High-temperature Raman spectroscopy of lawsonite: constraints on bonding changes and dehydration

The Raman spectrum of lawsonite is characterized in situ at high temperatures through its dehydration temperature under vacuum. The structural reorganization associated with dehydration appears to initiate in the Si and Al-dominated layer of the structure, and proceeds between 525–550 and 725 °C. The loss of water from the structure appears complex, with the first structural reorganization associated with dehydration occurring at 550 °C, coexistence of lawsonite and the initial dehydration products up to 700 °C, and rapid disappearance of lawsonite between 675 and 725 °C. The dehydration is likely preceded by increased disorder of the hydroxyls within the structure. Indications of enhanced disorder of hydrogen ions at high temperatures include a dramatic decrease in intensity of an O–H librational vibration, large peak broadening of the principal hydroxyl stretch, and the disappearance of a shoulder on the strongest hydroxyl stretching vibration. The thermal anharmonicity of a range of Raman active modes is determined, and a revision to mode assignments is suggested based on the thermal shifts. Under low pressure conditions and our laboratory timescale, the observed product of lawsonite dehydration is the metastable pseudo-hexagonal polymorph of CaAl₂Si₂O₈, dmisteinbergite. Production of dmisteinbergite through intermediate temperature lawsonite dehydration represents a previously unrecognized pathway for its formation, which previously has been limited to low-temperature hydrothermal environments, pyrometamorphic environments, materials rapidly quenching from melts (including in pseudotachylites), and as a retrograde phase from high grade metamorphic environments. The formation of metastable dmisteinbergite from lawsonite is likely facilitated by a topotactic relation between its layered structure and that of lawsonite.