High-Resolution In Situ XRD Monitoring of Structural and Crystal-Chemical Evolution in Glauconite During Heating from 25 to 900 °C

Thermal transformation of glauconite involves coupled changes in interlayer organization, octahedral-sheet geometry, and the state of structural hydroxyl groups. In this study, the structural evolution of glauconite was investigated by high-temperature in situ X-ray diffraction from 25 to 900 °C with a temperature increment of approximately 10 °C. Temperature-resolved diffraction data were collected in the low- and high-angle regions to monitor basal periodicity and the 060-related reflection, respectively. The ~10.1–10.2 Å basal periodicity persisted throughout the heating sequence, although the corresponding diffraction profile changed progressively. A transient reflection at d ≈ 4.81 Å appeared at approximately 150–160 °C, reached its maximum intensity at ~180–300 °C, and disappeared above ~340–360 °C, indicating the development of an intermediate structural state during the low-temperature transformation of glauconite. The apparent b parameter, derived from the principal maximum of the 060-related high-angle region using b = 6d060, evolved non-monotonically over the range 9.078–9.108 Å, initially increasing to 9.096 Å by ~155 °C, decreasing to 9.084 Å at 505–575 °C, and then increasing again to 9.108 Å at 635–795 °C. These changes are consistent with successive structural relaxation, dehydroxylation-related reorganization, and subsequent post-dehydroxylation rearrangement of the 2:1 structure. FTIR and SEM-EDS observations after heating to 900 °C independently confirmed structural and morphological reorganization. The results show that temperature-resolved laboratory in situ XRD can detect transient and non-monotonic structural changes in glauconite and demonstrate the potential of this approach to investigate thermal transformations in other 2:1 phyllosilicates.