High-pressure behavior of anorthite: Compression and amorphization

We present the results of a detailed study of the high-pressure behavior of anorthite (CaAl₂Si₂O₈) between ambient pressure and 30 GPa under static conditions, using in situ Raman spectroscopy and energy-dispersive X ray diffraction. On increasing pressure under hydrostatic conditions, the PĪ- 71 transition occurs first at 2.6 GPa. At 10 GPa, a reversible polymorphic transition is observed which transforms the I1̄ polymorph into a phase of higher symmetry. With further pressure increase, large changes occur in the Raman and X ray spectra between 14 and 16 GPa, premonitory to the onset of pressure-induced amorphization. Above 16 GPa, anorthite is fully amorphous. However, only samples pressurized to above 22 GPa remain amorphous on recovery to ambient conditions. The high-pressure behavior of anorthite is highly sensitive to deviatoric stresses. Under less hydrostatic conditions, the 10-GPa transition described above occurs below 9 GPa, amorphization begins below 11.2 GPa, and samples decompressed from peak pressures as low as 17 GPa are fully amorphous. We have also carried out a thermodynamic analysis of the pressure-induced amorphization of anorthite, using a two state model for the equation of state (EOS) of CaAl₂Si₂O₈ glass in order to account for the increase in Al coordination. With this EOS, we calculate that the free energies of the crystalline and amorphous materials are equal at about 30 GPa, or slightly lower when effects of structural relaxation in the glass are taken into account, well above the observed crystalline-amorphous transition at 16 GPa.