The crystallization and precipitation of calcium carbonate minerals is the subject of intensive research due to the existence of its polymorphic and morphological varieties in geological and biological systems, as well as due to its application both in industrial fields, in particular, in the production of plastics, rubbers, and paper production, and for the creation of biomedical implants and drug delivery systems.

The existence of a new polymorphic modification of calcium carbonate, monoclinic aragonite CaCO₃, has been experimentally discovered [1]. Its crystal structure has been solved and its crystal structure has been discussed. It was found that, unlike the previously known modification, orthorhombic aragonite with cell parameters a = 4.961 Å, b = 7.967 Å, and c = 5.740 Å, the new polymorph belongs to monoclinic symmetry, crystallizes in the space group P2₁/c, and has cell parameters a = 12.732 Å, b = 5.740 Å, c = 9.378 Å, and b =96.91°.

The crystal cell of the new polymorph is formed by three Ca²⁺ cations and three carbonate anions occupying general positions. In the structure, carbonate anions form stacks along the b axis, in which they are arranged in a mutually overlapping manner. The stacks are surrounded by Ca²⁺ cations coordinated by nine oxygen ions. The new monoclinic polymorph has pseudohexagonal symmetry, and this effect is observed only along the b direction of the cell and is absent in the direction of other axes. As in the case of vaterite, the existence of a supercell can be assumed in the structure of the new aragonite, resulting in the high R-factor.

A three-dimensional set of diffraction reflections was obtained for a single crystal at room temperature using a Rigaku OD XtaLAB Synergy-S single-crystal diffractometer on MoK_α radiation (λ = 0.71073 Å). The experimental data were processed using the CrysAlisPro v. 1.171.39.46 software package. The crystal structure was determined based on direct methods using the SHELX [2] software package.