Pavel, Hercynite is stable with quartz down to 750°C. Its reaction to form garnet or cordierite of course depends on P as well. On the other hand Mg-rich spinel is not stable with quartz. I wonder if your matrix assemblage is a migmatite that is introduced later, or whether it is a quartzose metasediment. eric >Dear geo-metamorphists, > >I would like read your opinion on rather usual texture: inclusions >of Mg-spinel or hercynite in the core of cordierite porphyroblasts >in quartziferous rocks (generally, gneisses). Sometimes these >porphyroblasts contain also fibrolite needles. The direct contact >between spine/hercynite and quartz is invariably absent. > >Spinel-bearing cordierites are known mostly in granulitic gneisses >however without evidences for temperatures above 1000°C. I have >met also hercynite inclusions in cordierite from an assemblage >Qtz+Pl+Bt+Chl+Crd+St+And. The mineral composition and geological >setting (central part of greenstone belt) of this rock indicates >that it did not heat up to 1000°C. > >So, spinel or hercynite in silica-saturated rock below this >temperature must be metastable. What can you say about origin of >such inclusions? Why they forms? Why spinel is located in >cordierite, not in other minerals? > > Pavel. >-- > > Pavel Azimov, PhD > Institute of Precambrian Geology and Geochronology > Russian Academy of Sciences > 2, Makarov Embankment > St.Petersburg, 199034, Russia > Phone: +7(812)328-03-62 > Fax: +7(812)328-48-01 > E-Mail: [log in to unmask] --