Phase transitions similar to those observed at low temperatures in lawsonite, CaAl2[Si2O7]·(OH)2 H2O, were recorded at ∼95 and ∼150 °C in hennomartinite, SrMn2[Si2O7](OH)2·H2O. The room temperature structure of the natural phase was refined from X-ray single-crystal data in space group P21cn and that of the > 150 °C phase at 245 °C in space group Cmcm. The structures are isotypic to the low-temperature and room temperature structures of lawsonite. The P21cn structure of hennomartinite exhibits twinning parallel to (100) with additional disorder mainly involving the H positions. As indicated by a split O position (O5), disorder and twinning must also be considered in the Cmcm structure. Similar to lawsonite, the driving force of the phase transitions is apparently the development of cooperative hydrogen bonds. At low temperatures the H atoms shift toward neighboring O atoms forming strong hydrogen bonds.

The transition temperatures were determined by monitoring the intensities of critical reflections over the temperature range from −163 to 245 °C. The low-temperature boundary of the Cmcm phase is characterized by the appearance of the 017 reflection (among other C-centering forbidden reflections) below 150(5) °C. The high-temperature boundary of the twinned room temperature phase is characterized by a dramatic enhancement of the intensity of the 406 reflection below 95(5) °C. Between 95 and 150 °C (in analogy to lawsonite) the average space group Pmcn is proposed, which is the mediating sub-super group between P21cn and Cmcm.

After one heating and cooling cycle, splitting of hk0 reflections (observed in ω scans) below ∼100 °C indicates that the true, relaxed room temperature structure represents a monoclinic derivative of the natural, initially determined P21cn average structure. Moreover, a decrease of the a unit-cell parameter and the unit-cell volume during the 245 °C data collection suggests a temperature-controlled structural evolution of hennomartinite from disorder and small twin domains in the natural phase to large twin domains caused by annealing at elevated temperatures.

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