A New Mineral Calcioveatchite, SrCaB₁₁O₁₆(OH)₅·H₂O, and the Veatchite–Calcioveatchite Isomorphous Series

Abstract

The new mineral calcioveatchite, ideally SrCaB₁₁O₁₆(OH)₅·H₂O, is a Ca-Sr-ordered analogue of veatchite. It was found at the Nepskoe potassium salt deposit, Irkutsk Oblast, Siberia, Russia in halite-sylvite and sylvite-carnallite rocks, with boracite, hilgardite, kurgantaite, hydroboracite, volkovskite, veatchite, anhydrite, magnesite, and quartz. Calcioveatchite forms prismatic or tabular crystals up to 1 × 1.5 × 3 mm³ and crystal clusters up to 3 mm across. It is transparent and colourless with vitreous lustre. Calcioveatchite is brittle, cleavage is perfect on {010}, the Mohs’ hardness is ca 2, D_meas is 2.58(1), and D_calc is 2.567 g cm⁻³. Calcioveatchite is optically biaxial (+), α = 1.543(2), β = 1.550(5), γ = 1.626(2), 2V_meas = 30(10)°, and 2V_calc = 35°. The average chemical composition (wt.%, electron microprobe, H₂O calculated by stoichiometry) is: CaO 7.05, SrO 20.70, B₂O₃ 61.96, H₂O 10.22, and total 99.93. The empirical formula, calculated based on 22 O apfu = O₁₆(OH)₅(H₂O) pfu, is Sr_1.23Ca_0.78B_10.99O₁₆(OH)₅·H₂O. Calcioveatchite is monoclinic, space group P2₁, a = 6.7030(3), b = 20.6438(9), c = 6.6056(3) Å, β = 119.153(7)°, V = 798.26(8) ų, and Z = 2. Polytype: 1M. The strongest reflections of the powder XRD pattern [d,Å(I,%)(hkl)] are: 10.35(100)(020), 5.633(12)(110), 5.092(10)(120), 3.447(14)(060), 3.362(13)(101, 051), 3.309(38)(–102), 2.862(10)(012), and 2.585(19)(080). The crystal structure was solved based on single-crystal XRD data, R1 = 0.0420. Calcioveatchite (calcioveatchite-1M) is an isostructural analogue of veatchite-1M with the 11-fold cation polyhedron occupied mainly by Sr [Sr_0.902(8)Ca_0.098(8)] whereas the 10-fold polyhedron is Ca dominant [Ca_0.686(7)Sr_0.314(7)]. The chemical composition of veatchite from five localities in Russia (Nepskoe), Kazakhstan (Shoktybay and Chelkar in the North Caspian Region), and the USA (Tick Canyon and Billie Mine in California) was studied, and it is shown to exist in nature as a continuous, almost complete isomorphous series which extends from Ca-free veatchite, Sr₂B₁₁O₁₆(OH)₅·H₂O, to calcioveatchite with the composition Sr_1.14Ca_0.87B_10.99O₁₆(OH)₅·H₂O.

1. Introduction

Veatchite is a rare strontium borate mineral known in several boron deposits and occurrences related to sedimentary rocks of different types [1].

The history of the identification and study of veatchite is complicated. It was described as a new mineral species with the idealised formula Ca₂B₆O₁₁·2H₂O in 1938 from the old Lang boron (colemanite) mine, Tick Canyon, Lang, Los Angeles country, CA, USA. In the holotype specimen, veatchite occurs as white fibrous veinlets (up to 6 mm thick) which cut howlite and limestone [2]. At the type locality, it also occurs in cavities of colemanite aggregates as well-shaped platy crystals [3]. However, the chemical analysis of veatchite reported in the first description of the mineral [2] was wrong: Sr was erroneously determined as Ca that resulted in an incorrect formula. Later veatchite was re-analysed and redefined as a Ca-poor strontium borate with the idealised chemical formula 3SrO·8B₂O₃·5H₂O or, alternately, SrO·3B₂O₃·2H₂O [4]. The formula SrO·3B₂O₃·2H₂O was confirmed by chemical analytical data for samples found in evaporitic rocks at Aislaby, Yorkshire, England, UK [5], whereas the formula Sr₃B₁₆O₂₇·5H₂O was confirmed by new analyses of a sample from the type locality [6]. The monoclinic unit cell with the parameters a = 20.81, b = 11.74, c = 6.63 Å, β = 92°02′, and V = 1620 ų and space group A2/a (#15) or Aa (#9) was reported for veatchite from the type locality [7]. In 1959, a new mineral species dimorphous with veatchite was described from boron-bearing evaporitic rocks at Reyershausen, Bovenden, Lower Saxony, Germany. This mineral, closely related to veatchite and named p-veatchite (“primitive veatchite”), was characterized as monoclinic, with the space group P2₁/m (#11) or P2₁ (#4) and the unit-cell parameters a = 6.72, b = 20.70, c = 6.58 Å, β = 119°40′, and V = 796 ų [8]. In 1960, a comparative study of samples from several localities showed that the recently found specimen from the Four Corners area, Kramer, San Bernardino Co., California, USA, was veatchite, identical to the type material from Tick Canyon, whereas the sample from Aislaby was p-veatchite, an analogue of the type specimen from Reyershausen [9,10]. At the same period, veatchite and Ca-free p-veatchite were described from boron-bearing evaporites of Western Kazakhstan [11,12,13] (unfortunately, in the cited papers no exact locality was reported), and the crystal structure of p-veatchite was first studied on this material [14,15]. The structure of veatchite was independently studied by Clark and Christ [16,17] and Rumanova et al. [18]. Results of all these works gave the same idealised formula for both veatchite and p-veatchite, Sr₂[B₁₁O₁₆(OH)₅]·H₂O; the final data were summarized in three papers published in 1971 [15,17,18]. In 1979, the third natural modification of Sr₂[B₁₁O₁₆(OH)₅]·H₂O was discovered: triclinic, with the space group A1 (#1) or A–1 (#2) and the following unit-cell parameters: a = 20.80, b = 11.72, c = 6.63 Å, α = 90°, β = 90°48′, γ = 91°57′, and V = 796 ų. It was found in association with colemanite and hydroboracite in the Killik and Hizarcik boron (colemanite) mines, Emet, Kütahya, Turkey, and named veatchite-A [19]. At present day, according to the IMA-accepted nomenclature rules for polymorphs and polytypes, these three modifications are considered as polytypes (polytypic varieties) of the mineral species veatchite. These polytypes have been named veatchite-2M (formerly veatchite, first described in [2]), veatchite-1M (formerly p-veatchite), and veatchite-1A (formerly veatchite-A) [20].

Reliable chemical data for all samples of veatchite (veatchites) studied before the 1980s demonstrated a low content of calcium. The Ca-richest veatchite of all samples studied in this period was the sample from Tick Canyon analysed by Kramer: it contained 1.7 wt.% CaO [6]; its empirical formula, being recalculated based on 22 O atoms per formula unit (apfu) = O₁₆(OH)₅·H₂O, was (Sr_1.94Ca_0.20)B_10.91O₁₆(OH)₅·1.18H₂O. Samples containing much more calcium were found by one of the authors of the present paper (V.N.A.) in the mid-1980s at the Nepskoe potassium salt deposit, Siberia, Russia. This mineral was described in 1989 as “Ca-veatchite”, a variety of veatchite containing 5.7–6.3 wt.% CaO [21]; the calculation of wet chemical analytical data published in this article gave the following empirical formula: (Sr_1.41–1.42Ca_0.63–0.70)B_{10.92–10.96}O₁₆(OH)₅·0.93–1.07H₂O. The crystal structure of this mineral from Nepskoe was studied by Rastsvetaeva et al. in 1993, and the ordering of Sr and Ca between two sites was found: the simplified crystal chemical formula of the mineral was presented as Sr(Ca_0.8Sr_0.2)B₁₁O₁₆(OH)₅·H₂O [22]. However, despite the Sr,Ca-ordered structure of this borate, called in the cited paper “p-veatchite with high calcium content”, the proposal to consider it as a new mineral species was never submitted to the IMA Commission on New Minerals, Nomenclature and Classification (CNMNC).

In 2019 we performed an additional study of this mineral from the Nepskoe deposit, including the refinement of its crystal structure, and submitted to the IMA CNMNC the proposal to accept it as a new mineral species with the idealised formula SrCaB₁₁O₁₆(OH)₅·H₂O. It was named calcioveatchite as an analogue of veatchite Sr₂B₁₁O₁₆(OH)₅·H₂O in which one of two independent cation sites is Ca dominant, unlike veatchite, a mineral with both cation sites Sr dominant. Both the mineral and its name have been approved by the IMA CNMNC, IMA No. 2020–011. The type specimen of calcioveatchite is deposited in the systematic collection of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, with the catalogue number 97013. We also studied the chemical composition of other specimens of calcioveatchite and veatchite from Nepskoe and veatchite from four other localities (situated in Western Kazakhstan and California) and found that veatchite and calcioveatchite form a continuous, almost complete solid solution. In this paper, we describe calcioveatchite as a new mineral species and for the first time characterise, based on the original data, a novel veatchite–calcioveatchite isomorphous series, with accentuation on chemical variation in this series.

2. Materials

2.1. Calcioveatchite and Veatchite from the Nepskoe Deposit

The type material of calcioveatchite originates from drillcore of a prospecting borehole no. 130 (depth 911 m) drilled at the Nepskoe potassium salt deposit, Irkutsk Oblast, Siberia, Russia [23]. Other specimens of calcioveatchite and veatchite from Nepskoe studied in this work were found in drillcores of boreholes nos. 130 (depths 880–900 m), 85 (depths 808–818 m), 124 (depth 881 m), and 67 (depth 854 m). These and other borate minerals were extracted from the insoluble rest after dissolution of a host saline rock in water. Calcioveatchite is a common mineral there.

Calcioveatchite occurs in a massive, granular halite-sylvite rock (sylvinite) or in a sylvite-carnallite rock and is also associated with boracite, hilgardite (hilgardite-1A), kurgantaite, hydroboracite, volkovskite, veatchite, anhydrite, magnesite, and quartz. Calcioveatchite forms separate prismatic or tabular crystals up to 1 × 1.5 × 3 mm³ and crystal clusters up to 3 mm across (Figure 1, Figure 2, Figure 3 and Figure 4). Sometimes it is intimately associated with boracite and/or anhydrite (Figure 3). The major forms of monoclinic (symmetry class 2) crystals of calcioveatchite are monohedra {010} and {0-10} and dihedra {110}, {120}, {041}, {031}, and {-111} (Figure 3); subordinate or rare forms are {100}, {1-10}, {1-20}, {130}, {140}, {021}, {011}, and {1-11}: see [21]. Simple twins with contact face {0-10} were observed.

Calcioveatchite is transparent and colourless, with white streaks and vitreous lustre. It is non-fluorescent under both ultraviolet rays and an electron beam. Calcioveatchite is brittle. Cleavage is perfect on {010}; the fracture is stepped. The Mohs’ hardness is ca 2. The density measured by flotation in heavy liquids (bromoform + ethanol) is 2.58(1) g cm⁻³. The density calculated using the empirical formula and the unit-cell volume refined from single crystal XRD data is 2.567 g cm⁻³.

Veatchite in specimens from the Nepskoe deposit is indistinguishable from calcioveatchite in mode of occurrence and general appearance, visually. Our data show that veatchite is a much rarer mineral at Nepskoe than calcioveatchite.

2.2. Veatchite from Other Localiies

Specimens of veatchite from four other localities were also studied using electron microprobe analysis.

Shoktybay boron occurrence, North Caspian Region, Western Kazakhstan. Veatchite occurs as well-shaped or crude tabular oblique-angled colourless transparent crystals up to 0.5 × 2.5 × 4 mm³ associated with axinite-(Mn). These minerals form the insoluble rest after dissolution in water of a host evaporitic rock mainly consisting of halite and sylvite.

Chelkar salt dome, North Caspian Region, Western Kazakhstan. Veatchite forms tabular colourless transparent grains up to 1 mm across associated with boracite and kurgantaite. They were found in the insoluble rest after dissolution in water of a rock which consisted of halite with clay admixture.

Tick Canyon, Lang, Los Angeles country, CA, USA (type locality of veatchite). Veatchite occurs as white lamellae up to 2 mm across in cavities of colemanite aggregate.

Billie Mine, Ryan, Inyo Co., CA, USA. Veatchite forms white divergent lamellae up to 1 mm across overgrowing colemanite crystals.

All samples studied in this work are from collections of the authors (I.V.P. and V.N.A.).

3. Methods

The IR spectra of the holotype calcioveatchite from Nepskoe and veatchite from Shoktybay were obtained in the Federal Research Center of Problems of Chemical Physics and Medicinal Chemistry, Russian Academy of Sciences, Chernogolovka, Russia. In order to obtain IR absorption spectra, powdered samples were mixed with anhydrous KBr (in the KBr-to-mineral ratio of about 150:1), pelletized and analysed using an ALPHA FTIR spectrometer (Bruker Optics, Ettlingen, Germany) with a resolution of 4 cm⁻¹. A total of 16 scans were collected for each spectrum. The IR spectrum of an analogous pellet of pure KBr was used as a reference.

The chemical composition of all studied samples was studied by electron microprobe analysis (EMPA) at the Laboratory of Analytical Techniques of High Spatial Resolution, Faculty of Geology, Moscow State University, using a JEOL JSM-6480LV scanning electron microscope equipped with an INCA-Wave 500 wavelength-dispersive spectrometer. The analytical conditions were as follows: an acceleration voltage of 20 kV and a beam current of 10 nA; the electron beam was rastered to the 5 × 5 μm² area. The following standards were used: Ca—diopside, Sr—SrSO₄, and B—BN. In the type material of calcioveatchite, H₂O content was measured by the Penfield method (for the “gross” sample consisting of several crystals previously checked by electron microprobe to evaluate the Sr–Ca ratio). The empirical formulae were calculated on the basis of 22 O apfu = O₁₆(OH)₅(H₂O) pfu.

Powder XRD data were collected at the X-Ray Diffraction Resource Center of St. Petersburg State University using a Rigaku R-AXIS Rapid II diffractometer with curved image plate detector, rotating anode with VariMAX microfocus optics, using CoKα radiation, in Debye–Scherrer geometry, at accelerating voltage of 40 kV, current of 15 mA, and exposure time 12 min for each sample. The distance between sample and detector was 127.4 mm. The data were processed using osc2xrd software (Department of Crystallography, St. Petersburg State University, St. Petersburg, Russia) [24].

Single-crystal XRD studies were carried out at Faculty of Geology, Moscow State University, Moscow, Russia, using an Xcalibur S diffractometer equipped with CCD detector (MoKα radiation); for details see Section 4.4.

4. Results

4.1. Optical Data

Calcioveatchite (the holotype specimen) is optically biaxial (+), α = 1.543(2), β = 1.550(5), γ = 1.626(2) (589 nm), 2V_meas = 30(10)°, and 2V_calc = 35°. Dispersion of optical axes: r < v, medium. Orientation: Y = b, Z ^ c ≈ 30°. Elongation is positive. In plane-polarized light the mineral is colourless and non-pleochroic.

4.2. Infrared Spectroscopy

The IR spectrum of calcioveatchite is rather close to that of veatchite (Figure 5). Bands in the range 3100–3500 cm⁻¹ correspond to O–H stretching vibrations. The doublet in the region 1640–1670 cm⁻¹ corresponds to the nondegenerate mode of H–O–H bending vibrations and consequently indicates the presence of two nonequivalent H₂O molecules. Two groups of strong bands observed in the ranges 1200–1500 and 900–1150 cm⁻¹ are due to ^[3]B–O and ^[4]B–O stretching vibrations, respectively. The bands at 826 and 860 cm⁻¹ correspond to O–B–H bending modes, and the bands in the range 600–800 cm⁻¹ are mainly due to O–B–O bending vibrations.

The weak absorptions in the ranges 1150–1200 and 2270–2480 cm⁻¹ correspond to overtones and combination modes. The relatively weak bands observed below 600 cm⁻¹ may correspond to mixed lattice modes, in particular those involving translational and librational vibrations of the isolated groups [B(OH)₃ and H₂O] as a whole.

The positions of the low-frequency ^[3]B–O stretching bands (at 1245 and 1274 cm⁻¹ for the calcioveatchite and at 1240 and 1266 cm⁻¹ for the veatchite) correspond to vibrations involving the longest ^[3]B–O bonds (i.e., ^[3]B2–O5 and ^[3]B5–O5, see the description of the crystal structure below). The positions of the low-frequency ^[4]B–O stretching bands (at 937 cm⁻¹ for the calcioveatchite and at 934 cm⁻¹ for the veatchite) may correspond to vibrations involving an elongate ^[4]B–O bond where O is an atom coordinating Ca in the calcioveatchite (i.e., the bond ^[4]B1–O9, ^[4]B4–O11, or ^[4]B9–O6). Thus the substitution of Sr²⁺ with the lighter cation Ca²⁺ among cations coordinating O5, O6, O9, and O11 results in shifts of these bands towards higher frequencies. These shifts are small because the force characteristics of the Ca–O and Sr–O bonds are much lower than those of the B–O bonds.

According to the equation ν (cm⁻¹) = 3592 – 304·10⁹·exp[–d(O···O)/0.1321] for hydrogen bonds [25], the bands of O–H stretching vibrations at 3441, 3345, 3281, and 3235 cm⁻¹ correspond to the O···O distances of 2.83, 2.76, 2.73, and 2.72 Å which corresponds well to the D···A distances 2.800, 2.789, 2.738, and 2.711 determined as a result of the crystal structure refinement (see below).

The broad and asymmetric band with the maximum at 3133 cm⁻¹ (which corresponds to d(O···O) = 2.68 Å) may correspond to the D···A values of 2.636 and 2.647 Å. Thus the IR spectrum indicates some weakening of corresponding hydrogen bonds as compared to the H-bond strengths expected for the short distances of 2.636 and 2.647 Å. Obviously, this fact is due to the low value of the O14 - H14 ··· O1 angle (134°).

4.3. Chemical Composition

Chemical data for all samples studied in this work are given in

Table 1. Chemical composition of minerals of the calcioveatchite (1–9)–veatchite (10–18) isomorphous series.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
wt.%
CaO	7.05 [6.31–8.02]	9.26	8.10	7.29	6.57	6.42	5.51	5.36	5.11	4.22	3.70	2.86	2.34	1.42	1.14	0.23	0.11	0
SrO	20.70 [19.37–21.83]	17.11	19.56	20.06	21.83	22.67	22.75	23.57	24.56	22.53	27.24	27.38	29.44	31.61	29.01	31.48	35.17	31.73
B2O3	61.96 [61.28–62.52]	63.22	63.14	61.93	62.10	60.70	61.90	60.76	59.46	59.94	60.03	59.90	58.31	57.49	60.99	59.26	56.44	58.62
Total	99.93 *	100 *	90.80	89.28	90.50	89.79	90.16	89.69	89.13	89.69	90.97	90.14	90.09	90.52	91.14	90.97	91.72	100 *
formulae calculated based on 22 O atoms per formula unit (apfu) **
Ca	0.78	1	0.87	0.80	0.72	0.68	0.61	0.60	0.56	0.48	0.38	0.33	0.25	0.16	0.14	0.03	0.01	0
Sr	1.23	1	1.14	1.20	1.30	1.31	1.36	1.43	1.46	1.57	1.64	1.68	1.75	1.87	1.83	1.96	2.00	2
B	10.99	11	10.99	11.00	10.99	11.01	11.03	10.97	10.98	10.95	10.98	10.99	11.00	10.97	11.03	11.01	10.99	11

Note: 1, 3–9, 12, 14—Nepskoe deposit, Siberia, Russia (1—calcioveatchite holotype, averaged for four spot analyses, ranges are in brackets); 10, 11, 13—Shoktybay, North Caspian Region, Western Kazakhstan; 15—Chelkar, North Caspian Region, Western Kazakhstan; 16—Tick Canyon, Lang, Los Angeles country, CA, USA; 17—Billie Mine, Ryan, Inyo Co., California, USA; 2—calculated for the ideal formula SrCaB₁₁O₁₆(OH)₅·H₂O; 18—calculated for the ideal formula Sr₂B₁₁O₁₆(OH)₅·H₂O. Analyses 2–18 are ordered by Ca content decrease. * Total also includes H₂O_calc value calculated for (OH)₅(H₂O) = 7 H apfu: 1–10.22, 2–10.41, 18–9.65 wt.%. ** 22 O apfu = O₁₆(OH)₅(H₂O) pfu.

. Contents of other elements with atomic numbers >6 are below detection limits. For the holotype specimen (no. 1 in Table 1), we gave water content calculated based on the structure data (see below) because it was not possible determine H₂O content directly for this small single crystal. It seems correct because these values are very close: the calculated H₂O content is 10.22 wt.% while the average H₂O content measured using the Penfield method for the “gross” sample was 10.07 wt.%, as averaged from the results of three analyses: 9.85, 10.12, and 10.25 wt.%. For other samples, water content was not measured due to the scarcity of pure material.

The empirical formula of the holotype calcioveatchite calculated using H₂O_calc. for the structurally studied crystal (10.22 wt.%) is Sr_1.23Ca_0.78B_10.99O₁₆(OH)₅·H₂O, whereas the empirical formula calculated using H₂O_meas. for the “gross” sample (10.07 wt.%) on the basis of 21 O apfu = O₁₆(OH)₅ pfu (i.e., by stoichiometry of the anionic part) is Sr_1.23Ca_0.78B_10.99O₁₆(OH)₅·0.94H₂O. The simplified formula of calcioveatchite, taking into account the structure data, is Sr(Ca,Sr)B₁₁O₁₆(OH)₅·H₂O. The ideal formula is SrCaB₁₁O₁₆(OH)₅·H₂O.

The values of the Gladstone–Dale compatibility index [26] are –0.043 if Dmeas. is used and −0.048 if Dcalc. is used; they rated as ‘good’ in both cases.

As our EMPA data show (Table 1, Figure 6), the Ca–Sr ratio in the studied samples demonstrated wide variation: CaO content varied from 0.1 to 8.1 wt.% and SrO from 35.2 to 17.1 wt.%; that corresponds to a range from Sr_2.00Ca_0.01B_10.99O₁₆(OH)₅·H₂O to Sr_1.14Ca_0.87B_10.99O₁₆(OH)₅·H₂O.

4.4. X-ray Crystallography and Crystal Structure of Calcioveatchite

Powder XRD data of calcioveatchite are reported in

Table 2. Powder X-ray diffraction data (d in Å) of calcioveatchite.

Iobs	dobs	Icalc *	dcalc **	h k l
100	10.35	100	10.322	020
12	5.633	8	5.632	110
3	5.503	2	5.527	-111
10	5.092	5	5.092	120
4	4.481	1, 2	4.458, 4.421	130, 031
14	3.447	17	3.441	060
13	3.362	6, 3	3.369, 3.357	101, 051
38	3.309	4, 3, 33	3.325, 3.307, 3.303	111, -211, -102
6	3.215	3	3.203	121
3	3.181	4	3.187	-221
3	2.997	3	3.012	-231
2	2.971	2	2.966	160
3	2.923	2	2.926	200
2	2.903	1	2.898	210
10	2.862	7	2.857	012
7	2.839	9	2.841	-212
3	2.751	2	2.763	-222
3	2.695	2	2.693	230
2	2.631	2	2.626	071
19	2.585	23	2.580	080
3	2.548	1	2.546	240
9	2.390	1, 7	2.400, 2.388	-261, 250
2	2.364	1, 1	2.365, 2.361	052, 180
5	2.197	6	2.199	211
5	2.152	6	2.155	-113
6	2.070	1, 2, 5	2.077, 2.066, 2.064	270, -133, 0.10.0
6	2.031	3, 10	2.033, 2.033	-182, 241
2	2.002	4	1.998	-143
3	1.943	3	1.940	152
5	1.918	1, 2, 1	1.916, 1.918, 1.915	-282, -153, -253
2	1.863	4	1.860	261
3	1.776	1	1.769	271
1	1.761	1	1.760	1.10.1
2	1.653	2	1.651	-204
1	1.591	1	1.592	291
1	1.583	1	1.582	-314
1	1.506	1	1.507	-451
1	1.425	1	1.423	-3.11.2
1	1.398	1	1.396	2.13.0

Note: the strongest reflections are marked in bold type; * for the calculated pattern, only reflections with intensities ≥1 are given; ** for the unit-cell parameters obtained from single-crystal data

. Parameters of the monoclinic unit cell calculated from the powder data are a = 6.695(6), b = 20.689(7), c = 6.609(6) Å, β = 119.12(6)º, and V = 800(1) ų.

Single-crystal X-ray diffraction studies of calcioveatchite were carried out on a crystal of 0.10 × 0.41 × 0.60 mm³ in size. A full sphere of three-dimensional data was collected. Data reduction was performed using CrysAlisPro version 1.171.39.46 [27]. The data were corrected for Lorentz factor and polarization effect. The crystal structure of calcioveatchite was solved by direct methods and refined using the SHELX software package [28] to R = 0.0420 for 3727 unique reflections with I > 2σ(I). The H atoms of the OH groups and the H₂O molecule were located in a difference Fourier map and refined with a distance restraint O–H = 0.85(1) Å [additionally, H–H = 1.37(1) Å for a water molecule to hold near-optimal geometry] and with U_iso(H) = 1.5U_eq(O). The crystal data and the experimental details are presented in

Table 3. Crystal data, data collection information and structure refinement details for calcioveatchite.

Formula	(Sr0.90Ca0.10)(Ca0.69Sr0.31)B11O16(OH)5·H2O
Formula weight	615.89
Temperature, K	293(2)
Radiation and wavelength, Å	MoKα; 0.71073
Crystal system, space group, Z	Monoclinic, P21, 2
Unit-cell parameters, Å/°	a = 6.7030(3) b = 20.6438(9) β = 119.153(7) c = 6.6056(3)
V, Å3	798.26(8)
Absorption coefficient μ, mm-1	4.470
F000	600
Crystal size, mm	0.10 × 0.41 × 0.60
Diffractometer	Xcalibur S CCD
Absorption correction	Gaussian
θ range, °	3.480–28.280
Index ranges	−8 ≤ h ≤ 8, −27 ≤ k ≤ 27, −8 ≤ l ≤ 8
Reflections collected	13,834
Unique reflections	3947 (Rint = 0.0566)
Unique reflections [I > 2σ(I)]	3727
Refinement method	Full-matrix least-squares on F2
Number of refined parameters	334
Final R indices [I > 2σ(I)]	R1 = 0.0420, wR2* = 0.0890
R indices (all data)	R1 = 0.0462, wR2* = 0.0912
GoF	1.040
Largest diff. peak/hole, e/Å3	0.82 and −0.56

Note: *w = 1/[σ²(F_o²) + (0.0489P)² + 0.0101P]; P = {[max of (0 or F_o²)] + 2F_c²}/3.

; atom coordinates, thermal displacement parameters, and site occupancies in

Table 4. Atom coordinates and equivalent displacement parameters (U_eq, in Ų) and site occupancy factors (s.o.f.) for calcioveatchite.

Site	x	y	z	Ueq	s.o.f.
Sr	0.38068(10)	0.73563(3)	0.19425(11)	0.01048(18)	Sr0.902(8)Ca0.098
Ca	0.92530(17)	0.34818(5)	0.47827(18)	0.0116(3)	Ca0.686(7)Sr0.314
B1	0.3913(13)	0.8851(5)	0.1961(14)	0.0120(18) *	B1.00
B2	0.3381(12)	0.3625(4)	0.3774(12)	0.0113(15)	B1.00
B3	0.1879(13)	0.9880(4)	0.1075(16)	0.0170(17)	B1.00
B4	0.9719(12)	0.8858(4)	0.0165(13)	0.0121(15)	B1.00
B5	0.4243(13)	0.3682(4)	0.0629(13)	0.0125(15)	B1.00
B6	0.5209(12)	0.2046(4)	0.2988(13)	0.0114(15)	B1.00
B7	0.7903(11)	0.7300(4)	0.0937(12)	0.0112(14)	B1.00
B8	0.1243(12)	0.2236(3)	0.2206(12)	0.0118(16)	B1.00
B9	0.0576(12)	0.7080(4)	0.5189(12)	0.0093(16)	B1.00
B10	0.4168(15)	0.5814(5)	0.1823(16)	0.0191(19)	B1.00
B11	0.7540(14)	0.1039(4)	0.4017(15)	0.0155(16)	B1.00
O1	0.0476(8)	0.6379(3)	0.4991(9)	0.0154(12)	O1.00
O2	0.7324(6)	0.2351(3)	0.4632(7)	0.0129(9)	O1.00
O3	0.8584(7)	0.7389(3)	0.3233(7)	0.0122(9)	O1.00
O4	0.5959(7)	0.8573(3)	0.3921(8)	0.0140(11)	O1.00
O5	0.4981(7)	0.3683(2)	0.3019(8)	0.0126(11)	O1.00
O6	0.9487(7)	0.2292(3)	0.2682(7)	0.0148(10)	O1.00
O7	0.4362(7)	0.2291(3)	0.0615(7)	0.0131(10)	O1.00
O8	0.3910(9)	0.9555(3)	0.2088(10)	0.0167(13)	O1.00
O9	0.6018(7)	0.3630(2)	0.0168(8)	0.0133(11)	O1.00
O10	0.3485(7)	0.2198(2)	0.3741(7)	0.0128(11)	O1.00
O11	0.8000(7)	0.8699(2)	0.0931(8)	0.0135(10)	O1.00
O12	0.9482(7)	0.7239(3)	0.0185(8)	0.0126(11)	O1.00
O13	0.1124(7)	0.3599(2)	0.2232(8)	0.0135(10)	O1.00
O14	0.5993(8)	0.6213(3)	0.2671(11)	0.0250(13)	O1.00
H14	0.727(8)	0.607(5)	0.372(12)	0.037 *	H1.00
O15	0.1853(7)	0.8564(2)	0.1844(8)	0.0097(10)	O1.00
O16	0.1753(9)	0.0540(3)	0.1018(13)	0.0341(15)	O1.00
H16	0.301(9)	0.074(4)	0.165(16)	0.051 *	H1.00
O17	0.9847(9)	0.9561(3)	0.0050(9)	0.0177(12)	O1.00
O18	0.2207(9)	0.6148(3)	0.0953(12)	0.0302(15)	O1.00
H18	0.092(8)	0.596(4)	0.043(17)	0.045 *	H1.00
O19	0.7687(8)	0.0377(3)	0.4080(11)	0.0304(14)	O1.00
H19	0.657(11)	0.012(4)	0.363(16)	0.046 *	H1.00
O20	0.8675(10)	0.4601(3)	0.4460(12)	0.0340(15)	O1.00
H2OA	0.960(11)	0.492(3)	0.48(2)	0.051 *	H1.00
H2OB	0.733(6)	0.475(4)	0.366(16)	0.051 *	H1.00
O21	0.5436(8)	0.1335(3)	0.2970(8)	0.0153(11)	O1.00
O22	0.4334(10)	0.5155(3)	0.1902(13)	0.0340(16)	O1.00
H22	0.302(8)	0.498(5)	0.132(17)	0.051 *	H1.00

Note: * U_iso.

; and selected interatomic distances in

Table 5. Selected interatomic distances (Å) in the structure of calcioveatchite.

Sr - O7                    2.530(4)      - O10                 2.548(4)      - O12                 2.556(4)      - O18                 2.668(6)      - O14                 2.695(6)      - O2                    2.714(4)      - O6                    2.766(4)      - O15                 2.802(5)      - O4                    2.874(5)      - O3                    2.885(4)      - O9                    3.005(5)   <Sr - O>              2.731 B1 - O8                     1.455(11)    - O4                     1.468(9)    - O15                   1.469(9)    - O9                     1.501(9)    <B1 - O>                  1.47 B2 - O13                    1.352(8)      - O4                      1.367(8)      - O5                      1.391(8)     <B2 - O>                1.370 B3 - O17                    1.360(10)      - O16                    1.363(10)      - O8                      1.366(10)   <B3 - O>                    1.36 B4 - O15                    1.449(9)        - O17                    1.459(10)      - O13                    1.497(9)      - O11                    1.503(8)    <B4 - O>                    1.48 B5 - O11                    1.347(9)      - O9                      1.368(8)      - O5                      1.406(9)     <B5 - O>                1.374

Ca - O20              2.335(6)       - O5                2.545(4)        - O13              2.555(4)        - O11              2.555(5)       - O2                2.648(6)       - O3                2.656(6)      - O15              2.675(4)      - O9                2.765(5)      - O6                2.863(6)      - O4                2.897(4)    <Ca - O>            2.649 B6 - O2               1.444(9)      - O7               1.472(9)      - O21             1.476(9)      - O10             1.497(8)      <B6 - O>            1.472 B7 - O7                1.356(8)      - O3                 1.367(8)         - O12              1.376(7)    <B7 - O>           1.366 B8 - O10               1.345(8)     - O6                 1.363(8)       - O12              1.408(8)       <B8 - O>             1.372 B9 - O1               1.453(10)   - O2                1.465(8)   - O3                1.476(8)   - O6                1.493(8)   <B9 - O>           1.47 B10 - O18                1.341(10)        - O14                1.349(11)        - O22                1.364(11)      <B10 - O>             1.35 B11 - O1                 1.357(10)         - O19               1.369(10)       - O21               1.376(9)       <B11 - O>               1.37

. H-bonding is presented in

Table 6. Hydrogen-bond geometry (Å,°) in the structure of calcioveatchite.

D – H ··· A	D – H	H ··· A	D ··· A	∠(D – H ··· A)
O14 - H14 ··· O1	0.849(15)	1.99(7)	2.647(7)	134(9)
O16 - H16 ··· O21	0.850(15)	1.87(3)	2.711(7)	169(10)
O18 - H18 ··· O16	0.847(15)	1.79(2)	2.636(8)	172(10)
O19 - H19 ··· O8	0.852(15)	1.94(2)	2.789(8)	171(9)
O20 - H20A ··· O19 O20 - H20B ··· O22	0.848(15) 0.847(15)	1.86(3) 1.95(2)	2.674(8) 2.800(8)	161(8) 175(9)
O22 - H22 ··· O17	0.852(15)	1.887(18)	2.738(8)	177(11)

, and bond-valence calculations are given in

Table 7. Bond-valence calculations for calcioveatchite.

Site	Sr	Ca	B1	B2	B3	B4	B5	B6	B7	B8	B9	B10	B11	Σ	H-bonding	Σ
O1											0.80		1.04	1.84	+0.26(O14)	2.10
O2	0.20	0.18						0.82			0.77			1.97		1.97
O3	0.14	0.18							1.01		0.75			2.08		2.08
O4	0.14	0.10	0.76	1.01										2.01		2.01
O5		0.23		0.95			0.91							2.09		2.09
O6	0.18	0.12								1.03	0.71			2.04		2.04
O7	0.29							0.76	1.05					2.10		2.10
O8			0.79		1.02									1.81	+0.19(O19)	2.00
O9	0.11	0.14	0.70				1.01							1.96		1.96
O10	0.28							0.70		1.08				2.06		2.06
O11		0.23				0.69	1.07							1.99		1.99
O12	0.28								0.99	0.90				2.17		2.17
O13		0.23		1.06		0.70								1.99		1.99
O14 = OH	0.20											1.07		1.27	−0.26(O1)	1.01
O15	0.16	0.17	0.76			0.81								1.90		1.90
O16 = OH					1.03									1.03	−0.22(O21) +0.26(O18)	1.07
O17					1.03	0.78								1.81	+0.21(O22)	2.02
O18 = OH	0.22											1.09		1.31	−0.26(O16)	1.05
O19 = OH													1.01	1.01	−0.19(O8) +0.24(O20)	1.06
O20 = H2O		0.38												0.38	−0.20(O19) −0.18(O22)	0.00
O21								0.75					0.99	1.74	+0.22(O16)	1.96
O22 = OH												1.02		1.02	+0.18(O20) −0.21(O17)	0.99
Σ	2.20	1.96	3.01	3.02	3.08	2.98	2.99	3.03	3.05	3.01	3.03	3.18	3.04

Note: the values were calculated taking into account the assigned occupancy of the sites. Parameters were taken from [29] and for H-bonding from [30].

.

5. Discussion

5.1. Crystal Structure and Crystal Chemical Features

As mentioned above, three polytypes of veatchite Sr₂B₁₁O₁₆(OH)₅·H₂O are known, namely veatchite-1M, veatchite-2M, and veatchite-1A (

Table 8. Crystal data of calcioveatchite and three known polytypes of veatchite.

Mineral	Calcioveatchite	Veatchite
Ideal Formula	SrCaB11O16(OH)5·H2O	Sr2B11O16(OH)5·H2O
Polytype	1M	1M	2M	1A
Crystal system Space group	Monoclinic P21	Monoclinic P21	Monoclinic Cc	Triclinic P–1
a, Å b, Å c, Å α, ° β, ° γ, ° V, Å3 Z	6.7030 (3) 20.6438 (9) 6.6056 (3) 90 119.153 (7) 90 798.26 (8) 2	6.7127 (4) 20.704 (1) 6.6276 (4) 90 119.209 (1) 90 805.4 (2) 2	6.6070 (3) 11.7125 (5) 20.6848 (9) 90 91.998 (1) 90 1599.7 (2) 4	6.6378 (6) 6.7387 (6) 20.982 (2) 87.860 (1) 82.696 (1) 60.476 (1) 809.7 (2) 2
Source	This work	[20]

). The crystal structure of any of them is based upon three-layer sheets with the central layer formed by Sr-centred eleven-fold and ten-fold polyhedra sandwiched between two borate layers [20]. According to the structural classification of borates [31], veatchite is considered as a sheet borate, and two types of fundamental building blocks (FBBs) are distinguished in its structure [20]. FBB I can be presented using the code 3∆2⎕,1∆:<∆2⎕>-<2∆⎕>,∆ which means two three-membered rings, one consisting of one B-O triangle and two B-O tetrahedra and another consisting of two triangles and one tetrahedron; an isolated triangle [B(OH)₃] is also a part of FBB I. Closely related FBB II corresponds to the code 3∆2⎕:<∆2⎕>-<2∆⎕> (Figure 7a) which means the same construction of two three-membered rings but without an isolated triangle [B(OH)₃]. Triclinic and monoclinic polytypes are characterized by different sequences of FBBs which result in centrosymmetric (veatchite-1A) or non-centrosymmetric (veatchite-1M and -2M) structures. Both monoclinic polytypes have an identical sequence of FBBs but differ from one another by a slight shift between layers [20]. The sequence of FBBs I and II in the structure of veatchite-1M is shown in Figure 7b. The modular approach to veatchite polytypes and structurally related natural and synthetic borates is reported in detail in [32].

Veatchite-1M = p-veatchite [8,9,14,15,17,20] and calcioveatchite SrCaB₁₁O₁₆(OH)₅·H₂O are isostructural. The difference between these minerals is only in the content of cation sites. In veatchite-1M both sites are occupied only or mainly by Sr. In calcioveatchite (= calcioveatchite-1M), the site which centres an eleven-fold polyhedron is Sr-dominant [Sr_0.902(8)Ca_0.098(8)] whereas the site in a smaller ten-fold polyhedron is Ca-dominant [Ca_0.686(7)Sr_0.314(7)] (Table 4 and Table 5). The substitution of a significant part of Sr for Ca causes some decrease of unit-cell parameters of calcioveatchite in comparison with veatchite-1M (Table 8). The crystal structure of calcioveatchite is depicted in Figure 8. Our results are in agreement with the data for the “p-veatchite with high calcium content” from Nepskoe reported in [22].

The ordering of Sr and Ca between two independent cation sites different in volume is also known for two other natural borates formed, like calcioveatchite, in evaporitic rocks. Along with the pair veatchite Sr₂B₁₁O₁₆(OH)₅·H₂O–calcioveatchite SrCaB₁₁O₁₆(OH)₅·H₂O, the pairs ginorite Ca₂B₁₄O₂₀(OH)₆·5H₂O–strontioginorite SrCaB₁₄O₂₀(OH)₆·5H₂O [33,34] and hilgardite Ca₂B₅O₉Cl·H₂O–kurgantaite (formerly “strontiohilgardite”) SrCaB₅O₉Cl·H₂O [35,36] demonstrate this crystal chemical feature.

5.2. The Veatchite–Calcioveatchite Isomorphous Series

Table 1 contains a representative selection of the EMP analyses of the specimens studied in this work while in Figure 6 all our point analyses of minerals of the veatchite–calcioveatchite isomorphous series are given. They demonstrate that this series in nature is continuous and almost complete: from Ca-free [11] or almost Ca-free ([19]; nos. 16 and 17 in Table 1) to the composition Sr_1.14Ca_0.87B_10.99O₁₆(OH)₅·H₂O (no. 3 in Table 1). The formal border between the mineral species veatchite and calcioveatchite corresponds to the composition Sr_1.5Ca_0.5B₁₁O₁₆(OH)₅·H₂O.

We detected calcioveatchite only in the samples from Nepskoe (Table 1; Figure 6); however, some earlier published analyses of veatchite from other localities suggest that this mineral is not endemic of this deposit. The averaged for ten spot EMP analyses chemical composition of the veatchite from boron-bearing evaporitic (essentially halite) rocks of the Penobsquis potassium salt deposit, Kings Co., New Brunswick, Canada, corresponds to Sr_1.51Ca_0.48Fe_0.01B₁₁O₁₆(OH)₅·H₂O (4.34 wt.% CaO and 25.21 wt.% SrO) [37], i.e., it is a Ca-rich variety of veatchite chemically very close to the formal border with the calcioveatchite compositional field. However, the range given in the cited paper for CaO is 1.30–6.63 and for SrO is 21.25–30.45 wt.%; therefore, we believe that at least zones chemically corresponding to calcioveatchite occur in the veatchite crystals from Penobsquis. In the book [38], the following wet chemical analysis of ‘veatchite’ from an unspecified locality in Western Kazakhstan is reported (wt.%): CaO 9.59, SrO 21.69, B₂O₃ 59.19, H₂O 10.50, and total 100.97. Its calculation gives the empirical formula Sr_1.30Ca_1.06B_10.56O₁₆(OH)₅·1.12H₂O which demonstrates significant deviation from the stoichiometric ratio (Sr + Ca):B = 2:11. Probably this analysis is of low quality (the sample was polluted by another Ca borate?), and we cannot take it into consideration.

All reliable localities of calcioveatchite and the Ca-rich (>2 wt.% CaO) variety of veatchite are related to evaporites: there are Nepskoe, Shoktybay (nos. 1, 3–13 in Table 1) and Penobsquis [37]. At the same time, a Ca-poor veatchite also occurs in deposits of this genetic type: there are Nepskoe, Chelkar (nos. 14–15 in Table 1), and an unspecified locality in Western Kazakhstan [11]. Thus, a complete solid-solution series between veatchite and calcioveatchite can be formed in evaporites. On the contrary, in boron deposits in other sedimentary rocks (California and Turkey: [4,6,19]; nos. 16–17 in Table 1) only Ca-poor veatchite is known to date.

Veatchite and, especially, calcioveatchite are common accessory minerals in some layers of sylvinites and sylvite-carnallite rocks at the Nepskoe deposit. Borates with veatchite-type structure have strong affinity to the Sr²⁺ cation. Due to this, calcioveatchite turned out to be one of the major, together with borates of the hilgardite-kurgantaite series, concentrators of Sr in the evaporitic rock of this huge salt deposit.

6. Conclusions and Implications

The new mineral calcioveatchite, ideally SrCaB₁₁O₁₆(OH)₅·H₂O, was studied in detail, involving electron microprobe analysis, single-crystal and powder X-ray diffraction, and infrared spectroscopy and optical methods. Its crystal structure, determined on a single crystal, showed significant ordering of Sr and Ca between two independent cation sites which centre eleven-fold and ten-fold polyhedra, respectively. The chemical compositions of veatchite and calcioveatchite from five localities in Russia (Nepskoe deposit in Siberia), Kazakhstan (Shoktybay and Chelkar in the North Caspian Region), and USA (Tick Canyon and Billie Mine in California) were studied, and it is shown that these minerals form a continuous, almost complete isomorphous series which extends from Sr₂B₁₁O₁₆(OH)₅·H₂O to, at least, Sr_1.14Ca_0.87B_10.99O₁₆(OH)₅·H₂O.

As it is demonstrated in the example of the huge Nepskoe potassium salt deposit, borates of the veatchite–calcioveatchite series can be geochemically significant minerals, important concentrators of Sr in the evaporitic formation.