*2.2. Methods*

## 2.2.1. X-Ray Powder Diffraction (PXRD)

The method was used to determine the phase composition of the precipitates. The measurements were performed using a Bruker « D2 Phaser » powder diffractometer (CuKα radiation of wavelength λ = 1.54178 Å). X-ray diffraction patterns were collected at room temperature in the range of 2θ = 3–60◦ with a step of 0.02◦. Phase identification was carried out using the ICDD PDF-2 Database (release 2016). The unit cell parameters and coherently scattering domain (CSD) size were refined by using TOPAS 4.2 software [26].

## 2.2.2. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray (EDX) Spectroscopy

Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used for the identification of calcium oxalates and for estimating calcium and strontium content in their crystals. Tetragonal weddellite and monoclinic whewellite crystals and their intergrowths were identified on SEM images by their previously described morphological features [12,27].

The study was carried out by means of a Zeiss Supra 40MP electron microscope, equipped with a variable-pressure secondary electron (VPSE) detector and Hitachi S-3400N with energy dispersive attachment AzTec Energy 350, at an accelerating voltage of 2 or 5 kV (depending on the image resolution). Magnification range varied from 100x to 1000x. Two SE detectors (secondary electron Everhart–Thornley), as well as a BSE detector (scintillation detector based on the highly sensitive YAG crystal with the resolution of 0.1Z of the atomic number) were used. The specimens were applied on two sided conducting tape and were coated with carbon (~15 nm). EDX analysis was performed by a standardless method that is generally reliable for elements with Z > 10. The mineral standards used were diopside (Ca) and celestine (Sr).
