Synthesis and Stability Relations of Mg--Al Pumpellyite, Ca4Al5MgSi6O21(OH)7

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Abstract
Hydrothermal synthesis and investigations of stability relations of Mg—Al pumpellyite were conducted using high-pressure cold-seal apparatus over the temperature range 250–600 °C and 2–8 kb Pfluid. Mg—Al pumpellyite Ca4Al5MgSi6O21(OH)7 was synthesized from partially crystalline gel mixtures of stoichiometric composition at 275–410 °C, 6–9 kb Pfluid, and runs of 7–90 days. Pure monomineralic synthetic Mg—Al pumpellyite has refractive index nβ = 1.624 (2) and cell dimensions α = 8.825 (8) Á, b = 5.875 (5) Á, c = 19.10 (1) Á, and β = 97.39 (7)°. The high temperature assemblage of the equivalent bulk composition consists of clinozoisite, hydrogrossular/grossular, aluminous septechlorite/chlorite, quartz, and H2O. Hydrogrossular was synthesized in the presence of quartz at 8 kb from 400–500 °C, and hydrogrossular + quartz are unstable with respect to grossular + H2O at 400 °C and 8 kb Pfluid. At 8 kb Pfluid, aluminous septechlorite forms at temperatures below 500 °C whereas aluminous 14 Á chlorite crystallizes at 500–600 °C. The equilibrium relations of Mg—Al pumpellyite were determined using subequal mixtures of synthetic Mg—Al pumpellyite and its high temperature assemblage. The reaction 9 Mg—Al pumpellyite = 9 clinozoisite + 6 grossular + 2 chlorite + 4 quartz + 19 H2O occurs at temperatures of 390 °C at 8 kb, 368 °C at 5 kb, and near 325 °C at 2 kb Pfluid. The reversal data yield an approximate value of −3141 joules/mole °K for the standard entropy of formation for the synthetic Mg—Al pumpellyite. The Schreinemakers' relations for pumpellyite, prehnite, clinozoisite, tremolite, grossular, and amesite in the presence of excess quartz and fluid were constructed in the pseudo-ternary system CaO−Al2O3−MgO(SiO2−H2O). The results, together with reconnaissance experiments on the reaction 4 Mg—Al pumpellyite + 2 quartz = 8 prehnite + aluminous septechlorite + 2 H2O, locate the invariant point [TR] at approximately 5.7 kb Pfluid and 375 °C. The results of the present study are not compatible with previous experimental data on the invariant point [GR]. The P–T oriented phase relations are used to interpret some natural parageneses developed in low-grade metabasaltic rocks recrystallized under conditions of low αco2. The high-temperature stability relations of Mg—Al pumpellyite are useful to denote the onset of greenschist facies metamorphism in rocks of basaltic composition.

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