• journal_title：European Journal of Mineralogy
• Contributor：Pouyan SHEN ; Shyh-lung HWANG ; Hao-Tsu CHU ; Tzen-Fu YUI ; Chiennan PAN ; Wuu-Liang HUANG
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2005-
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2005/0017-0543
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：17
• issue：4
• firstpage：543
• section：Articles

Extraction and electron irradiation (under transmission electron microscopy) of an epitaxial nanometer-thick α-PbO₂-type TiO₂ slab between twinned rutile bicrystals in ultra-high pressure metamorphic rock caused phase changes into a modified fluorite-type 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 TiO₂-Al₂O₃ 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 α-PbO₂-type TiO₂ 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 α-PbO₂-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.