Abstract: China has selected highly compacted Gaomiaozi (GMZ) bentonite as buffer/backfill material for deep geological repositories. The crystal chemical characteristics of montmorillonite, the dominant mineral constituent of GMZ bentonite, play a decisive role in controlling the performance of the engineered barrier system. This study characterizes the crystal chemical properties and crystalline swelling of GMZ montmorillonite via X-ray diffraction and X-ray fluorescence analyses, which confirmed a Cheto-type montmorillonite with an interlayer charge of 0.98 e⁻ per unit cell. A novel ex-situ X-ray diffraction protocol was developed to control the relative humidity (RH = 12% ~ 100%) of the testing environment, revealing basal spacing expansion from 12.6 to 18.7Å. Results show hydration is largely heterogeneous due to structural/compositional variability. Comparative analysis indicates that interlayer charge and exchangeable cations critically affect swelling, though their impact diminishes under high RH and with divalent cations. These findings advance the fundamental understanding of crystalline swelling mechanisms of montmorillonite in GMZ bentonite and provide a theoretical framework for elucidating the linkage between crystallographic hydration processes and the macroscopic hydro-mechanical behavior of bentonite barriers.