*3.2. Single-Crystal X-ray Di*ff*raction and Crystal Structure Determination*

The single-crystal X-ray diffraction data were indexed in the *P*21 space group with the following unit-cell parameters: *a* = 8.73980 (19), *b* = 14.4129 (3), *c* = 11.3060 (3) Å, β = 106.665 (2)◦, and *V* = 1364.35 (5) Å<sup>3</sup> (Table 2). Details on data collection and structure refinement are also given in Table 2. The final structure refinement converged to *R*<sup>1</sup> = 0.0557 for 6142 unique observed reflections with *I* > 2σ(*I*). The H atoms of OH groups and H2O molecules were located from the difference Fourier synthesis. The studied crystal is microtwinned with the inversion center as a twin operation: twinning by merohedry Class I [13] with the twin domain ratio of 68/32. Coordinates and equivalent thermal displacement parameters of atoms are given in Table 3, selected interatomic distances in Table 4, and H-bonding in Table 5. Other crystal structure information for krasnoshteinite has been deposited with the Editors and is available as Supplementary Materials (see below): anisotropic displacement parameters of non-hydrogen atoms in the structure are presented in Table S1 and bond valence calculations in Table S2; crystallographic information file (CIF) is given as a separate Supplementary Material. Bond-valence parameters for Al-O and B-O were taken from [14] and for H-bonding from [15,16].


**Table 2.** Crystal data, data collection information and structure refinement details for krasnoshteinite.


**Table 3.** Coordinates and equivalent displacement parameters (*U*eq, in Å2) of atoms in krasnoshteinite.


**Table 3.** *Cont.*

\* The positions of H atoms were located from the difference Fourier map and refined with O-H and H-H distances softly restrained to 0.95(1) and 1.50(1) Å, respectively, to hold near-optimal geometry. Uiso (H) = 1.2 Ueq (O).


**Table 4.** Selected interatomic distances (Å) in the structure of krasnoshteinite.

**Table 5.** Hydrogen-bond geometry (Å,◦) in the structure of krasnoshteinite.



**Table 5.** *Cont.*
