Synthesis and Stability Relations of Mg--Al Pumpellyite, Ca4Al5MgSi6O21(OH)7 |
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Authors: | SCHIFFMAN PETER; J G LIOU |
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Institution: | Tuttle-Jahns Laboratory for Experimental Petrology, Dement of Geology, Stanford University Stanford, CA 94305 |
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Abstract: | Hydrothermal synthesis and investigations of stability relationsof MgAl pumpellyite were conducted using high-pressurecold-seal apparatus over the temperature range 250600°C and 28 kb Pfluid. MgAl pumpellyite Ca4Al5MgSi6O21(OH)7was synthesized from partially crystalline gel mixtures of stoichiometriccomposition at 275410 °C, 69 kb Pfluid, andruns of 790 days. Pure monomineralic synthetic MgAlpumpellyite has refractive index nß = 1.624 (2) andcell dimensions = 8.825 (8) Á, b = 5.875 (5) Á,c = 19.10 (1) Á, and ß = 97.39 (7)°. The high temperature assemblage of the equivalent bulk compositionconsists of clinozoisite, hydrogrossular/grossular, aluminousseptechlorite/chlorite, quartz, and H2O. Hydrogrossular wassynthesized in the presence of quartz at 8 kb from 400500°C, and hydrogrossular + quartz are unstable with respectto grossular + H2O at 400 °C and 8 kb Pfluid. At 8 kb Pfluid,aluminous septechlorite forms at temperatures below 500 °Cwhereas aluminous 14 Á chlorite crystallizes at 500600°C. The equilibrium relations of MgAl pumpellyite were determinedusing subequal mixtures of synthetic MgAl pumpellyiteand its high temperature assemblage. The reaction 9 MgAlpumpellyite = 9 clinozoisite + 6 grossular + 2 chlorite + 4quartz + 19 H2O occurs at temperatures of 390 °C at 8 kb,368 °C at 5 kb, and near 325 °C at 2 kb Pfluid. Thereversal data yield an approximate value of 3141 joules/mole°K for the standard entropy of formation for the syntheticMgAl pumpellyite. The Schreinemakers' relations for pumpellyite, prehnite, clinozoisite,tremolite, grossular, and amesite in the presence of excessquartz and fluid were constructed in the pseudo-ternary systemCaOAl2O3MgO(SiO2H2O). The results, togetherwith reconnaissance experiments on the reaction 4 MgAlpumpellyite + 2 quartz = 8 prehnite + aluminous septechlorite+ 2 H2O, locate the invariant point TR] at approximately 5.7kb Pfluid and 375 °C. The results of the present study arenot compatible with previous experimental data on the invariantpoint GR]. The PT oriented phase relations are used to interpretsome natural parageneses developed in low-grade metabasalticrocks recrystallized under conditions of low co2. The high-temperaturestability relations of MgAl pumpellyite are useful todenote the onset of greenschist facies metamorphism in rocksof basaltic composition. |
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