A STUDY OF THE STABILITY OF ANORTHITE IN THE PT CONDITIONS OF EARTHS TRANSITION ZONE

The laser-heated diamond anvil cell has been used to synthesize anorth ite samples under the PT conditions of the transition zone. Samples re covered from high-pressure and high-temperature runs have been studied by X-ray diffraction (XRD) and analytical transmission electron micro scopy (ATEM). Below 17.5 GPa, the structure of the anorthite remains s table. The lattice parameters of anorthite recovered at ambient pressu re and temperature have been determined by lattice parameter refinemen t, whereby it is shown that the distortion of the structure increases with increasing pressure of synthesis. Above 17.5 GPa, anorthite decom poses into three phases, one with the composition Ca1.33Al1.33Si2.33O8 and a hollandite-type structure, a phase with the composition Al2SiO5 with a kyanite structure, and an amorphous phase with the approximate composition (Ca0.8,Al0.2)(Si0.8,Al0.2)O3. This last phase is assumed to be the result of the amorphization of Ca-perovskite on pressure rel ease. The geophysical implications of the stability of these high-pres sure aluminocalcic phases in the PT conditions of the transition zone are discussed.