Pressure-induced amorphization in plagioclase feldspars: A time-resolved powder diffraction study during rapid compression

The pressure-induced amorphization of the two endmembers of the plagioclase ((Na_1−xCa_x)Al_1+xSi_3−xO₈) solid-solution, anorthite (CaAl₂Si₂O₈) and albite (NaAlSi₃O₈), has been studied as a function of compression rate by means of time-resolved powder diffraction. Anorthite and albite were compressed in a diamond anvil cell to 80 GPa at multiple rates from 0.05 GPa/s to 80 GPa/s. The amorphization pressure decreases with increasing compression rate. This negative strain rate sensitivity indicates a change in deformation mechanism in the plagioclase solid-solution endmembers from brittle to ductile with increasing compression rate. The presented data support the previously proposed shear deformation mechanism for the amorphization of plagioclase. Furthermore, amorphization progresses over a wide pressure range suggesting heterogeneous amorphization, similar to observations based on recovered material from shock-compression experiments of plagioclase. Our experiments support the contention that amorphization pressures for plagioclase may occur at lower pressures than usually considered.