*Article* **Krasnoshteinite, Al8[B2O4(OH)2](OH)16Cl4**·**7H2O, a New Microporous Mineral with a Novel Type of Borate Polyanion**

**Igor V. Pekov 1,\*, Natalia V. Zubkova 1, Ilya I. Chaikovskiy 2, Elena P. Chirkova 2, Dmitry I. Belakovskiy 3, Vasiliy O. Yapaskurt 1, Yana V. Bychkova 1, Inna Lykova 4, Sergey N. Britvin 5,6 and Dmitry Yu. Pushcharovsky <sup>1</sup>**


Received: 6 April 2020; Accepted: 12 April 2020; Published: 15 April 2020

**Abstract:** A new mineral, krasnoshteinite (Al8[B2O4(OH)2](OH)16Cl4·7H2O), was found in the Verkhnekamskoe potassium salt deposit, Perm Krai, Western Urals, Russia. It occurs as transparent colourless tabular to lamellar crystals embedded up to 0.06 × 0.25 × 0.3 mm in halite-carnallite rock and is associated with dritsite, dolomite, magnesite, quartz, baryte, kaolinite, potassic feldspar, congolite, members of the goyazite–woodhouseite series, fluorite, hematite, and anatase. *D*meas = 2.11 (1) and *D*calc = 2.115 g/cm3. Krasnoshteinite is optically biaxial (+), α = 1.563 (2), β = 1.565 (2), γ = 1.574 (2), and 2*V*meas = 50 (10)◦. The chemical composition (wt.%; by combination of electron microprobe and ICP-MS; H2O calculated from structure data) is: B2O3 8.15, Al2O3 46.27, SiO2 0.06, Cl 15.48, H2Ocalc. 33.74, –O=Cl –3.50, totalling 100.20. The empirical formula calculated based on O + Cl = 33 *apfu* is (Al7.87Si0.01)Σ7.88[B2.03O4(OH)2][(OH)15.74(H2O)0.26]Σ16[(Cl3.79(OH)0.21]<sup>Σ</sup>4·7H2O. The mineral is monoclinic, *P*21, *a* = 8.73980 (19), *b* = 14.4129 (3), *c* = 11.3060 (3) Å, β = 106.665 (2)◦, *V* = 1364.35 (5) Å3, and *Z* = 2. The crystal structure of krasnoshteinite (solved using single-crystal data, *R*<sup>1</sup> = 0.0557) is unique. It is based upon corrugated layers of Al-centered octahedra connected via common vertices. BO3 triangles and BO2(OH)2 tetrahedra share a common vertex, forming insular [B2O4(OH)2] 4− groups (this is a novel borate polyanion) which are connected with Al-centered octahedra via common vertices to form the aluminoborate pseudo-framework. The structure is microporous, zeolite-like, with a three-dimensional system of wide channels containing Cl- anions and weakly bonded H2O molecules. The mineral is named in honour of the Russian mining engineer and scientist Arkadiy Evgenievich Krasnoshtein (1937–2009). The differences in crystal chemistry and properties between high-temperature and low-temperature natural Al borates are discussed.

**Keywords:** krasnoshteinite; zeolite-like borate; hydrous aluminum chloroborate; new mineral; crystal structure; microporous crystalline material; evaporitic salt rock; Verkhnekamskoe potassium salt deposit; Perm Krai

## **1. Introduction**

Boron is a rare chemical element in nature; its average content in the upper continental crust of the Earth is 0.0011 wt.% [1]. Despite its rarity, boron demonstrates diverse and complicated mineralogy and mineral crystal chemistry. Three hundred minerals with species-defining B are known, including 160 borates and oxoborates [2], and some of these minerals form huge and sometimes extremely rich deposits. Unusual geochemical and mineralogical features of boron are due to its very bright crystal chemical individuality which causes strong ability to separate from other elements in crystal structures and form very specific, unique structural units [3,4]. Unlike boron, aluminum is one of the most abundant elements in the lithosphere, however, natural Al borates are not numerous (only twelve borate and oxoborate minerals with species-defining Al are known: see Discussion) and are classified as rare minerals.

In the present article, we characterize the new mineral species krasnoshteinite (Cyrillic: крaснoштейнит), a hydrous aluminum chloroborate, and its unusual crystal structure. The mineral is named in honour of the Russian mining engineer and scientist, corresponding member of the Russian Academy of Sciences, Arkadiy Evgenievich Krasnoshtein (1937–2009), an outstanding specialist in the mining of potassium salts who made a great contribution to the exploitation of underground mines at the Verkhnekamskoe deposit. Dr. Krasnoshtein was the founder (1988) and first director of the Mining Institute of the Ural Branch of the Russian Academy of Sciences in Perm. Both the new mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association, IMA No. 2018-077.

The type specimen of krasnoshteinite was deposited in the systematic collection of the Fersman Mineralogical Museum of the Russian Academy of Sciences (Moscow, Russia), under the catalogue number 96274.
