Strontioborite: revalidation as a mineral species and new data

Abstract

Strontioborite, which was first described in 1960 and later discredited by the then named Commission on New Minerals and Mineral Names of the International Mineralogical Association (IMA CNMMN), has been re-investigated (electron microprobe, single-crystal and powder X-ray diffraction, crystal structure determination and IR spectroscopy) on two specimens, including the holotype, and revalidated by the IMA Commission on New Minerals, Nomenclature and Classification (CNMNC). Strontioborite is known only at the Chelkar salt dome (North Caspian Region, Western Kazakhstan), in halite rocks with bischofite, magnesite, anhydrite, halurgite, boracite, ginorite and celestine. It forms colourless lamellar, scaly or tabular crystals up to 2 mm across. The chemical composition (wt.%, H₂O is calculated for (OH)₄ = 4 H apfu, according to structural data; holotype/neotype) is: CaO 1.42/0.27, SrO 23.10/23.79, B₂O₃ 67.37/67.57, H₂O 8.73/8.72, total 100.62/100.37. The empirical formulae [calculated based on 15 O apfu = O₁₁(OH)₄ pfu] of the holotype and neotype specimens are Sr_0.92Ca_0.10B_7.98O₁₁(OH)₄ and Sr_0.95Ca_0.02B_8.02O₁₁(OH)₄, respectively. The idealised formula is Sr[B₈O₁₁(OH)₄]. Strontioborite is monoclinic, space group P2₁, a = 7.6192(3), b = 8.1867(2), c = 9.9164(3) Å, β = 108.357(4)°, V = 587.07(3) ų and Z = 2. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I)(hkl)] are: 7.22(100)(100), 5.409(61)(110), 4.090(64)(020), 3.300(48)(210), 2.121(30)($\bar{1}$24) and 2.043(37)(040, 024, $\bar{2}$24). The crystal structure, solved from single-crystal X-ray diffraction data (R = 0.0372), is based upon the (100) layers of polymerised B–O–OH polyanions [B₈O₁₁(OH)₄]^2– and Sr-centred nine-fold polyhedra SrO₆(OH)₃. The B–O–OH polyanion is the cluster of three tetrahedra and three triangles; these clusters are decorated by the [B₂O₂(OH)₃] pyro-group consisting of two triangles. The layers are linked via vertices of Sr-centred polyhedra, which share seven vertices with B-centred polyhedra of one layer and two vertices with B-centred polyhedra of the adjacent layer, and by the system of H bonds. The crystal chemistry of strontioborite is discussed in comparison with other natural and synthetic borates.

Introduction

This paper reports the revalidation of strontioborite, a borate mineral which was discovered in 1960, and named without approval of the Commission on New Minerals and Mineral Names of the International Mineralogical Association (IMA CNMMN). This occurred only a year after the foundation of the IMA CNMMN (1959) and the IMA rule to submit any proposal for a new mineral species to the Commission for its approval before publication was not yet in wide practice. In 1962, the published data on strontioborite were critically considered by the IMA CNMMN and the name strontioborite was rejected. On the basis of this IMA Commission decision, strontioborite was listed with the status D (discredited) in The IMA/CNMNC List of Mineral Names compiled by Nickel and Nichols (2004) and subsequently the later The official IMA-CNMNC List of Mineral Names issued by the IMA Commission on New Minerals, Nomenclature and Classification (CNMNC), http://cnmnc.units.it/ (Pasero, 2024).

Regardless of data obtained in the period of 1964–1975 which demonstrated its chemical and structural individuality, strontioborite was not added to the ‘official’ IMA status of a valid mineral species for almost sixty years because of a formality: these new data had not been submitted to the IMA CNMMN/CNMNC. In addition, the data from this period were incomplete. The missing data e.g. correct quantitative chemical data for this mineral, definitely obtained on pure material, are first reported in the present paper.

We carried out a revision study of strontioborite using type material and submitted a proposal to revalidate this mineral to the IMA CNMNC. In particular, we proposed to keep the name strontioborite given by the pioneer discoverer, Lobanova (1960) to the mineral because it is a strontium borate. Our proposal (IMA2020–017) was approved (Pekov et al., 2020) and, thus, since 2020 strontioborite has the status of valid mineral species (symbol Srbo, Warr, 2021). Type specimens are deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia with the catalogue numbers 69851 (holotype) and ST-7069 (neotype).

History of previous studies

Strontioborite was discovered by Lobanova (1960) in samples from cores of several boreholes drilled in boron-bearing evaporitic rocks of the Chelkar (another spelling: Shalkar) salt dome in the North Caspian Region, Western Kazakhstan. Note that the geographic information in the cited paper was limited by the term ‘Caspian Region’ due to the secrecy of materials on boron deposits in the Soviet Union. Pekov (1998) was the first to publish a full geographic account of the strontioborite locality. The first description of strontioborite contains information on the occurrence and associated minerals, its general appearance and some physical properties, as well as a wet chemical analysis [CaO 4.15, SrO 21.66, MgO 5.75, B₂O₃ 57.85, H₂O 11.52, total 100.93 wt.%] and powder X-ray diffraction (XRD) data (Table 1). The formula 4(Sr,Ca)O⋅2MgO⋅12B₂O₃⋅9H₂O with Sr:Ca = 3:1 was suggested for the mineral (Lobanova, 1960). In his abstract of Lobanova (1960) for New Mineral Names, M. Fleischer in 1961 concluded that strontioborite ‘requires verification’ and that ‘some of the data could be construed as indicating a mixture of strontioginorite, boracite, and anhydrite’ (Fleischer, 1961; Hey, 1961). Probably, the doubts concerning the correctness of the chemical composition together with the absence of unit-cell data caused the rather negative voting in the IMA CNMMN: strontioborite ended up in the category of mineral names “rejected by 60% or more of the Commission” (IMA, 1962) and, as a result, was subsequently discredited.

Table 1. Formula and crystal data on strontioborite: historical overview.

*The formula was first reported in the paper by Kondrat'eva (1969). **In papers by Kondrat'eva (1964, 1969) and Brovkin et al. (1975), the unit cell of strontioborite was reported in the setting with the a and c parameters reversed compared to that given here.

The first single-crystal XRD data for strontioborite were reported by Kondrat'eva (1964), who studied several crystals of the mineral separated from the type material (received from the discoverer, V.V. Lobanova) and obtained reproducible results. It was found that strontioborite is monoclinic, with a unique unit cell and space group either P21 or P21/m. The presence of a piezoelectric effect confirmed the non-centrosymmetric space group P21. The improved powder XRD pattern of strontioborite (Table 1) was also reported (Kondrat'eva, 1964; English abstract: Fleischer, 1965). Kondrat'eva (1969) also mentioned that the simplified formula 3SrO⋅CaO⋅2MgO⋅11B₂O₃⋅8.5H₂O better corresponds to the original chemical analysis than 4(Sr,Ca)O⋅2MgO⋅12B₂O₃⋅9H₂O. However, in general, both formulae were considered as doubtful in the light of the obtained unit-cell data: the calculated Z values were not whole-number ones (Kondrat'eva, 1964, 1969).

In the same period, strontioborite was found independently in drillcores of other boreholes at the Chelkar salt dome by I.I. Khalturina (Avrova et al., 1968). On the single crystal separated from this material, Brovkin et al. (1975) solved the crystal structure of the mineral (R = 0.11), which turned out to be unique. The structure obtained by Brovkin et al. (1975) confirmed both the space group and unit-cell parameters reported by Kondrat'eva (1964) for strontioborite and made it possible to radically revise its formula (Table 1). Based on the crystal structure refinement, even despite the absence of new quantitative chemical data, Brovkin et al. (1975) suggested for strontioborite the idealised formula Sr[B₈O₁₁(OH)₄] and described a novel borate polyanion in this mineral. This idealised formula of strontioborite entered into the reference books of Malinko et al. (1991), Pekov (1998), Anthony et al. (2003) and Chukanov (2014).

Material used for our studies

Samples from cores of several boreholes drilled at the Chelkar salt dome in 1950s – 1960s remain the only source of strontioborite to date. The type specimen of strontioborite collected by V.V. Lobanova in 1950s was given by her in 1960s (catalogued in 1967) to the Fersman Mineralogical Museum of the Russian Academy of Sciences (in that period, the Academy of Sciences of the Soviet Union), Moscow, Russia and is deposited in the systematic collection of the Museum with the catalogue no. 69851 (Pekov, 1998). The strontioborite specimen collected by I.I. Khalturina, is preserved in the collection of the outstanding Russian mineralogist and mineral collector V.I. Stepanov. He donated his collection to the Fersman Mineralogical Museum in the 1980s (Pekov et al., 2015) and this specimen of strontioborite, received by V.I. Stepanov directly from I.I. Khalturina in 1968, is now deposited in the Museum with the catalogue no. ST-7069. Both samples are glass vials with white, loose water-insoluble residues remaining after dissolution of rock salt in water. After examination the residues turned out to be mixtures of small crystals of strontioborite and the associated water-insoluble minerals listed below.

We studied both these specimens and detected in them the same mineral, strontioborite, with identical XRD characteristics and physical properties and only a slight difference in the amount of minor Ca (Table 2). In specimen ST-7069, we found single crystals of strontioborite larger and more perfect than in the specimen 69851. A single crystal separated from the specimen ST-7069 was used by us for the crystal structure determination; note, the first study of the strontioborite structure was carried out by Brovkin et al. (1975) also on the crystal separated from the material collected by I.I. Khalturina (see above). Thus, the material originally studied by Lobanova (1960) and represented in the Fersman Mineralogical Museum by specimen no. 69851 retains its status as the holotype for strontioborite, whereas specimen ST-7069 is now considered as the neotype of this mineral.

Table 2. Chemical composition of strontioborite (wt.%, two samples from the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences – FMM: see text).

*Calculated for (OH)₄ = 4 H atoms per formula unit, according to structural data. S.D. – standard deviation.

Occurrence and general appearance

Strontioborite occurs in rocks mainly consisting of halite. It is associated with bischofite, magnesite, anhydrite, halurgite, boracite, ginorite and celestine. Strontioborite occurs embedded in halite as crude crystals that are flattened on {100}. They are lamellar, scaly or, rarely, tabular. The major crystal form is the pinacoid {100}, other faces are not indexed. Some scales show a hexagonal outline, however, strontioborite crystals are commonly polygonal, irregular in shape, some of them are divergent or blocky (Fig. 1). Crystals are typically 0.1–0.2 mm, rarely up to 2 mm across. Aggregates (up to 1 cm in size) of strontioborite intimately intergrown with halurgite, boracite, ginorite and/or magnesite were observed (Lobanova, 1960; Avrova et al., 1968; our data).

Figure 1. Blocky lamellar crystal of strontioborite enclosing aggregates of small crystals of magnesite (grey) and minor celestine (bright white). SEM (BSE) image, neotype ST-7069.

Strontioborite is a sedimentary mineral or was formed as a result of the diagenesis processes in boron-bearing evaporitic rocks.

Physical properties and optical data

Strontioborite is transparent, colourless, with white streak and vitreous lustre. Some crystals are white and semi-transparent due to abundant micro-inclusions of other minerals. Strontioborite is non-fluorescent under ultraviolet light or an electron beam. The mineral is brittle. Its Mohs’ hardness is ca 2½. Perfect, mica-like cleavage on {100} is observed. The fracture is stepped or laminated. The density measured by flotation in heavy liquids (bromoform + ethanol) is 2.40(2) g cm^–3. The density calculated for the holotype using the empirical formula and unit-cell volume obtained from the single-crystal XRD data is 2.35 g cm^–3 (Lobanova, 1960; our data).

Strontioborite is optically biaxial (+), α = 1.470(2), β = 1.510(2), γ = 1.579(2) (589 nm), 2V_meas. = 85(5)° and 2V_calc. = 77.5°. Dispersion of optical axes was not observed. Elongation is positive or negative (for different crystals), inclined. Under the microscope the mineral is colourless and non-pleochroic (Lobanova, 1960).

Infrared spectroscopy

In order to obtain an infrared (IR) absorption spectrum, a powdered sample of strontioborite was mixed with anhydrous KBr, pelletised, and analysed using an ALPHA FTIR spectrometer (Bruker Optics) at a resolution of 4 cm^–1. 16 scans were collected in the wavenumber range from 360 to 3800 cm^–1. The IR spectrum of an analogous pellet of pure KBr was used as a reference.

In the IR spectrum of strontioborite (Fig. 2), four bands of O–H stretching vibrations of OH groups are observed in the range from 3000 to 3500 cm^–1. This is in agreement with structural data (Tables 6, 7). According to the equation ν (cm^–1) = 3592–304⋅10⁹⋅exp[–d(O⋅⋅⋅O)/0.1321] for hydrogen bonds (Libowitzky, 1999), the O–H stretching bands with the absorption maxima at 3013, 3161, 3355 and 3401 cm^–1 correspond to the O⋅⋅⋅O distances of 2.65, 2.69, 2.77 and 2.80 Å, respectively. These values are rather close to the D⋅⋅⋅A distances of 2.620, 2.698, 2.852 and 2.919 Å determined from the crystal structure refinement (Table 5). Some discrepancies for the long d(O⋅⋅⋅O) distances are due to a high inaccuracy of the above-mentioned correlation as applied to weak hydrogen bonds.

Figure 2. Powder infrared absorption spectrum of strontioborite.

Two groups of strong bands observed in the ranges 1300–1500 and 900–1230 cm^–1 correspond to ^[3]B–O and ^[4]B–O stretching vibrations, respectively. The bands in the range 590–900 cm^–1 are mainly due to O–B–O bending vibrations. Weak bands observed below 550 cm^–1 correspond to mixed lattice modes. Weak bands in the range 2000–2800 cm^–1 are overtones and combination modes.

The IR spectrum of strontioborite is unique and can be used as a reliable diagnostic tool.

It should be noted that two IR spectra of samples labelled as ‘strontioborite’ in the book by Chukanov (2014) were obtained, as is now clear, on the material significantly contaminated by halurgite and boracite. Thus, the first correct IR spectrum of strontioborite is reported in the present paper (Fig. 2).

Chemistry

The chemical composition of both above-described samples of strontioborite was studied using 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). The WDS mode was used (with an acceleration voltage of 20 kV and a beam current of 10 nA; electron beam was rastered to the 5 × 5 μm area) and gave detectable contents of Ca, Sr and B. The contents of other elements with atomic numbers > 4, except oxygen, are below detection limits. Analytical data (in wt.%, average of four spot analyses for each sample) and standards used are given in Table 2. H₂O was not determined because of the paucity of pure material. H₂O content was calculated from the structure data (see below) that showed a good agreement with electron microprobe data (Table 2). CO₂ was not analysed because the structure data show the absence of this constituent. The absence of gas release in hydrochloric acid also indicates that strontioborite does not contain carbonate groups.

The empirical formulae calculated on the basis of 15 O atoms per formula unit (apfu) = O₁₁(OH)₄ pfu, taking into account the structure data (see below), are: Sr_0.92Ca_0.10B_7.98O₁₁(OH)₄ for sample FMM 69851; and Sr_0.95Ca_0.02B_8.02O₁₁(OH)₄ for sample FMM ST-7069. They clearly correspond to the structure-confirmed idealised formula SrB₈O₁₁(OH)₄.

The values of the Gladstone–Dale compatibility index 1 – (K _p/K _c) (Mandarino, 1981) for the holotype calculated with D _meas. and D _calc. are –0.033 (excellent) and –0.055 (good), respectively. Strontioborite is insoluble in water and easily dissolves in cold dilute HCl aqueous solution without effervescence. The solution obtained shows characteristic colour reaction with quinalizarin clearly indicating the presence of boron.

X-ray crystallography and crystal structure determination details

Powder XRD data for both samples were collected with a Rigaku R-AXIS Rapid II single-crystal diffractometer equipped with cylindrical image plate detector (radius 127.4 mm) using Debye-Scherrer geometry, CoKα radiation (rotating anode with VariMAX microfocus optics), 40 kV, 15 mA and 15 minutes exposure. Angular resolution of the detector is 0.045 2θ (pixel size 0.1 mm). The data were integrated using the software package Osc2Tab (Britvin et al., 2017). Powder XRD data for the neotype sample FMM ST-7069 are reported in Table 3. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I)(hkl)] are: 7.22(100)(100); 5.409(61)(110); 4.090(64)(020); 3.300(48)(210); 2.121(30)($\bar{1}$24) and 2.043(37)(040, 024, $\bar{2}$24). The powder XRD pattern for the holotype sample FMM 69851 is very close. The monoclinic unit cell parameters refined from the powder data for the holotype are: a = 7.622(3), b = 8.183(2), c = 9.919(4) Å, β = 108.36(3)° and V = 587.2(5) ų.

Table 3. Powder X-ray diffraction data (d in Å) of strontioborite.

*For the calculated pattern, only reflections with intensities ≥1 are given; **for the unit-cell parameters obtained from single-crystal data. The strongest reflections are highlighted in bold.

Single-crystal XRD studies of neotype strontioborite were carried out using an Xcalibur S diffractometer equipped with a CCD detector. A full sphere of three-dimensional data was collected. Data reduction was performed using CrysAlisPro Version 1.171.39.46 (Rigaku Oxford Diffraction, 2018). The data were corrected for Lorentz, polarisation and absorption effects. The structure was solved by direct methods and refined using the SHELX software package (Sheldrick, 2015) to R = 0.0372 for 2751 unique reflections with I > 2σ(I). H atoms were located in a difference-Fourier map and refined with O–H distances restrained to 0.90(1) Å and U _iso (H) = 1.2 U _eq (O). Minor Ca was taken into account in the Sr site. The crystal data, data collection information and structure refinement details are given in Table 4, atom coordinates, equivalent and anisotropic displacement parameters in Table 5, selected interatomic distances and H-bonding scheme in Tables 6 and 7 and bond-valence calculations in Table 8. The crystallographic information file has been deposited with the Principal Editor of Mineralogical Magazine and is available as Supplementary material (see below).

Table 4. Crystal data, data collection information and structure refinement details for strontioborite.

Table 5. Atom coordinates, equivalent and anisotropic displacement parameters (in Ų) for strontioborite.

*U _iso.

Table 6. Selected interatomic distances (Å) in the structure of strontioborite.

Table 7. H-bonding scheme (Å,°) in the structure of strontioborite.

D – donor; A – Acceptor

Table 8. Bond valence calculations for strontioborite.

Bond-valence parameters were taken from Gagné and Hawthorne (2015) and from Ferraris and Ivaldi (1988) for H-bonding.

*The occupancy of Sr site of Sr_0.866(8)Ca_0.134(8) was taken into account.

Discussion

Crystal structure and comparative crystal chemistry of strontioborite and related borates

Our results confirmed the correctness of the structure data obtained for strontioborite by Brovkin et al. (1975). The crystal structure of this mineral (Fig. 3a) is based upon the (100) layers of polymerised B–O–OH polyanions [B₈O₁₁(OH)₄]^2– and Sr-centred nine-fold polyhedra SrO₆(OH)₃. The layers are linked via vertices of Sr-centred polyhedra, which share seven vertices with B-centred polyhedra of one adjacent layer and two vertices with B-centred polyhedra of the other adjacent layer, and by the system of H bonds. According to the classification of fundamental building blocks (FBB) in borates (Grice et al., 1999), the FBB in strontioborite (Fig. 3b) could be presented as 5Δ3□:[ϕ]<Δ2□> | <Δ2□> | <Δ2□> | 2Δ. This means that three <Δ2□> rings containing two B-centred tetrahedra and one B-centred triangle are linked sharing tetrahedra, each ring sharing one tetrahedron with two adjacent rings. Thus, three tetrahedra share common vertices and the hexaborate group with three tetrahedra and three triangles is formed. These hexaborate groups are decorated by the [B₂O₂(OH)₃] pyro-group 2Δ consisting of two triangles.

Figure 3. The crystal structure of strontioborite projected along the b axis (a) and the FBB in the structure of strontioborite (b). H atoms of OH groups are shown as blue spheres. The unit cell is outlined. Drawn using Diamond, Version 3.2k (Crystal Impact, 2024).

In terms of crystal structure, strontioborite is unique among minerals but has the isostructural synthetic analogue with Ca instead of Sr, Ca[B₈O₁₁(OH)₄] (Zayakina and Brovkin, 1978; Yamnova et al., 2005; Wiggin and Weller, 2005). Topologically the same FBB was reported in the structures of some other synthetic borates, namely PbB₈O₁₁(OH)₄ (Belokoneva et al., 1999; Wang et al., 2006), BaB₈O₁₁(OH)₄ (Sun et al., 2010), apparently, BaB₈O₁₁(OH)₄⋅3H₂O (Wang and Liang, 2019) and SnB₈O₁₁(OH)₄ (Schönegger et al., 2018). The difference between the compounds AB₈O₁₁(OH)₄ with A = Ca, Sr vs. A = Pb, Ba, Sn is in the configuration of the layer and in the arrangement of the neighbouring layers (Yamnova et al., 2005; Schönegger et al., 2018).

Among minerals, FBB 5Δ3□:[ϕ]<Δ2□> | <Δ2□> | <Δ2□> | 2Δ is known only in strontioborite. At the same time, FBB 3Δ3□:[ϕ]<Δ2□> | <Δ2□> | <Δ2□> | representing a part of the FBB in strontioborite (i.e. the cluster of three tetrahedra and three triangles formed by three rings consisting of two tetrahedra and one triangle with three tetrahedra share common vertex) is rather common (Grice et al., 1999). As an isolated cluster it occurs in the structures of mcallisterite Mg₂[B₆O₇(OH)₆]₂⋅9H₂O (dal Negro et al., 1969), aksaite Mg[B₆O₇(OH)₆]⋅2H₂O (dal Negro et al., 1971) and rivadavite Na₆Mg[B₆O₇(OH)₆]₄⋅10H₂O (dal Negro et al., 1973). In polymerised forms this FBB can be detected in chains in aristarainite Na₂Mg[B₆O₈(OH)₄]₂⋅4H₂O (Ghose and Wan, 1977) and in sheets in tunellite Sr[B₆O₉(OH)₂]⋅3H₂O (Clark, 1964; Burns and Hawthorne, 1994) and nobleite Ca[B₆O₉(OH)₂]⋅3H₂O (Karanović et al., 2004). In the structures of ginorite Ca₂B₁₄O₂₀(OH)₆⋅5H₂O (Pankova et al., 2018) and strontioginorite SrCaB₁₄O₂₀(OH)₆⋅5H₂O (Konnert et al., 1970; Grice, 2005), the strontioborite-type FBB is a part of a more complex FBB 8Δ6□:[ϕ]<Δ2□> | <Δ2□> | <Δ2□> | - [ϕ]<Δ2□> | <Δ2□> | <Δ2□> | 2Δ (Grice et al., 1999).

Two borates with Sr as the only species-defining metal cation are known as valid mineral species to date, namely veatchite Sr₂B₁₁O₁₆(OH)₅⋅H₂O (represented by three polytypes: –1M, –2M and –1A, Grice and Pring, 2012) and tunellite SrB₆O₉(OH)₂⋅3H₂O (Erd et al., 1961; Clark, 1964; Burns and Hawthorne, 1994). Veatchite and strontioborite are quite different in terms of crystal structure, whereas tunellite demonstrates similarity with strontioborite in unit-cell parameters [tunellite: a = 14.415(3), b = 8.213(1), c = 9.951(2) Å, β = 114.05(1)°, V = 1075.8 ų and Z = 4, Burns and Hawthorne, 1994] and some structural features (Fig. 4). Heteropolyhedral layers formed by Sr-centred ten-fold polyhedral in tunellite and the hexaborate FBBs 3Δ3□:[ϕ]<Δ2□> | <Δ2□> | <Δ2□> are topologically related to those formed in strontioborite even though Sr in the latter is nine-fold coordinated and FBBs forming the layer are decorated by additional [B₂O₂(OH)₃] pyro-groups. The kinds of linkage of the layers in these minerals are different: in tunellite, Sr-centred polyhedra of adjacent layers share a vertex occupied by an H₂O molecule (Burns and Hawthorne, 1994) whereas in strontioborite Sr-centred polyhedra of one layer share two vertices with B-centred polyhedra of the adjacent layer. The powder XRD patterns of these minerals are markedly different.

Figure 4. Crystal structure of tunellite SrB₆O₉(OH)₂⋅3H₂O projected along the b axis (drawn after Burns and Hawthorne, 1994). H atoms of OH groups and H₂O molecules are shown as blue spheres. The unit cell is outlined.

Correspondence between earlier published data on strontioborite and our results

The results of our studies of strontioborite clearly confirm the correctness of crystallographic and crystal-structure data reported by Kondrat'eva (1964) and Brovkin et al. (1975). The powder XRD data of the mineral published by Lobanova (1960) and Kondrat'eva (1964, 1969) are in agreement with both our measured and calculated powder XRD patterns (Tables 1 and 3). We used optical data of strontioborite reported by Lobanova (1960) for the calculation of the Gladstone–Dale compatibility index (Mandarino, 1981) and obtained the values corresponding to excellent / good rates for D _meas / D _calc, respectively. Thus, the samples studied by Lobanova (1960), Avrova et al. (1968), Kondrat'eva (1964, 1969), Brovkin et al. (1975), and our team (this work) undoubtedly belong to the same mineral species, strontioborite. Its idealised formula is Sr[B₈O₁₁(OH)₄] and, thus, we confirmed the assumptions by Fleischer (1965), Kondrat'eva (1964, 1969) and Brovkin et al. (1975) that the original chemical analysis of strontioborite reported by Lobanova (1960) is wrong, having been carried out on a mixture of minerals. Based on the IR spectra published in the book (Chukanov, 2014) – see above, we assume that the original sample could have been contaminated by halurgite Mg₄[B₈O₁₃(OH)₂]₂⋅7H₂O and/or boracite Mg₃B₇O₁₃Cl, which can be a source of Mg impurity. Another source of Mg could be admixed magnesite (see Fig. 1). It is not excluded that a source of Ca impurity could be ginorite Ca(Ca,Sr)B₁₄O₂₀(OH)₆⋅5H₂O. Halurgite, ginorite and magnesite, all intimately associated with strontioborite, form colourless lamellae visually resembling strontioborite individuals.