Low- and high-temperature evolution of löweite, Na ₁₂ Mg ₇ (SO ₄ ) ₁₃ ⸱15H ₂ O

Hydrated sodium and magnesium sulfates are fairly common minerals.This study is focused on lweite from the fumaroles of the Tolbachik volcano in Kamchatka.The crystal structure of lweite was refined using single-crystal X-ray diffraction, and the hydrogen atoms were localized for the first time.In situ single-crystal (temperature range -173 to 227 C) and powder (temperature range -173 to 900 C) X-ray studies were performed.Both techniques show that lweite remains stable up to about 220 C without showing any signs of potential phase transitions.The mineral is also stable under a vacuum of 600 Pa.The following transformation sequence for lweite was observed upon heating: lweite metathnardite + vanthoffite + "x-phase" metathnardite + periclase.The thermal expansion of lweite demonstrates two distinct patterns in two temperature ranges.There is virtually no expansion in the structure until -93 C.Following this, the structure rapidly expands, exhibiting a highly anisotropic behavior.Shear deformations of the