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.