*3.3. Powder X-ray Di*ff*raction*

The indexed powder X-ray diffraction data are given in Table S3 in Supplementary Materials (see below). The powder X-ray diffraction pattern of krasnoshteinite is unique and can be used as a good diagnostic tool of the mineral. The parameters of a monoclinic unit cell refined from the powder data are as follows: *a* = 8.740 (4), *b* = 14.409 (4), *c* = 11.316 (4) Å, β = 106.58 (3)◦, and *V* = 1366 (1) Å3.
