A high-pressure structural study of lawsonite using angle-dispersive powder-diffraction methods with synchrotron radiation

Structural refinements of lawsonite have been obtained at pressures up to 16.5 GPa using angle-dispersive powder diffraction with synchrotron radiation on a natural sample contained in a diamond anvil cell. Lawsonite compresses smoothly and relatively isotropically up to 10 GPa. Its bulk modulus is 126.1(6) GPa (for K' = 4), consistent with previous results. A trend of decreasing Si-O-Si angle indicates that compression is accommodated partly through the narrowing of the cavities containing Ca and H2O in the [001](ortho) direction. At 10-11 GPa there is a phase transition from Cmcm to P2(1)/m symmetry. The occurrence of a mixed-phase region, spanning >1 GPa, indicates that the transition is first order in character. The phase transition occurs through a shearing of(010)(ortho) sheets containing AlO6 octahedral chains in the [100](ortho) direction, which causes an increase in beta (mono). Across the transition, the number of oxygens coordinated to Ca increases from 8 to 9, causing an increase in the average Ca-O bond length. The compressibility of P2(1)/m lawsonite could not be determined due to solidification of the methanol/ethanol pressure-transmitting medium. On the basis of an experiment in which the P2(1)/m lawsonite structure was heated to 200 degreesC at 12.0 Cpa, we predict a shallow positive P-T slope for the phase transition, and therefore no stability field for P2(1)/m lawsonite in the Earth.