The nucleation and growth processes of titanian clinohumite (Ti-Cl) and titanian chondrodite (Ti-Ch) during hydrous alteration of olivine at 8 GPa, 1300 K are described in detail. We observed two different nucleation sites and orientation relationships for Ti-Cl: (1) heterogeneous nucleation at olivine grain or phase boundaries, yielding semicoherent (001) lamellae with (001)ol//(001)Ti-Cl and (010)ol//(010)Ti-Cl; (2) formation of coherent nanometer-sized nuclei inside olivine with the orientation relationship (100)ol//(001)Ti-Cl and (011)ol//(020)Ti-Cl. Only the former, for which the interface with olivine facilitates the transport of OHand Ti necessary for the formation of the humite-group minerals, survives to large sizes. These results suggest that Ti-Cl should form readily in a Ti- and H-rich environment at mantle temperatures, independent of F content, from shallow environments to those at depths exceeding 250 km, and could play a role in recycling water back into the deep mantle.

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