Lower temperature stability limit of Mg cordierite in the range 1–7 kb water pressure: A redetermination |
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| Authors: | F Seifert W Schreyer |
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| Institution: | (1) Institut für Mineralogie der Ruhr-Universität, Bochum |
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| Abstract: | Hydrothermal experiments utilizing the seeding technique show conclusively that the lower temperature stability limit of Mg cordierite so far accepted (Schreyer and Yoder, 1964) represents metastable equilibria throughout the pressure range 1–7 kb. The stable equilibrium curve generally lies at somewhat higher temperatures and is not subdivided by an invariant point involving the phase pyrophyllite within this pressure range. Instead of pyrophyllite+andalusite+chlorite, hitherto assumed to be the breakdown assemblage of cordierite below 5 kb water pressure, the paragenesis Al silicate+chlorite+quartz was found to be stable over the entire pressure range investigated. The andalusite/kyanite transition, as determined by Richardson et al. (1969), is confirmed by a minor change of slope of the cordierite breakdown curve. Direct breakdown of cordierite into pyrophyllite-bearing assemblages must be limited to conditions below about 1 kb and 450° C. This gives indirect support to Kerrick's (1968) stability data on pyrophyllite, as opposed to those of Althaus (1969) and others. These new results offer an explanation for the common assemblage chlorite+andalusite (or kyanite)+quartz occuring in spotted slates, greenschists, and also as retrograde alteration products of natural cordierites.We thank Dr. K. Langer, Bochum, for partial analyses of some of the natural starting materials. Financial support of this work by Deutsche Forschungsgemeinschaft, Bad Godesberg, is gratefully acknowledged. |
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