The crystal structure of dypingite: understanding the long-range disorder

Dypingite, a hydrated magnesium carbonate hydroxide mineral [Mg₅(CO₃)₄(OH)₂·<i>X</i>H₂O, <i>X</i> = 5-6], exhibits promising catalytic and purification properties. Although it was discovered 55 years ago, the crystal structure of this compound has remained unknown due to its aggregated morphology and structural disorder. This work investigates the origin of this phenomenon through a systematic analysis of synthetic and natural mineral samples, using synchrotron powder X-ray diffraction, thermogravimetric analysis and transmission electron microscopy. The findings reveal that ambient humidity significantly influences dypingite's structural properties at room temperature. High humidity (80% relative humidity at 22 °C) causes inhomogeneous expansion of the unit cell along the crystallographic <i>c</i> axis, leading to long-range structural disorder. Conversely, at 20% relative humidity at 22 °C, the mineral structure exhibits a shorter <i>c</i> lattice constant and reduced structural disorder. Chemical analysis reveals that samples kept at 80% and 20% relative humidity for 10 days differ by one molecule of water of hydration, yielding Mg₅(CO₃)_4.5(2)(OH)_0.96(3)·6.0(2)H₂O and Mg₅(CO₃)_4.5(2)(OH)_1.02(4)·5.0(2)H₂O, respectively. The results obtained demonstrate that the crystal structure of dehydrated dypingite [Mg₅(CO₃)_4.5(2)(OH)_1.02(4)·5.0(2)H₂O] derives from hydro-magnesite's unit cell tripled along the <i>a</i> axis. The analysis of the mineral crystal structure provides insight into the role of humidity on the structural properties of dypingite, including unit-cell dimensions and long-range disorder.