A. Proyer 1 TiO 2 exsolution from garnet by open system precipitation: Evidence from crystallographic and shape preferred orientation of rutile inclusions Alexander Proyer* 1 , Gerlinde Habler 2 , Rainer Abart 2 , Richard Wirth 3 , Kurt Krenn 1 and Georg Hoinkes 1 1 Institute of Earth Sciences, University of Graz, Universitaetsplatz 2/II, A-8010 Graz, AUSTRIA *Corresponding author, e-mail: alexander.proyer@uni-graz.at Tel.: 0043-316-380-5541; Fax: 0043-316-380-9865 2 Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria 3 Geoforschungszentrum Potsdam, Am Telegrafenberg, D-14473 Potsdam, Germany Abstract 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 Back Scatter Diffraction and TEM-techniques. A definite though complex crystallographic orientation relationship between the garnet host and rutile was identified in that Rt[001] is either parallel to Grt<111> or describes cones with opening angle 27.6° around Grt<111>. Each Rt[001] 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 Fe 2+ to Fe 3+ and transfer from the dodecahedral to the octahedral site just vacated by Ti 4+ : 6 M 2+ 3 TiAl[AlSi 2 ]O 12 + 6 M 2+ 2,5 TiAlSi 3 O 12 = 10 This is a preprint. The final publication is available at Springer via http://dx.doi.org/10.1007/s00410-013-0872-7