SHOCK-INDUCED CRYSTAL-GLASS TRANSITION IN SOLID SILICA

Solid silica SiO2 occurs under a variety of forms, alpha-quartz being the stable one at ambient conditions. Shock wave compression induces in this mineral typical defects which appear at the optical microscope as very straight and narrow (less than or equal to 1 mu m) lamellae parallel to {10(1) over bar n} planes. Transmission electron microscopy on both naturally (meteorite impacts) and experimentally (gun shots) shocked quartz shows that they are thin lamellae of amorphous silica. Theoretical computations show that the shock-induced crystal-amorphous transition results from instabilities of the shear modulus in {10(1) over bar n} planes for P>10 GPa. The elastic misfit energy in the shock front is relaxed by the nucleation of amorphous and more compressible zones the growth of which is then driven by the propagating shock front. The boundaries delineating these defects are sharp and move at very high speed.