*2.2. Chemical Composition*

The chemical composition of krasnoshteinite was studied using a Jeol JSM-6480LV scanning electron microscope equipped with an INCA-Wave 500 wavelength-dispersive spectrometer (Laboratory of Analytical Techniques of High Spatial Resolution, Dept. of Petrology, Moscow State University). Electron microprobe analyses were obtained in the wavelength-dispersive spectroscopy mode (20 kV and 20 nA; the electron beam was rastered to the 5 × 5 μm area to avoid damage of the highly hydrated mineral) and gave contents of Al, Si, O, and Cl. The standards used were: Al2O3 (Al), wollastonite (Si), YAl3(BO3)4 (O), and NaCl (Cl). The contents of other elements with atomic numbers higher than 8 are below detection limits.

The presence of significant amount of chlorine in krasnoshteinite prevents the quantitative determination of boron by electron microprobe, due to the overlap of X-ray emission lines of the *K* series of B with *L* lines of Cl. The boron content was determined using ICP-MS. The measurements were carried out with the Element-2 (Thermo Fisher Scientific) instrument which has high resolution (that avoids interference of components) and sensitivity. Several crystals of the mineral were dissolved in 10 cm<sup>3</sup> of 3% HNO3 solution (Merck, Suprapur®) in deionized water (EasyPure). Since the mass of the mineral was too small for accurate weighing, we have determined contents of B and Al in relative units and further used averaged Al content, obtained by electron microprobe, for B content calculation. The obtained value is in good agreement with the boron content determined from the crystal structure refinement. Contents of Li and Be in krasnoshteinite are below detection limits.

H2O was not analysed because of the paucity of material. Hydrogen (H2O) content was calculated based on the structure data (see below) and taking into account the charge balance requirement. The analytical total is close to 100 wt.% (Table 1) that demonstrates a good agreement between electron microprobe data for Al, Si, O, and Cl, ICP-MS data for B and calculated value for H. The correctness of the obtained chemical data was also confirmed by the superior value of the Gladstone–Dale compatibility index [8]: 1 − (Kp/Kc) = 0.003 (superior) with measured density value, or 0.006 (superior) with calculated density value.

CO2 was not analysed because the structure data showed the absence of this constituent. The absence of gas release in HCl (see below) also indicated that krasnoshteinite does not contain carbonate groups.
