Shock induced formation of MgAlzO4 spinel from oxides

The physics of mineral grain sliding, which occurs upon dynamic compression of rocks, is investigated by shock loading single crystals of corundum (A1203) and periclase (MgO) in contact obliquely in impact experiments. Energy dispersive X-ray analysis and X-ray diffraction studies of samples recovered from 26 36 GPa, 800 ns experiments indicated that under certain conditions a spinel phase of composition MgA1204 and thickness _< 20 !.tm was produced at the interface between the two crystals. Although the computed shock (continuum) temperatures were below those necessary to melt the initial oxides, the spinel nonetheless appears to have formed as a result of localised melting, via grain boundary sliding friction, followed by rapid quenching. Scanning electron microscopy (SEM) revealed some evidence for such melting. Moreover, the timescale of the experiments is too short for solid state diffusion (during the shock state) to explain the observed spinel thickness, although defect enhanced solid state diffusion, subsequent o loading and unloading, remains a possibility. The results also reinforce other recent observations and theories of heterogeneous deformation in minerals.