The restricted stability of osumilite under hydrous conditions in the system K2O-MgO–Al2O3–SiO2–H2O |
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Authors: | Martin Olesch Friedrich Seifert |
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Affiliation: | (1) Mineralogisches Institut, Universität Kiel, Olshausenstraße 40-60, 2300 Kiel, Federal Republic of Germany;(2) Present address: Institut für Mineralogie, Am Hubland, 8700 Würzburg, Federal Republic of Germany |
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Abstract: | Osumilitess was synthesized as a single phase product in the model system K2O-MgO-Al2O3-SiO2 at 800° C/ 0.5 Kbar water pressure and at 800° to 840° C/1.0 Kbar total pressure with
0.3 in the gas phase. The experimentally determined solid solubility range of synthetic osumilites can be expressed by the formula KMg2(Al3-xMgx) (Al2–xSi10+x)O30 with 0x0.4. A survey of sixteen chemical analyses of natural osumilites from eleven occurrences shows a solid solubility characterized by 0x0.6. Reversed stability experiments for the synthetic osumilite KMg2(Al2.75Mg0.25)(Al1.75Si10.25)O30 determined at water pressure equal to total pressure demonstrate its restriction to water pressures below 0.8 Kbar (at 0.5 Kbar, the stability range is between 765° and 800° C). At the lower thermal stability limit osumilite+H2O vapor break down to cordierite+K feldspar+phlogopitess+quartz, at the higher one to cordierite+K feldspar+phlogopite+liquid. Reduction of water fugacity will expand the stability field largely by shifting the lower and higher thermal stability limits to lower and higher temperatures, respectively. The dependence of osumilite stability on water fugacity makes osumilite a sensitive indicator mineral for dry conditions in rocks formed at total pressures higher than about 0.8 Kbar. |
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