Abstract β anhydrite, the stable form of CaSO4, is a common mineral and a key phase in gypsum’s dehydration mechanism. An in-depth multi-scale investigation of its structure by powder X-ray diffraction revealed a very slight distortion of the crystal cell at room temperature (β = 90.092(9) °), that led to rule out the commonly admitted orthorhombic model (space group Cmcm) in favor of monoclinic C2/c subgroup. According to high-temperature XRD, the structure resumes however the orthorhombic symmetry around 400 °C.

中文翻译：

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β-硬石膏CaSO4的单斜畸变

摘要 β硬石膏是CaSO4的稳定形式，是一种常见的矿物，是石膏脱水机理的关键相。通过粉末 X 射线衍射对其结构进行了深入的多尺度研究，揭示了室温下晶胞的非常轻微的畸变（β = 90.092(9) °），这导致排除了普遍承认的正交模型（空间群 Cmcm) 偏向于单斜 C2/c 亚群。根据高温 XRD，该结构在 400 °C 左右恢复正交对称性。