Graham, C. M. and Tareen, J. A. K. and Mc millan, P. F. and Lowe, B. M. (1992) An experimental and thermodynamic study of cymrite and celsian stability in the system BaO-Al 2 O 3 -SiO 2 -H 2 O. European Journal of Mineralogy, 4 (2). pp. 251-269.
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The stabilities and growth kinetics of monoclinic celsian feldspar (BaAl2Si2O8) and of cymrite (BaAl2Si2O8.nH2O, where 0 < n < 1) have been investigated in the system BaO-Al2O3-SiO2-H2O at pressures up to 10 kbar and temperatures up to 800-degrees-C. Synthesis experiments and growth-rate studies using gel starting materials have identified a broad area of P-T space at low pressures and temperatures in which cymrite grows rapidly and metastably in short-duration runs, being replaced by celsian in longer runs. The equilibrium: cymrite = celsian + H2O has been tightly bracketed by reversal experiments at about 9.7 kbar and 800-degrees-C, 9.0 kbar and 710-degrees-C, and 8.0 kbar and 580-degrees-C. Broad brackets on the equilibrium at lower P and T, due to slow reaction, are thermodynamically consistent with these results, and confirm that cymrite is only stable at high pressures, providing a valuable metamorphic geobarometer. Down-temperature extrapolation from these brackets demonstrates that cymrite is not stable in low P-T sedimentary and hydrothermal environments. Numerous documented natural occurrences of cymrite in these environments are therefore considered to be the result of rapid metastable growth and persistence in the celsian stability field. The molecular water contents of cymrites, determined by thermogravimetric analysis, have been found to vary systematically with the P(H2O) and T of synthesis, values increasing P(H2O) and decreasing T to a maximum of one H2O per formula unit. A small component of probable non-molecular (?structural) OH has also been identified, which becomes increasingly abundant at high temperatures. Micro-Raman and micro-infrared studies reveal increasing orientational ordering of the molecular H2O groups with increasing temperature of synthesis. Thermodynamic analysis of molecular water incorporation in anhydrous cymrite (equivalent to hexacelsian) by the reaction BaAl2Si2O8 + H2O = BaAl2Si2O8.H2O yields an enthalpy change of about -59 kJ mol-1 and an entropy change of -133 J K-1 mol-1, comparable to analogous data for the hydration of anhydrous cordierite. Isopleths have been calculated for the equilibrium water content of cymrite as a function of P(H2O) and T.
| Item Type: | Article |
|---|---|
| Subjects: | F Earth Science > Geology |
| Divisions: | Department of > Earth Science |
| Depositing User: | Users 23 not found. |
| Date Deposited: | 15 May 2021 06:15 |
| Last Modified: | 27 Jul 2022 11:33 |
| URI: | http://eprints.uni-mysore.ac.in/id/eprint/16448 |
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