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On the transformation pathways of -PbO₂-type TiO₂ at the twin boundary of rutile bicrystals and the origin of rutile bicrystals

¹ Institute of Materials Science and Engineering, National Sun Yat-sen University, Kaohsiung, Taiwan, ROC
² Department of Materials Science and Engineering, National Dong Hwa University, Hualien, Taiwan, ROC
³ Central Geological Survey, P.O. Box 968, Taipei, Taiwan, ROC
⁴ Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC
⁵ Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC

^* Corresponding author, e-mail: pshen{at}mail.nsysu.edu.tw

Extraction and electron irradiation (under transmission electron microscopy) of an epitaxial nanometer-thick -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 -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 -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.

Key-words: -PbO₂-type TiO₂, transformation path, twinned rutile, ultra-high pressure, metamorphism.