Low-temperature phase transitions of the high-pressure low-temperature silicate lawsonite, CaAl2(Si2O7)(OH)2·H2O, were discovered by Libowitzky & Armbruster (1995) using single-crystal X-ray diffraction and optical microscopy. The space group symmetry is changed in two steps from Cmcm (>273 K) via Pmcn (<273 K) to P21cn (<155 K) as a consequence of different H2O and OH arrangements in the cavities of an almost unchanged framework that consists of chains of edge-sharing AlO6 octahedra interconnected by di-silicate groups. The phase transitions were interpreted using polarized IR spectroscopy of...
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