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All, Oriented rutile needles are common in garnets from both granulites and eclogites. However, there is no easy way to show that they "exsolved" (=unmixed), even though this is widely assumed. A second question is how minor Ti substitutes in garnet, and this too is not well established for Ca-poor garnets. Almandine has difficulty in exsolving rutile because garnet is R2O3 and rutile is RO2 (with minor ROOH). Only if the initial garnet has cation vacancies, Fe2+ becomes partially oxidized during rutile growth, or if some Ti is 3+, can rutile by itself form from garnet. We need experiments on garnet-rutile. eric >Horst, > >Many of the references in the literature of the diatremes of the Four >Corners region of the U.S. refer to garnet xenocrysts from the mantle >containing oriented exsolved rutile in pyropic garnets. In the paper >on another subject by me and Armand Chase (1961) there is a >photograph of a thick section of one of these. A number of papers by >the late Tom McGetchin are material to any discussion of these >xenocrysts. Also Eric Essene and colleagues have done considerable >recent work on these xenocrysts. Eric has my thick section of the >photographed xenocryst referred to above and has discovered other >minerals that exsolve as oriented extensions of the oriented rutiles. >I hope this helps. > >John Rosenfeld > >***** >See Figure 4 on p. 529 in: > >1961 Rosenfeld, John L. and Armond B. Chase. "Pressure and >Temperature of Crystallization from Elastic Effects Around Solid >Inclusions in Minerals?," Amer. Jour. Sci., 259, p. 519-541. > > > >>Dear Listmembers, >> >>Granulite facies garnets often show relatively high Ti contents in their >>cores and zonations to nearly Ti-free rims. Sometimes one can find >>finegrained rutile needles inside the garnets in strong orientation. Both >>features may be results of higher solubility of Ti in garnet at high >>pressures (or high temperatures?). >>I was searching for a reference dealing with Ti solubility in garnet, but >>did'nt find anything. Help! >> >>thanks in advance! >> >>horst >> >>================================= >>Horst Marschall >>Mineralogisches Institut >>Universitaet Heidelberg >>INF 236 >>69120 Heidelberg >>++49-(0)6221-546021 >>[log in to unmask] >>================================= > >-- >John L. Rosenfeld >Department of Earth & Space Sciences >University of California, Los Angeles >Los Angeles, California 90095-1567 > >Phone: 310-825-1505 >e-mail: [log in to unmask] >website: <http://www.ess.ucla.edu/facpages/rosenfel.html> Eric Essene Professor of Geology Department of Geological Sciences 2534 C.C. Little Bldg. 425 E. University Ave. University of Michigan Ann Arbor MI 48109-1063 USA fx: 734-763-4690 ph: 734-764-8243