*2.2. Single-Crystal X-Ray Di*ff*raction Study*

A single crystal of guilleminite (0.08 <sup>×</sup> 0.04 <sup>×</sup> 0.01 mm3) was selected under binoculars, encased in viscous cryoprotectant, and mounted on cryo-loop. Diffraction data were collected using a Bruker Kappa Duo diffractometer (Bruker AXS, Madison, WI, USA) equipped with a CCD (charge-coupled device) Apex II detector operated with monochromated microfocused MoKα radiation (λ[MoKα] = 0.71073 Å) at 45 kV and 0.6 mA. Diffraction data were collected at 100 K with frame widths of 0.5◦ in ω and ϕ, and an exposure of 70 s per frame. Diffraction data were integrated, and background, Lorentz, and polarization correction were applied. An empirical absorption correction based on spherical harmonics implemented in the SCALE3 ABSPACK algorithm was applied in the CrysAlisPro program [11]. The unit-cell parameters were refined using the least-squares techniques. The crystal structure of guilleminite was solved by a dual-space algorithm and refined using the SHELX programs [12,13] incorporated in the OLEX2 program package [14]. The final model includes coordinates and anisotropic displacement parameters for all non-H atoms. The H atoms of H2O molecules were localized from difference Fourier maps and were included in the refinement, with *U*iso(H) set to 1.5*U*eq(O) and O–H restrained to 0.95 Å.

A dark-olive green prismatic crystal of demesmaekerite (0.034 <sup>×</sup> 0.032 <sup>×</sup> 0.022 mm3) was mounted on a glass fiber, and diffraction intensities were measured at room temperature with a Rigaku SuperNova (Oxford, UK) single-crystal diffractometer. The diffraction experiment was done using MoKα radiation from a micro-focus X-ray source collimated and monochromatized by mirror-optics and the detection of the reflected X-rays was done by an Atlas S2 CCD detector. X-ray diffraction data were collected at room-temperature with frame widths of 1.0◦ in ω and an exposure of 80 s per frame. Diffraction data were integrated, and background, Lorentz, and polarization correction were applied. An empirical absorption correction based on spherical harmonics implemented in the SCALE3 ABSPACK algorithm was applied in the CrysAlisPro program [11]. The structure was solved by the charge-flipping algorithm [12] and refined using the Jana2006 program [15]. The final refinement cycles were undertaken considering all atoms (except of hydrogen) refined with anisotropic atomic displacement parameters. The H atoms of H2O molecules were localized from the difference Fourier maps and were subsequently refined with *U*iso(H) set to 1.2\**U*eq of the donor O atom and O–H softly restrained to 0.95 Å.

Supplementary crystallographic data were deposited in the Inorganic Crystal Structure Database (ICSD) and can be obtained by quoting the depository numbers CSD 1963864 and 1964420 for guilleminite and demesmaekerite, respectively, at https://www.ccdc.cam.ac.uk/structures/ (see Supplementary Materials).
