We investigated rutile needles with a clear shape preferred orientation in garnet from (ultra) high-pressure metapelites from the Kimi Complex of the Greek Rhodope by electron microprobe, electron backscatter diffraction and TEM techniques. A definite though complex crystallographic orientation relationship between the garnet host and rutile was identified in that Rt is either parallel to Grt <111 > or describes cones with opening angle 27.6A degrees around Grt <111 >. Each Rt small circle representing a cone on the pole figure displays six maxima in the density plots. This evidence together with microchemical observations in TEM, when compared to various possible mechanisms of formation, corroborates a precipitate origin. A review of exchange vectors for Ti substitution in garnet indicates that rutile formation from garnet cannot occur in a closed system. It requires that components are exchanged between the garnet interior and the rock matrix by solid-state diffusion, a process we refer to as "open-system precipitation" (OSP). The kinetically most feasible reaction of this type will dominate the overall process. The perhaps most efficient reaction involves internal oxidation of Fe2+ to Fe3+ and transfer from the dodecahedral to the octahedral site just vacated by OSP is likely to occur at conditions where the transition of natural systems to open-system behaviour becomes apparent, as in the granulite and high-temperature eclogite facies.
Proyer, A., Habler, G., Abart, R., Wirth, R., Krenn, K., & Hoinkes, G. (2013). TiO2 exsolution from garnet by open-system precipitation: evidence from crystallographic and shape preferred orientation of rutile inclusions. Contributions to Mineralogy and Petrology, 166(1), 211-234. https://doi.org/10.1007/s00410-013-0872-7