**4. Discussion**

The crystal structure of krasnoshteinite (Figure 2) is unique. It is based upon the (010) corrugated layers of Al-centered octahedra connected via common vertices to form a pseudo-framework. There are eight crystallographically non-equivalent octahedrally coordinated Al sites: Al(1) and Al(7) cations center octahedra AlO(OH)4(OH2), Al(2,3) – AlO(OH)5, Al(4) – AlO2(OH)4, Al(5) – Al(OH)5(OH2), Al(6) – Al(OH)6, and Al(8) – Al(OH)4(OH2)2. These Al-centered octahedra play different structural roles. Al(1–4)- and Al(6,7)-centered octahedra share edges to form six-membered clusters. Al(8)-centered octahedra link adjacent clusters along the *c* axis sharing two corners with each cluster, while Al(5)-centered octahedra play the same role linking the clusters along the *a* axis to form octahedral layers (Figure 3a). Adjacent layers are connected via the common O(3) vertex of Al(7)- and Al(1)-centered octahedra, forming the three-dimensional octahedral motif.

Boron atoms occupy two crystallographically non-equivalent sites and center B(1)O3 triangles and B(2)O2(OH)2 tetrahedra, which share a common vertex to form insular [B2O4(OH)2] <sup>4</sup><sup>−</sup> groups (Figure 3b). According to the classification of fundamental building blocks (FBB) in borates [17,18], FBB in krasnoshteinite is 1Δ-:Δ-, i.e., the block with one triangle and one tetrahedron sharing corner. Krasnoshteinite is the first borate with such FBBs. These groups are connected with clusters of Al-centered octahedra via common vertices. Thus, a BO3 triangle shares one O vertex with a

B-centered tetrahedron, one vertex with two Al-centered octahedra [Al(4) and Al(7)] of the layer, and one vertex with Al(1)- and Al(2)-centered octahedra of an adjacent layer (thus reinforcing the linkage between neighboring octahedral layers). A BO2(OH)2 tetrahedron shares one O vertex with a BO3 triangle, one O vertex with two Al-centered octahedra [Al(3) and Al(4)], and one O=OH vertex with a Al(5)-centered octahedron; all Al(3,4,5) octahedra belong to the same layer. The resultant aluminoborate pseudo-framework contains three-membered [2B + Al] rings. Such Al-B-O units are known in the porous aluminoborate frameworks as being crucial to stabilizing them [19–21]. The same configuration of the three-membered [2B + Al] ring was described in the crystal structures of satimolite, KNa2(Al5Mg2)[B12O18(OH)12](OH)6Cl4·4H2O [22], and synthetic porous Al borates, PKU-3 H24.3Al9B18O51Cl3.3·6.8H2O [23] and PKU-8 (H18Al7B12O36)Cl3(NaCl)2.4·6.5H2O [19].

The aluminoborate pseudo-framework in krasnoshteinite is microporous and zeolite-like (Figure 2). The three-dimensional system of wide channels contains Cl<sup>−</sup> anions and H2O molecules. Together with OH groups and H2O molecules belonging to Al- and B-centered polyhedra, they form a complicated system of hydrogen bonds (Table 5).

Among 160 natural borates and oxoborates, known to date as valid mineral species, only twelve minerals contain species-defining Al, namely aluminomagnesiohulsite, (Mg,Fe2<sup>+</sup>)2 (Al,Mg,Sn)(BO3)O2; jeremejevite, Al6(BO3)5(F,OH)3; johachidolite, CaAlB3O7; krasnoshteinite, Al8[B2O4(OH)2](OH)16Cl4·7H2O; londonite, CsAl4Be4B12O28; mengxianminite, (Ca,Na)2Sn2(Mg,Fe)3 Al8[(BO3)(BeO4)O6]2; painite, CaZrAl9(BO3)O15; peprossiite-(Ce), CeAl2B3O9; pseudosinhalite, Mg2Al3O(BO4)2(OH); rhodizite, KAl4Be4B12O28; satimolite, KNa2(Al5Mg2) [B12O18(OH)12] (OH)6Cl4·4H2O; and sinhalite, MgAl(BO4) [2]. Satimolite and krasnoshteinite are low-temperature (LT) borates formed in evaporitic rocks, whereas the other ten minerals are known only in high-temperature (HT) geological formations: granitic pegmatites, HT metamorphic or metasomatic rocks, or post-volcanic HT assemblages. These HT Al borates and oxoborates do not contain H2O molecules and have compact crystal structures that cause high hardness and mechanical and chemical stability in the majority of them. Data on the Mohs hardness of peprossiite-(Ce) and pseudosinhalite are absent in literature, aluminomagnesiohulsite has the Mohs hardness value of 6, and jeremejevite, johachidolite, londonite, mengxianminite, painite, rhodizite, and sinhalite demonstrate the Mohs hardness values between 7 to 8 [24]. All ten minerals have crystal structures with only one type of B-centered polyhedra, BO3 triangles [aluminomagnesiohulsite, jeremejevite, mengxianminite, and painite], or BO4 tetrahedra [johachidolite, londonite, peprossiite-(Ce), pseudosinhalite, rhodizite, and sinhalite], without OH groups coordinating B [25]. In LT formations, the crystal chemistry and properties of borate minerals with species-defining Al change drastically. Satimolite and krasnoshteinite are highly hydrated chloroborates which have low Mohs hardness values, 2 (satimolite) or 3 (krasnoshteinite), and dissolve even in diluted HCl. They contain complex borate polyanions composed of both B-centered triangles and tetrahedra with OH groups which participate in the tetrahedra. The structures of both satimolite and krasnoshteinite are microporous and zeolite-like. Thus, under LT conditions, aluminum octahedra in borates became a building unit of open-work aluminoborate structure motifs.

**Figure 2.** The crystal structure of krasnoshteinite in three projections. The unit cell is outlined.

**Figure 3.** Octahedral layer (**a**) and insular [B2O4(OH)2] <sup>4</sup><sup>−</sup> group (**b**) in the structure of krasnoshteinite. For legend, see Table 3.
