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Pavel: This probably is not the case for your samples, but it is possible to get intergrowths of spinel in cordierite via the pseudomorphous replacement of aluminosilicate porphyroblasts via a prograde (c. 550°C) reaction of the type: andalusite + Mg-rich chlorite = spinel + cordierite + fluid. In the Reynolds Range, Northern Territory, Australia, this happened. I think it is a result of a complex history, involving later prograde metamorphism, to low-P granulite facies, affecting areas where I think earlier "diagenetic hydrothermal" alteration had produced Mg-rich chlorite. In the most Mg-rich areas, the pseudomorphs have aluminosilicate cores partly pseudomorphed at the rims by a cordierite + corundum intergrowth; in slightly more iron-rich areas, the blocky pseudomorphs consist of intergrown cordierite + green spinel, inside a clear cordierite rim. The pseudomorphs occur in rocks that contain quartz. Julie Vry >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] -- Dr. Julie Vry Senior Lecturer - Geology School of Earth Sciences Victoria University PO Box 600 Wellington, New Zealand phone, office: 64 4 463 6432 please leave messages at: 025 644 5598 fax 64 4 463 5186 email: [log in to unmask]