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Trisrota, John, Peter et al.: I wonder if Cr in spinel of altered peridotite stems from olivine, which is not a Cr-loving silicate. Kees L. Op 17/09/2011 15:41, Robert Tracy schreef: > Trisrota, John, Peter et al.: > > I believe the old term (and maybe current as well) among opaque-mineral folks is "ferrit-chromit", since this phase that John and Peter refers to is typically a magnetite - Cr-spinel solid solution. I have commonly seen it as well in serpentinized ultramafic rocks, and sometimes it can even persist through the prograde metamorphic metamorphic process into amphibolite facies and possibly even beyond. > > Bob T. > > On Sep 17, 2011, at 5:10 AM, Clemens, JD, Prof<[log in to unmask]> wrote: > >> Hi Trisrota, >> >> This is an almost universal feature of rocks in which mafic minerals have been serpentinised or altered to talc. The cause is simple, as you suspect. The new magnesian hydrous minerals are unable to accomodate all the Fe released by the reaction of the olivine or pyroxene into their crystal lattices, at the conditions of alteration. It thus expresses itself as magnetite-rich spinel oxide. >> >> Cheers, >> JC >> >> >> John D. Clemens >> Professor in Geology& Exec. Head >> Dept of Earth Sciences, University of Stellenbosch, >> Private Bag X1, 7602 Matieland, South Africa >> tel: +27 (0)21 808 3159 fax: +27 (0)21 808 3129 >> e-mail:[log in to unmask] >> web page:http://www.sun.ac.za/geo/people/clemens_e.htm >> ________________________________________ >> From: Metamorphic Studies Group [[log in to unmask]] On Behalf Of Trisrota Chaudhuri [[log in to unmask]] >> Sent: 17 September 2011 10:25 >> To:[log in to unmask] >> Subject: Texture in olivine cumulate >> >> Dear all, >> I'm getting a typical texture of olivine cumulate rocks in thin sections (photo attached), in which olivines are extremely altered into serpentine and/or talc. But the interesting thing is that every altered olivine grain has a thin rim of opaques along their grain boundaries. Is it possible that the olivine, due to alteration, released some extra Fe/Mg, which might not been needed to form the altered product, and hence concentrated along olivine grain boundaries? Or something else like grain boundary exsolution has taken place? Please enlighten me. >> Regards, >> >> -- >> Trisrota Chaudhuri, >> JRF of Indian Statistical Institute and Jadavpur University, >> Kolkata, India. > Robert Tracy > Professor of Geosciences > Virginia Tech > Blacksburg VA 24061-0420 > O: 540-231-5980 > F: 540-231-3386 >