EFFECTS OF TEMPERATURE AND PRESSURE ON THE STRUCTURE OF LAWSONITE

Independent isobaric data on thermal expansion and isothermal compress ibility data for lawsonite, CaAl2Si2O7(OH)(2) . H2O, were determined b etween 23 and 598 degrees C and 0.001 and 37.7 kbar using single-cryst al X-ray diffraction in a microfurnace and a diamond-anvil cell, respe ctively. The crystal structures of lawsonite were also refined from in tensity data collected at 23, 444, and 538 degrees C and 0.5 and 28.7 kbar. Both expansion and compression patterns are slightly anisotropic , with minor changes along c with respect to a and b. Unit-cell dimens ions vary linearly with T and P: alpha(a) = 1.26(4) x 10(-5), alpha(b) = 1.12(4) x 10(-5), alpha(c) = 7.6(3) x 10(-6) degrees C-1, and beta( a) = 3.4(1) x 10(-4), beta(b) = 3.0(1) x 10(-4), beta(c) = 2.8(2) x 10 (-4) kbar(-1). Bulk modulus, calculated as the reciprocal of cell-volu me compressibility, is 1100(40) kbar. The cavities of the framework ac commodating Ca and H2O molecules change by only about +/-5% in the inv estigated rand P ranges. Thus, at room pressure H2O molecules can be h osted in the lawsonite structure at least up to above 500 degrees C, w here the reduction of reflection intensities shows the beginning of de hydration and breakdown of the phase. Like other dense phases, structu ral changes with T and P essentially affect bond lengths, whereas inte rpolyhedral variations mainly concern the Si-O-Si' angle between tetra hedral pairs, which increases with temperature and decreases with pres sure. The present data define the ''geometric'' equation of state for lawsonite on the basis of cell-volume variations: V/V-o = 1 + 3.13(9) x 10(-5)T - 9.1(3) x 10(-4)P, where T is in degrees Celsius, P is in k ilobars, and the alpha/beta ratio is 34 bar/degrees C. This indicates that the cell volume of lawsonite remains unchanged with geothermal gr adients of about 10 degrees C/km, a condition actually observed in dow n-going subduction slabs. Therefore, the results of high-T and high-P structure refinements are in agreement with results from multi-anvil e xperiments and confirm that lawsonite is a good candidate for carrying water down to mantle depths.