Dense gamma-Al(2)O(3)condensates, with residual stress up to 3 GPa and ranging from nanometer to an unexpected micrometer size, were formed by pulsed laser ablation on Al target under oxygen background gas for a very rapid heating and cooling effect. Analytical electron microscopic observations indicated such nanoparticles tended to coalesce over {111} facets to form multiple twin and tilt boundary. The micrometer-size particles changed, upon electron irradiation, into metastable orthorhombic delta form full of twin variants and faults parallel to {100}.

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Extraction and electron irradiation (under transmission electron microscopy) of an epitaxial nanometer-thick [-PbO 2-type TiO 2 slab between twinned rutile bicrystals in ultra-high pressure metamorphic rock caused phase changes into a modified fluoritetype and then an amorphous phase. This martensitic-type transition process accounts for the dislocations and stacking faults of the slab and disordering of Ti in the adjoined rutile bicrystals. Additional hydrothermal experiments of sol-gel TiO 2-Al 2 O 3 performed at 8.5-9 kbar and 675-800°C in the piston-cylinder apparatus indicated that twinned rutile bicrystals were shaped in mirror image without the formation of [-PbO 2-type TiO 2 slab at the twin boundary and with no other planar defects for the bicrystals. The twinned bicrystals can be rationalized by growth and/or coalescence processes. Accordingly, it is not justified to assume a precursor phase of [-PbO 2-type structure for twinned rutile bicrystals when there is no such relic. Rutile, unless exsolved epitaxially from a host mineral such as garnet, does not constitute evidence for unusually deep burial for ultra-high pressure terranes.