**5. Conclusions**

This paper is devoted to the new mineral species, krasnoshteinite. No mineral or synthetic compound related to it, in terms of crystal structure, has been found in literature and databases. Krasnoshteinite contains an earlier unknown borate polyanion, the insular [B2O4(OH)2] <sup>4</sup><sup>−</sup> group consisting of one BO3 triangle and one BO2(OH)2 tetrahedron sharing corner. It was a surprise that such a simple anionic complex turned out novel for borates, both natural and synthetic, which is one of the most structurally diverse and best-studied classes of chemical compounds.

Krasnoshteinite (Al8[B2O4(OH)2](OH)16Cl4·7H2O) is the second, after jeremejevite (Al6(BO3)5 (F,OH)3), natural borate with only Al as a metal cation; and the second, after satimolite (KNa2(Al5Mg2) [B12O18(OH)12](OH)6Cl4·4H2O), mineral with a zeolite-like aluminoborate framework motif in the structure. Due to the presence of a three-dimensional system of wide channels containing Cl− anions and weakly bonded H2O molecules, krasnoshteinite is of interest as a potential prototype of a novel family of microporous crystalline materials without large cations.

Borate minerals with species-defining Al formed in high-temperature and low-temperature geological formations are strongly different in crystal chemistry and physical and chemical properties. The high-temperature Al borates and oxoborates do not contain H2O molecules, have compact crystal structures, and are typically characterized by high hardness and mechanical and chemical stability. Their crystal structures contain only one type of B-centered polyhedral, BO3 triangles, or BO4 tetrahedra. Unlike them, the low-temperature Al borates are highly hydrated, have low hardness, and are chemically unstable. They contain complex borate polyanions composed of both triangular and tetrahedral borate polyhedra with OH groups which participate in boron tetrahedra. Their structures are zeolite-like, being based upon open-work aluminoborate motifs.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4352/10/4/301/s1.

**Author Contributions:** Conceptualization, I.V.P., N.V.Z., I.I.C., and D.Y.P.; Methodology, I.V.P., N.V.Z., and S.N.B.; Investigation, I.V.P., N.V.Z., I.I.C., E.P.C., D.I.B., V.O.Y., Y.V.B., and I.L.; Writing—Original Draft Preparation, I.V.P., N.V.Z., I.I.C., and D.Y.P.; Writing—Review and Editing, I.V.P. and N.V.Z.; Visualization, N.V.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Russian Foundation for Basic Research, grants 18-29-12007-mk (I.V.P., N.V.Z., and V.O.Y. for electron microprobe, XRD, and crystal structure studies) and 18-05-00046 (I.I.C. and E.P.C. for fieldwork and SEM studies).

**Acknowledgments:** The research has been carried out using facilities at the XRD Research Center of St. Petersburg State University in part of powder XRD study.

**Conflicts of Interest:** The authors declare no conflict of interest.
