Powder diffraction and Raman spectroscopic study of lawsonite at high water pressure

The structural behavior of natural lawsonite CaAl 2 Si 2 O 7 (OH) 2 · H 2 O in aqueous and nonaqueous media (pressure up to 9.5 GPa) has been studied by synchrotron powder diffraction and Raman spectroscopy. The volume compressibility of lawsonite is found to be similar in both aqueous and nonaqueous media, while irreversible amorphization is observed only under compression in the aqueous medium to a pressure of about 6 GPa. Along with the observed increase in framework vibration frequencies, this reveals that the lawsonite structure is unstable when hydrostatic conditions of compression differ from those provided by crystallization of ice VII in an aqueous medium.