different orientations (rotation by 90˚ to the starting position) we noticed that there is a change in intensity between the Raman bands at 606 cm− 1 and 768 cm− 1 related to the symmetric stretching mode (A1g) of the Mn-O-Mn vibration and the breathing mode (A1g) of [TiO6], respectively, compared to the spectrum presented in Fig. 3a. There is also an additional band at 1542 cm− 1, which is an overtone (Fig. 3b). The assignment of the karlleuite Raman bands is in good agreement with the previous spectroscopic studies carried out on the Ca2(Mn, Ti)O4 compound (Surace et al. 2014; Oka and Hayakawa 2022).

Fig. 3
figure 3

Raman spectra of karlleuite obtained at the two orientations relative to a polarised laser beam, along the elongation of crystal (a) and rotated by 90˚ (b). The analysis points are shown as white spots in the inset optical images

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X-ray crystallography

The crystal structure of karlleuite exhibits a Ruddlesden-Popper type structure characteristic of perovskite-like layered oxides. It is well known for synthetic phases, and karlleuite is the first natural compound to exhibit this structure type.

According to empirical formula A(Ca1.97Ce3+0.06)2.03B(Mn4 + 0.39Ti0.36Fe3+0.19Al0.09)1.03O4, the A site in karlleuite structure is occupied mainly by Ca with minor Ce, while the B site is occupied by a mixture of Mn4+, Ti, Fe and Al. The structure refinement aimed at determining the accurate electron densities at the A and B positions. Using the Ca atom, the occupancy refinement of the A site converged to 1.001(8). Consequently, the Ce substitution was found to be insignificant, if present at all. Therefore, the occupation parameter of Ca was fixed to 1.

Occupancy refinement of the B site using Mn converged to 0.842(7). The total B site scattering is ~ 21.05(14) electrons per site. The average electron microprobe analysis for the same sample yielded (Mn4 + 0.39Ti0.36Fe3+0.19Al0.09)1.03, corresponding to ~ 23.8 electrons. The individual site occupancies of four elements could not be freely refined, as the Mn and Fe scattering curves are too similar for X-ray. Furthermore, with such a small number of unique reflections (109/138 reflections with I > 3σI), the number of parameters that can be meaningfully refined is very limited. Therefore, the site occupancy of all four elements at the B site was fixed according to the empirical formula and the anisotropic temperature parameters were refined (Tables 2 and 3).

The crystal structure of karlleuite display a modular nature and consists of CaBO3 perovskite layers with B = (Mn, Ti, Fe, Al), packed between CaO rock-salt layers arranged along the c-axis (Fig. 4a, b). The Ca atoms, located at the boundary between the perovskite and rock salt layers, are coordinated by nine oxygen atoms. They have one short bond to the O1 site at 2.259(6) Å (along the c-axis), four bonds to O2 at 2.5653(10) Å, and four longer bonds to O1 equal to 2.6767(6) Å in the ab-plane (Fig. 4c). In perovskite layers, the octahedrally coordinated atom at the B site is bonded to four oxygen at O2 in the equatorial plane with bond lengths of 1.8842(2) Å, and two oxygen at O1 at the octahedra apex, with longer bond lengths equal to 1.994(5) Å (Fig. 4c).

Fig. 4
figure 4

(a) The layered arrangement of atoms and octahedra along the c-axis in the crystal structure of karlleuite; (b) Schematic arrangement of Ruddlesden-Popper type-structure characteristic of karlleuite, comprising rock-salt (CaO) and perovskite layers CaBO3 formed by Ca (green spheres) and BO6 octahedra; (c) Selected bond lengths in the karlleuite structure

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Due to the small quantity and size of the karlleuite crystals, X-ray powder diffraction data were not collected. However, the data were calculated using the VESTA program from the results of the single-crystal structure refinement (Table 6; Fig. 5a) and the observed PXRD patterns were obtained by collapsing the single-crystal reflection into two dimensions using a Gandolfi-like screening procedure (Fig. 5b).

Table 6 The calculated powder patterns of karlleuite
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Fig. 5
figure 5

(a) X-ray powder diffraction data calculated from the results of the single-crystal structure refinement; (b) An observed PXRD obtained by collapsing the single-crystal reflections into two dimensions using Gandolfi-like screening procedure powder Graph(1.2.1) in CrysAlisPro software, applying using baseline correction and smooth filter. Both diagrams are calculated using λ = 0.71073 Å

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BVS calculation and explanation

The adjustment of two different structural units, i.e., CaBO3 and CaO, within the lattice causes strong anisotropic features in the Ca2BO4 structure. The structures of Ruddlesden-Popper type n = 1, i.e., phase A2BO4, are known to exhibit bond valence sums (BVS) that deviate significantly from the theoretical one. This is due to the mismatch in size requirements of the perovskite and rock-salt type layers, leading to compression of the perovskite and elongation of the rock-salt type layers in the ab-plane (Brown 1992; Urusov and Orlov 1999).

For karlleuite structure, the BVS of Ca is 1.83 v.u. (valence units), indicating that Ca is slightly underbonded which is consistent with CaO with the rock-salt structure (Fm3m) and CaMnO3 perovskite. In the case of the perovskite layers and octahedrally coordinated B site, the B-O bond lengths differ due to the Jahn-Teller distortion. A distortion where four bonds contract and two expand, termed Q3, is typically observed in ordered perovskites (Kim et al. 2023). Distinguishing the substituents at the B site in the karlleuite structure, the BVS for each individual cation is: 3.857(11) v.u. for Mn4+, 4.529(14) v.u. for Ti4+, 3.964(12) v.u. for Fe3+, and 2.917(8) v.u. for Al, clearly showing a mismatch of ionic radii and predicted B-O distances for all cations at this site (Table 7). Therefore, the obtained calculation shows that the O2 atom with an average BVS of 2.258 v.u. is overbonded, while O1 is underbonded with an average BVS of 1.568 v.u. (Table 7).

Table 7 Selected interatomic distances (Å) and BVS of the karlleuite structure
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In CaO with a rock-salt structure (Fm3m), the O atom is coordinated by six Ca atoms, with bond lengths equal to 2.4048(1) Å, slightly larger than the predicted distance of 2.3735 Å. Therefore, in such structure type, oxygen atoms are usually slightly underbonded (1.838 v.u.) In the Ruddlesden-Popper structure, the Jahn Teller effect, and an elongation of the octahedra in the perovskite layers along the c-axis, causes longer B-O1 1.994(5) Å and shorter O1-Ca 2.259(6) Å bonds (Fig. 4c). Elongation of the rock-salt layers in the ab-plane leads to four long Ca-O1 bonds of 2.6767(6) Å and a very low bond valence sum of the O1 atom (Table 7).

In addition, the Goldschmidt tolerance factor (𝑡) for karlleuite, where A – O and B – O are the metal – oxygen bond lengths of the A and B site, respectively is 𝑡 = \(\:\frac{A-\text{O}}{\sqrt{2\:}(B-\text{O})}\) = 0.95. Following Goldschmidt (1926), the RP type n = 1 can be formed when 0.85 < 𝑡 < 1.05. When 𝑡 is close to unity, cubic perovskites (Pm3m) are stable at ambient conditions, when 𝑡 is greater or less than unity, disordered perovskite structures are expected.

The global instability index (GII), a bond valence-based metric originally designed to evaluate the total bond strain in a crystal (Brown 1992), of karlleuite is 0.3 v.u. which is much higher than the empirical threshold of 0.2, above which the feasibility of a structure should be doubted. Nevertheless, the majority of experimentally observed structures have GII values above the 0.2 v.u. threshold (Miller and Rondinelli 2023). Unlike ABO3 oxides, which are generally known as oxygen-deficient perovskites because oxygen vacancies are their dominant anion defect, RP oxides can be both oxygen-deficient and oxygen-rich, depending on the majority of oxygen defects. However, in karlleuite structure, where the O1 and O2 atoms are fully occupied, we did not find evidence of these defects.

Discussion

Ca2MnO4 space group consideration

Synthetic, polycrystalline Ca2MnO4 was solved by Ruddlesden and Popper (1957) in the tetragonal I4/mmm space group with the following lattice parameters a = 3.67 Å and c = 12.08 Å, which match the here reported karlleuite structure. However, Ca2MnO4 phase can also crystalize in the tetragonal space group I41/acd with lattice constants a = 5.183(1) Å and c = 24.117(4) Å, a super-cell (a = √2a′, c = 2c′) of the I4/mmm unit cell, as reported by Leonowicz et al. (1985) and later confirmed by others (Takahashi and Kamegashira 1993; Fawcett et al. 1998; Taguchi 2001; Autret et al. 2004). Further investigations were performed also for Bi-, La-, Y-, and Dy-doped Ca2MnO4 (Kawashima et al. 2009; Azulay et al. 2020; Bregiroux et al. 2021). The doping of the A site by trivalent cations causes the electron excess, which is balanced by the reduction of an equivalent amount of Mn4+ to Mn3+ at the B site (Azulay et al. 2020; Bregiroux et al. 2021).

The difference between the tetragonal I4/mmm and I41/acd superstructures corresponds to the rotation of MnO6 octahedra in opposite directions around the c-axis between two successive perovskite layers, and the rotation decreases with increasing temperature (Takahashi and Kamegashira 1993). The recent study confirms the occurrence of two tetragonal space groups for Ca2MnO4, wherein second order structural phase transition from I41/acd to I4/mmm is attained around 1050 K (Rocha-Rodrigues et al. 2020).

With this in mind, the ideal Ruddlesden-Popper type structure with I4/mmm space group and c unit-cell parameter around 12 Å was finally determined for karlleuite (Fig. 6a). Naturally, there is no rotation or tilting of the octahedra in the consecutive perovskite layers (Fig. 6b). In comparison, an unselected and more complex structure with the I41/acd space group is characterised by MnO6 octahedra tilted about their c-axes, which are also slightly contra-rotated in the ab plane (Fig. 6d, e). This contra-rotation is related to each octahedron set placed on the first and third, as well as the second and fourth perovskite layers (Fig. 6e). Moreover, the rotation of the MnO6 octahedra causes superlattice reflections and a doubling of the c parameter around 24 Å, which implies that the volume of the unit-cell with the I41/acd space group is almost four times larger than that of the karlleuite I4/mmm unit-cell (Fig. 6a, d). Superlattice reflexions can be a few magnitudes weaker than the main reflections. Therefore, sections of reciprocal space are constructed and extensively searched for any satellite reflections that can indicate possible superstructure. Still, no additional reflections were found. Furthermore, the phase transition between the two structures affects the Mn-O bond lengths together with the decrease of the Mn-O2-Mn angles between corner-connected octahedra from 180˚ in the I4/mmm (Fig. 6c) to 161.6˚ in I41/acd unit-cell (Fig. 6f; Leonowicz et al. 1985).

Fig. 6
figure 6

(a, d) Representation of the Ca2MnO4 structure in the I4/mmm (karlleuite) and supercell with I41/acd space groups (after Leonowicz et al. 1985); (b, e) Representation of undistorted and distorted MnO6 octahedra with characteristic rotation, view perpendicular and parallel to the c-axis; (c, f) Bonding scheme of the Ca2MnO4 structure with the relationship of the Mn-O2-Mn angle. Green and red spheres represent the Ca and O ions respectively, and Mn is shown as pink and purple octahedra (a-b, d-e) or spheres (c, f). The unit-cell is shown as a solid black line

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Considering that diffraction experiment was performed with powerful X-ray source (rotation anode) using the newest generation of hybrid pixel detector with excellent dynamic range, we are fairly sure that karlleuite exhibits undistorted I4/mmm structure, as the most of the oxides with RP n = 1 structures (Balachandran et al. 2014).

Genetic aspect of karlleuite formation and its relation to the perovskite supergroup

The formation of karlleuite is related to the Ca-rich pyrometamorphic rocks that occur as xenoliths in basaltic rocks in the Bellerberg volcanic region. Such rocks form under sanidine facies conditions, at temperatures above 1000˚C and low pressure ranging from a few kilobars to atmospheric conditions (Grapes 2010). Karlleuite is associated with Fe3+-bearing minerals such as brownmillerite, srebrodolskite, and sharyginite, confirming high oxygen fugacity conditions during crystallization. The presence of these accessory minerals indicates their co-crystallization under similar conditions from compounds (fragments of Mn-Ti-Fe minerals) present in the sedimentary protolith. This suggests that karlleuite could be formed under high-temperature conditions. Moreover, investigations of the Ca-Mn-O system show that the Ca2MnO4 phase is stable over the wide temperature range between 700 and even 1550˚C (White et al. 2008). Furthermore, several studies on the synthetic Ca2MnO4 compound confirm its formation in the temperature range of 900–1200˚C (Fawcett et al. 1998; Autret et al. 2004; Kawashima et al. 2009; Chihaoui et al. 2011a), and the recently established temperature of I41/acd → I4/mmm phase transition around 1050 K (> 750˚C) fits into the stability field of this compound in the Ca-Mn-O system (Rocha-Rodrigues et al. 2020). The high-temperature origin of karlleuite could also be supported by its crystal structure with the I4/mmm space group. This hypothesis is reinforced by the fact that many oxides with the higher-symmetry crystallize under high-temperature conditions (Skinner 2003; Nirala et al. 2020).

According to the nomenclature of the perovskite supergroup (Mitchell et al. 2017), karlleuite does not fit into the classification of stoichiometric or non-stoichiometric perovskites. It can be included in the layered perovskites with the Ruddlesden-Popper type structure, derived from the simple aristotype ABX3 structure, as presented in the hierarchical classification of synthetic compounds with the perovskite structure (Mitchell et al. 2017). Hitherto, no natural members with the Ruddlesden-Popper structure were previously known. As karlleuite is the first natural member characterised by this structure type, we propose to enrich the current classification with a potentially new, additional subgroup of minerals with the general formula An+1BnX3n+1, or AX(ABX3)n (n = 1), according to the commonly used formula of synthetic compounds (Sharma and Singh 1998; Mitchell 2002; Lacotte et al. 2014). Karlleuite will be the first representative of this subgroup within the Ruddlesden-Popper type structure layered perovskites.