Investigation of the α-β Moganite Transition under In-situ Heating/Cooling Using Synchrotron X-ray Diffraction and Pair Distribution Function Analysis

Moganite is a microcrystalline silica polymorph that serves as an indicator of evaporitic environments and a potential proxy for planetary water activity. In this study, we investigate its temperature-dependent structural variations using samples from the Mogán Formation, Gran Canaria, by combining in-situ synchrotron X-ray diffraction (XRD) and pair distribution function (PDF) analysis over 300–900 K. The results reveal a reversible phase transition from monoclinic I2/a (α phase) to orthorhombic Imab (β phase) at 560–580 K, marked by continuous convergence of the β angle to 90° and the disappearance of the (411) reflection. Thermal expansion is notably anisotropic: a and c increase up to the transition temperature and then plateau, while b expands continuously to 900 K, reflecting progressive un-kinking of the tetrahedral chains along [010] driven by cooperative SiO₄ rotations. Crystal structures of moganite at multiple temperatures across the α–β transition, including full anisotropic atomic displacement parameters (ADPs), were determined through the combined application of synchrotron XRD and PDF analyses. In particular, X-ray total scattering PDF analysis captures the progressive convergence and merging of the O2 and O3 sites across the symmetry change, as well as the relaxation of monoclinic strain tracked through the U₁₃ anisotropy of O3, consistent with a precession-like librational motion about the Si1–O3–Si2 bond. Additionally, X-ray total scattering PDF analysis reveals increasingly pronounced b-axis vibrational enhancement for Si2 and O2 that accelerates above the transition temperature (Tc), reflecting cooperative tetrahedral rotations along [010]. The integrated XRD–PDF methodology offers an effective approach for characterizing phase transitions in structurally complex terrestrial and planetary materials.