Introduction
Arsenates of fumarolic origin are specific and, as recent data show, numerous: more than 50 valid mineral species belonging to this chemical class are known in volcanic sublimates. All of them are hydrogen-free. The most prolific locality of these minerals is the Arsenatnaya fumarole situated at the apical part of the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption 1975–1976, Tolbachik volcano, Kamchatka Peninsula, Far-Eastern Region, Russia (55°41′N 160°14′E, 1200 m a.s.l.). One of the outstanding features of this fumarole is the abundance of new arsenate minerals (Pekov et al., Reference Pekov, Koshlyakova, Zubkova, Lykova, Britvin, Yapaskurt, Agakhanov, Shchipalkina, Turchkova and Sidorov2018a). Seventeen new species were characterised in the previous articles of this series, namely yurmarinite Na7(Fe3+,Mg,Cu)4(AsO4)6 (Pekov et al., Reference Pekov, Zubkova, Yapaskurt, Belakovskiy, Lykova, Vigasina, Sidorov and Pushcharovsky2014a), two polymorphs of Cu4O(AsO4)2, ericlaxmanite and kozyrevskite (Pekov et al., Reference Pekov, Zubkova, Yapaskurt, Belakovskiy, Vigasina, Sidorov and Pushcharovsky2014b), popovite Cu5O2(AsO4)2 (Pekov et al., Reference Pekov, Zubkova, Yapaskurt, Belakovskiy, Vigasina, Sidorov and Pushcharovsky2015a), structurally related shchurovskyite K2CaCu6O2(AsO4)4 and dmisokolovite K3Cu5AlO2(AsO4)4 (Pekov et al., Reference Pekov, Zubkova, Belakovskiy, Yapaskurt, Vigasina, Sidorov and Pushcharovsky2015b), katiarsite KTiO(AsO4) (Pekov et al., Reference Pekov, Yapaskurt, Britvin, Zubkova, Vigasina and Sidorov2016a), melanarsite K3Cu7Fe3+O4(AsO4)4 (Pekov et al., Reference Pekov, Zubkova, Yapaskurt, Polekhovsky, Vigasina, Belakovskiy, Britvin, Sidorov and Pushcharovsky2016b), pharmazincite KZnAsO4 (Pekov et al., Reference Pekov, Yapaskurt, Belakovskiy, Vigasina, Zubkova and Sidorov2017), arsenowagnerite Mg2(AsO4)F (Pekov et al., Reference Pekov, Zubkova, Agakhanov, Yapaskurt, Chukanov, Belakovskiy, Sidorov and Pushcharovsky2018b), arsenatrotitanite NaTiO(AsO4) (Pekov et al., Reference Pekov, Zubkova, Agakhanov, Belakovskiy, Vigasina, Yapaskurt, Sidorov, Britvin and Pushcharovsky2019a), the two isostructural minerals edtollite K2NaCu5Fe3+O2(AsO4)4 and alumoedtollite K2NaCu5AlO2(AsO4)4 (Pekov et al., Reference Pekov, Zubkova, Agakhanov, Ksenofontov, Pautov, Sidorov, Britvin, Vigasina and Pushcharovsky2019b), anatolyite Na6(Ca,Na)(Mg,Fe3+)3Al(AsO4)6 (Pekov et al., Reference Pekov, Lykova, Yapaskurt, Belakovskiy, Turchkova, Britvin, Sidorov and Scheidl2019c), zubkovaite Ca3Cu3(AsO4)4 (Pekov et al., Reference Pekov, Lykova, Agakhanov, Belakovskiy, Vigasina, Britvin, Turchkova, Sidorov and Scheidl2019d), pansnerite K3Na3Fe3+6(AsO4)8 (Pekov et al., Reference Pekov, Zubkova, Koshlyakova, Agakhanov, Belakovskiy, Vigasina, Yapaskurt, Britvin, Turchkova, Sidorov and Pushcharovsky2020a) and badalovite NaNaMg(MgFe3+)(AsO4)3 (Pekov et al., Reference Pekov, Koshlyakova, Agakhanov, Zubkova, Belakovskiy, Vigasina, Turchkova, Sidorov and Pushcharovsky2020b). The latter belongs to the alluaudite group, a subdivision of the alluaudite supergroup, as well as the new mineral calciojohillerite described in the present paper.
Calciojohillerite has the idealised formula NaCaMg3(AsO4)3, or, in accordance with the nomenclature of alluaudite-group arsenates (Hatert, Reference Hatert2019), NaCaMgMg2(AsO4)3. It is named as an analogue of johillerite NaCu2+MgMg2(AsO4)3 with species-defining calcium instead of copper in the ideal, end-member formula.
Both the new mineral and its name have been approved by the IMA Commission on New Minerals, Nomenclature and Classification, IMA2016–068 (Pekov et al., Reference Pekov, Koshlyakova, Agakhanov, Zubkova, Belakovskiy, Vigasina, Turchkova, Sidorov and Pushcharovsky2016c). The holotype specimen is deposited in the systematic collection of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow with the catalogue number 95621.
Occurrence and general appearance
At present, the Arsenatnaya fumarole is the largest and one of the hottest among strongly mineralised active fumaroles related to the Tolbachik volcano. The general description of Arsenatnaya and the data on zonation in distribution of mineral parageneses in this fumarole were reported by us earlier (Pekov et al., Reference Pekov, Zubkova, Yapaskurt, Belakovskiy, Lykova, Vigasina, Sidorov and Pushcharovsky2014a, Reference Pekov, Koshlyakova, Zubkova, Lykova, Britvin, Yapaskurt, Agakhanov, Shchipalkina, Turchkova and Sidorov2018a; Shchipalkina et al., Reference Shchipalkina, Pekov, Koshlyakova, Britvin, Zubkova, Varlamov and Sidorov2020).
Calciojohillerite is found in all middle and deep zones of the fumarole, from the so-called polymineralic zone (depth of 1.5–2 m from the surface) to the lowest, so-called anhydrite zone (3–4 m deep). Temperatures measured by us using a chromel–alumel thermocouple at the time of collecting (in 2015–2018) in different pockets with the new mineral varied from 360–380°C (polymineralic zone) to 450–490°C (anhydrite zone). Calciojohillerite is a constituent of sublimate incrustations formed in open space in fumarole chambers and cracks, or in vesicles of basalt scoria. We believe that the mineral crystallised at temperatures not lower than 500°C. It was deposited directly from the gas phase or, more probably, formed as a result of the interaction between hot volcanic gas and basalt scoria. The latter could be a source of Mg and Ca which have very low volatilities in such post-volcanic systems (Symonds and Reed, Reference Symonds and Reed1993).
The first specimen, which became the holotype of calciojohillerite, was collected in July 2015 from the polymineralic zone of Arsenatnaya. The new mineral is associated here with hematite, tenorite, johillerite, nickenichite, bradaczekite, hatertite, magnesiohatertite, badalovite, aphthitalite-group sulfates, langbeinite, calciolangbeinite, sanidine (As-bearing variety), fluorophlogopite, fluoborite, tilasite, anhydrite, pseudobrookite, rutile, sylvite, halite, lammerite, lammerite-β, urusovite, ericlaxmanite, kozyrevskite, arsmirandite, svabite, popovite, dmisokolovite, shchurovskyite, yurmarinite, krasheninnikovite, euchlorine, wulffite, alumoklyuchevskite, sellaite, gahnite and corundum. In the polymineralic zone, calciojohillerite is not a very common mineral.
Our fieldwork in 2016–2018 showed that calciojohillerite is a typical, sporadically main crust-forming mineral at deep levels of the Arsenatnaya fumarole. In the zone located at a depth of 2–3 m from the surface and intermediate between the polymineralic and anhydrite zones, calciojohillerite is the most abundant arsenate, the major concentrator of As. It forms, sometimes together with badalovite, crystal crusts up to 0.5 m2 in area and up to 1 cm thick which cover walls of cracks and cavities. Copper minerals, so rich and diverse further up, in the polymineralic zone, are almost absent here, except for minor amounts of johillerite and nickenichite. Calciojohillerite and other alluaudite-group arsenates are typically associated in this zone with fluorophlogopite, sanidine, diopside, hematite, cassiterite, anhydrite, metathénardite, belomarinaite, sylvite, tilasite, svabite and arsenowagnerite.
In the anhydrite zone, calciojohillerite also occurs in significant amounts. It is common in sublimate incrustations (up to 3 cm thick) which mainly consist of anhydrite, diopside and hematite and sporadically contain svabite and garnets of the berzeliite–schäferite series as important crust-forming constituents. Other typical minerals here are forsterite, magnesioferrite, spinel, ludwigite, rhabdoborite-group fluoroborates, powellite, baryte, fluorapatite, wagnerite (As-bearing variety), udinaite, arsenudinaite and paraberzeliite.
Calciojohillerite occurs as prismatic crystals, usually up to 1 mm in size, rarely up to 1 mm × 2 mm × 10 mm. Sometimes the crystals are short prismatic, almost isometric (Fig. 1a), but more commonly long prismatic (Figs 1b,c and 2) to acicular or spear-like, elongated along [001]. Crystals are crude or well-shaped, in some specimens divergent. They are typically assembled in bunches, sheaf-like or radial aggregates (Fig. 1c). Isolated crystals of calciojohillerite are rare; they commonly form chaotic groups, brushes or interrupted crusts (Figs 1 and 2) up to several cm2, rarely up to 0.5 m2 in area which overgrow incrustations of sanidine, fluorophlogopite, diopside and/or hematite or directly cover basalt scoria altered by fumarolic gas.
Fig. 1. Aggregates of calciojohillerite: (a, sample #4789: holotype specimen) crust of coarse short-prismatic, almost isometric colourless crystals on basalt scoria altered by fumarolic gas, (b, sample #4713) chaotic aggregates of coarse prismatic pale greenish-grey crystals with iron-black hematite on crust of As-bearing sanidine and (c, sample #4671) radial clusters of prismatic colourless to pale lilac crystals with deep blue johillerite and iron-black hematite. FOV widths: (a, b) 14.5 mm, (c) 2.8 mm. Photo: I.V. Pekov & A.V. Kasatkin.
Fig. 2. Group of calciojohillerite crystals. SEM (SE mode) image.
Physical properties and optical data
Calciojohillerite is transparent, typically colourless, pale green, pale yellow or light blue, sometimes pale lilac, pink, bluish-, greenish- or yellowish-grey. The streak is white and lustre is vitreous. The mineral is brittle. One direction of imperfect cleavage was observed under the microscope; parting was not observed. The fracture is uneven. The Mohs hardness is ca. 3½. Density calculated for the holotype using the empirical formula is 3.915 g cm–3.
In plane polarised transmitted light, calciojohillerite is colourless and non-pleochroic. It is optically biaxial (–), α = 1.719(3), β = γ = 1.732(3) (589 nm); 2Vmeas. = 15(10)° (estimated by the curve of the conoscopic figures on the sections perpendicular to the acute bisectrix), 2Vcalc. = 0°. Dispersion of optical axes is medium, r > v. Optical orientation: Y = b.
Raman spectroscopy
The Raman spectrum of the holotype calciojohillerite (Fig. 3) was obtained on a randomly oriented crystal using an EnSpectr R532 instrument with a green laser (532 nm) at room temperature. The output power of the laser beam was ~16 mW. The spectrum was processed using the EnSpectr expert mode program in the range from 100 to 4000 cm–1 with the use of a holographic diffraction grating with 1800 lines per cm and a resolution of 6 cm–1. The diameter of the focal spot on the sample was ~10 μm. The back-scattered Raman signal was collected with a 40× objective; signal acquisition time for a single scan of the spectral range was 1500 ms and the signal was averaged over 20 scans.
Fig. 3. The Raman spectrum of calciojohillerite.
The strongest bands in the region 750–950 cm–1 in the Raman spectrum of calciojohillerite correspond to As5+–O stretching vibrations of AsO43– anions. A weak band with a maximum at 574 cm–1 probably corresponds to Fe3+–O stretching vibrations caused by admixed Fe3+ in the holotype specimen. Bands with frequencies lower than 500 cm–1 correspond to bending vibrations of AsO4 tetrahedra, Mg–O, Ca–O and Na–O stretching vibrations and lattice modes. The bands in the spectrum are assigned according to Nakamoto (Reference Nakamoto1986). The absence of bands with frequencies higher than 950 cm–1 indicates the absence of groups with O–H, C–H, C–O, N–H, N–O and B–O bonds in the mineral.
Chemical composition
The chemical composition of calciojohillerite was determined with a Jeol JSM-6480LV scanning electron microscope equipped with an INCA-Wave 500 wavelength-dispersive spectrometer (Laboratory of Analytical Techniques of High Spatial Resolution, Dept. of Petrology, Moscow State University), with an acceleration voltage of 20 kV, a beam current of 20 nA and a 10 μm beam diameter. The following standards were used: jadeite (Na, Al and Si), KTiOPO4 (K, Ti and P), wollastonite (Ca), olivine (Mg), MnTiO3 (Mn), Cu (Cu), ZnS (Zn, S), FeS2 (Fe), V (V) and GaAs (As). Contents of other elements with atomic numbers higher than carbon are below detection limits. H2O was not analysed because both structure data and Raman spectrum show its absence.
The empirical formula of the holotype specimen (Table 1, #1) calculated on the basis of 12 O atoms per formula unit (apfu) is (Na1.30K0.01Ca0.67Mg2.78Mn0.05Cu0.02Al0.06Fe3+0.24)Σ5.13(As2.83P0.10S0.01V0.01)Σ2.95O12. In comparison with the holotype originated from the polymineralic zone of the Arsenatnaya fumarole, calciojohillerite from the anhydrite zone contains much less admixed Fe but is enriched with V and P, which both substitute As (Table 1, ##2–3).
Table 1. Chemical composition of calciojohillerite.
[1] Sample #4789: holotype, from polymineralic zone of the Arsenatnaya fumarole (average for five spot analyses, ranges are in parentheses); [2] sample #5358 and [3] sample #5348, both from anhydrite zone of the Arsenatnaya fumarole; [4] NaCaMg3(AsO4)3. ‘–’ indicates content is below the detection limit. *All Fe is considered as Fe3+ (see text).
X-ray crystallography and crystal structure determination
Powder X-ray diffraction data (XRD) of the holotype calciojohillerite (Table 2) were obtained with a DRON-2 diffractometer using CuKα radiation. Parameters of the monoclinic unit cell calculated from the powder XRD data are: a = 11.83(1), b = 12.772(5), c = 6.686(8) Å, β = 112.41(8)° and V = 934(2) Å3.
Table 2. Powder X-ray diffraction data (d in Å) of calciojohillerite.
*For the calculated pattern, only reflections with intensities ≥1 are given; **for the unit-cell parameters calculated from single-crystal data; the strongest reflections are marked in bold type.
Single-crystal XRD studies of the holotype calciojohillerite were carried out using an Xcalibur S diffractometer equipped with a CCD detector. A full sphere of three-dimensional data was collected. Intensity data were corrected for Lorentz and polarisation effects. Calciojohillerite is a monoclinic mineral which crystallises in the space group C2/c; unit-cell parameters are: a = 11.8405(3), b = 12.7836(2), c = 6.69165(16) Å, β = 112.425(3)°, V = 936.29(4) Å3 and Z = 4. The unit-cell data obtained from single-crystal and powder XRD data are in a good agreement. The crystal structure of the new mineral was solved by direct methods and refined using the SHELX-97 software package (Sheldrick, Reference Sheldrick2008) to R 1 = 0.0227 on the basis of 1825 independent reflections with I > 2σ(I). Crystal data, data collection information and structure refinement details are given in Table 3, coordinates and thermal displacement parameters of atoms and bond-valence sums in Table 4, refined site-scattering factors and assigned occupancies for mixed-occupied metal cation sites A and M in Table 5 and selected interatomic distances in Table 6. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).
Table 3. Crystal data, data collection information and structure refinement details for calciojohillerite.
Table 4. Сoordinates and equivalent displacement parameters (U eq, in Å2) of atoms, bond-valence sums (BVS) and site multiplicities (Q) for calciojohillerite.
*Bond-valence parameters are taken from (Brese and O'Keeffe, Reference Brese and O'Keeffe1991). All calculations were done taking into account the cation distribution shown in Table 5. **The A(1)’ site was localised from a difference-Fourier synthesis and could contain a small amount of Cu2+, as is typical for other Cu-bearing alluaudite-group arsenates (see text). ***U iso.
Table 5. Refined site-scattering factors and assignment for cation sites A and M in the structure of calciojohillerite.
SC – scattering curve used for refinement of site occupancy, s.o.f – refined site occupancy factor, SSFexp and SSFcalc – experimental and calculated site-scattering factors, respectively.
Table 6. Selected interatomic distances (Å) in the structure of calciojohillerite.
Discussion
The alluaudite supergroup includes monoclinic phosphate and arsenate minerals with the general formula A(2)’A(1)M(1)M(2)2(T 5+O4)3 in which T 5+ = P or As, whereas A and M are cations which occupy sites ranked in descending order of the size. The M sites are fully occupied while the A sites can be empty or partially occupied (Jebli et al., Reference Jebli, Badri and Ben Amara2016; Hatert, Reference Hatert2019). The alluaudite group forms the major part of the alluaudite supergroup. In particular, all 20 alluaudite-supergroup arsenates known to date belong to the alluaudite group. The species-defining M and A components known in alluaudite-group arsenates are as follows: M = Mg, Mn2+, Fe3+, Cu2+, Zn, Cd, Ca or Na; A = Na, Ca, Cu2+ or vacancy; some of these minerals are H-bearing, i.e. contain acidic arsenate anions or H2O (Hatert, Reference Hatert2019).
The crystal structure of calciojohillerite (Fig. 4) is typical for a member of the alluaudite group (Moore and Ito, Reference Moore and lto1979; Krivovichev et al., Reference Krivovichev, Vergasova, Filatov, Rybin, Britvin and Ananiev2013; Hatert, Reference Hatert2019 and references therein). It contains zig-zag chains of edge-sharing M(1)O6 and M(2)O6 octahedra; in calciojohillerite M = Mg (with admixtures of Fe3+, Al and Mn in the holotype). The octahedral chains consist of [M(2)2O10] dimers of the M(2)O6 octahedra connected via isolated M(1)O6 octahedra. Arsenic atoms occupy two crystallographically independent tetrahedrally coordinated sites. In both sites As is partially substituted by P; in accordance with electron-microprobe data, the As:P ratio in the sites was fixed as As0.95P0.05. As1O4 tetrahedra share all vertices with the M-centred octahedra forming (010) heteropolyhedral layers while each As(2)O4 tetrahedron shares three vertices with the MO6 octahedra of one layer and the fourth vertex with the octahedron of the adjacent layer, thus linking the layers to a three-dimensional framework (Fig. 4). The framework contains channels of two types in which the A sites are located. The A(1) site is coordinated by six O atoms with the distances in the range from 2.3500(16) to 2.5315(16) Å; two elongated A(1)–O(2) bonds [2.8695(18) Å] could also be included in the coordination sphere of A(1). The A(1) site is occupied by Ca with admixed Na (Tables 4 and 5). A small amount of Cu revealed by electron microprobe (Table 1) could be localised in the A(1)’ site (see below) found in the difference-Fourier synthesis. The A(1)’ site has square planar oxygen coordination with A(1)’–O distances varying from 1.87 to 1.99 Å. The A(1) and A(1)’ sites alternate in rows parallel to [001] with a formal A(1)–A(1)’ distance of 1.673 Å; thus, the neighbouring sites within one channel cannot be occupied simultaneously. If minor Cu (0.02 apfu) is located in A(1)’ then the A(1) site should be partially (at least for 2%) vacant. The A(2)’ site in calciojohillerite is occupied by Na, probably with traces of K, and centres an eight-fold polyhedron with A(2)’–O distances in the range from 2.419(2) to 2.999(2) Å (Table 6).
Fig. 4. The crystal structure of calciojohillerite in two projections: along the b axis (the upper drawing) and along the c axis (the lower drawing). As-centred tetrahedra are red, M-centred octahedra are purple, A(1), A(1)' and A(2)' sites are given as yellow, blue and green circles, respectively. The unit cell is outlined.
The presence of admixed Fe in trivalent state in the holotype calciojohillerite is confirmed by the bond-valence sum for M(2) (2.24 valence units: Table 4) and is in agreement with extremely oxidising conditions of mineral deposition in the Arsenatnaya fumarole: all other iron-bearing minerals, known from here, contain only Fe3+ (Pekov et al., Reference Pekov, Koshlyakova, Zubkova, Lykova, Britvin, Yapaskurt, Agakhanov, Shchipalkina, Turchkova and Sidorov2018a; Shchipalkina et al., Reference Shchipalkina, Pekov, Koshlyakova, Britvin, Zubkova, Varlamov and Sidorov2020).
Synthetic NaCaMg3(AsO4)3, an analogue of the end-member calciojohillerite, demonstrates in general, the same distribution of cations as the mineral, with only slight differences in the content of the A sites: (Na0.7Ca0.3) in A(2)’ and (Ca0.7Na0.3) in A(1) (Abdallah and Haddad, Reference Abdallah and Haddad2008).
Only three alluaudite-group arsenate minerals are characterised by Mg predominance at both M(1) and M(2) sites, i.e., have 3 apfu Mg in the idealised formula, namely johillerite NaCuMg3(AsO4)3, calciojohillerite NaCaMg3(AsO4)3 and nickenichite Na(Ca0.5Cu0.5)Mg3(AsO4)3 (Hatert, Reference Hatert2019). They differ from each other in species-defining constituents at positions belonging to the A1 group (Table 7) which includes three closely located sites labelled as A(1), A(1)’ and A(1)’’. Only one of them can have more than 50% occupancy due to a short cation–cation distance: <2 Å. The A(1)’-centred polyhedron is the smallest and has flat-square oxygen coordination and only the cation, Cu2+, is known to be located there in arsenate minerals of the group, with A(1)’–O distances lying in the range from 1.82 to 1.99 Å (Keller and Hess, Reference Keller and Hess1988; Auernhammer et al., Reference Auernhammer, Effenberger, Hentschel, Reinecke and Tillmanns1993; Krivovichev et al., Reference Krivovichev, Filatov and Burns2001; Tait and Hawthorne, Reference Tait and Hawthorne2004; Malcherek and Schlüter, Reference Malcherek and Schlüter2013; Schlüter et al., Reference Schlüter, Malcherek, Mihailova and Gebhard2013; Koshlyakova et al., Reference Koshlyakova, Zubkova, Pekov, Giester and Sidorov2018; Pekov et al., Reference Pekov, Lykova, Koshlyakova, Belakovskiy, Vigasina, Turchkova, Britvin, Sidorov and Scheidl2020c). This is in a good agreement with the ionic radius of Cu2+ and with a strong tendency of Cu2+-centred polyhedra for the Jahn–Teller distortion. Both A(1) and A(1)’’ are large-cation sites: eight-coordinated A(1) with A(1)–O distances from 2.32 to 3.06 Å [or six-coordinated in the case of hatertite with A(1)–O distances from 2.31 to 2.61 Å and two strongly elongated distances (3.22 Å) not included in the coordination sphere of A(1): Krivovichev et al., Reference Krivovichev, Vergasova, Filatov, Rybin, Britvin and Ananiev2013] and seven-coordinated A(1)’’ with A(1)’’–O from 2.34 to 2.83 Å. They are occupied by Na or Ca in all arsenate minerals of the alluaudite group (Hatert, Reference Hatert2019 and references therein).
Table 7. Comparative data for arsenate minerals of the alluaudite group with Mg prevailing in both M(1) and M(2) sites.
*Tait and Hawthorne (Reference Tait and Hawthorne2004) presented the crystal structure of johillerite in the space group I2/a with unit-cell parameters: a = 6.752, b = 12.739, c = 11.068 Å and β = 100.37°; here this setting is given transformed to the standard setting (C2/c) for better comparison with other alluaudite-group arsenates.
**The sites A(1)’’, A(2) and A(2)’’ vacant in all three minerals are not included.
Johillerite A (2)’Na[A (1)□A (1)’CuA (1)’’□]M (1)MgM (2)Mg2(TAsO4)3, calciojohillerite A (2)’Na[A (1)CaA (1)’□A (1)’’□]M (1)MgM (2)Mg2(TAsO4)3 and nickenichite A (2)’Na[A (1)(□0.5Ca0.5)A (1)’(□0.5Cu0.5)A (1)’’□]M (1)MgM (2)Mg2(TAsO4)3 thus demonstrate three different cases of content and, correspondingly, different distribution of species-defining constituents in positions of the A1 group (Hatert, Reference Hatert2019).
Typical calciojohillerite is almost Cu-free (Table 1; see also Pekov et al., Reference Pekov, Koshlyakova, Zubkova, Lykova, Britvin, Yapaskurt, Agakhanov, Shchipalkina, Turchkova and Sidorov2018a), which is reflected in its properties. Unlike johillerite and nickenichite which are usually bright or deep blue or violet, it is colourless or has pale colour (Fig. 1). Calciojohillerite also differs distinctly from johillerite and nickenichite in optical data, especially in optical sign and birefringence (Table 7).
Acknowledgements
We thank Uwe Kolitsch, Oleg I. Siidra and anonymous referee for valuable comments. This study was supported by the Russian Science Foundation, grant no. 19-17-00050.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1180/mgm.2021.2.
