Low- and high-temperature evolution of chlorothionite, K 2 Cu(SO 4 )Cl 2

The low-and high-temperature crystal chemistry of chlorothionite, K 2 Cu(SO 4 )Cl 2 , a fumarolic copper sulfate-chloride from the Yadovitaya fumarole (Tolbachik volcano, Kamchatka), and its synthetic analogue has been investigated.The study combined in situ single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD).SCXRD measurements were performed over the temperature range 100-700 K, while PXRD data were collected from 300 to 940 K.The synthetic analogue of chlorothionite has recently attracted considerable attention in the fields of solid-state physics and materials science due to its remarkable magnetic properties.No phase transitions are observed over the entire studied temperature range.However, PXRD reveals that the evolution of the diffraction pattern with increasing temperature can be divided into four distinct stages.Above ~600 K, reflections corresponding to tenorite (CuO) and orthorhombic K 2 SO 4 (Pmcn) appear in the powder patterns, followed by the formation of hexagonal K 2 SO 4 (P6 3 /mmc) above 680 K.The diffraction peaks of chlorothionite completely disappear after 820 K, leaving tenorite and hexagonal K 2 SO 4 as the sole crystalline phases.According to the SCXRD, the thermal expansion of chlorothionite is strongly anisotropic throughout the 100-700 K range.Between 100 and 400 K, the expansion is governed by the highly directional thermal response of bond lengths and angles within the coordination polyhedra: the largest expansion occurs along the b axis (i.e., along the Cu-polyhedral chains), whereas the c axis exhibits negative thermal expansion.Between 400 and 700 K, the expansion along c reverses sign, becoming strongly positive; this change is attributed to shear deformations driven by the displacement of potassium atoms.The [CuCl 2 (SO 4 )] complexes remain essentially rigid upon heating, functioning as stable structural blocks, while the K-O and K-Cl bonds behave as flexible hinges that control the macroscopic thermal response of the framework.The mineral and its synthetic analogue display virtually identical thermal expansion coefficients.This confirms that trace impurities in the natural sample have no measurable effect on its thermal behavior and validates the use of mineral samples as reliable proxies for the physical properties of their synthetic counterparts.