Experimentally derived thermochemical data for pargasite and reinvestigation of its stability with quartz in the system Na2O–CaO–>MgO–Al2O3–SiO2–H2O |
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| Authors: | Anurag Sharma |
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| Institution: | (1) Department of Geological Sciences and Environmental Studies, Binghamton University, Binghamton, NY 13902-6000, USA, US |
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| Abstract: | The stability of pargasite in the presence of excess quartz has been determined in the range of 0.5–6.0 kbar and 500–950 °C
in the system Na2O– CaO–MgO–Al2O3–SiO2–H2O, using synthetic minerals. The experimental results from this study indicate the presence of two distinct mineral assemblage
regions: (1) a high temperature supersolidus region containing tremolitic amphibole+melt+quartz; (b) a low temperature subsolidus region consisting of Al-rich amphibole+plagioclase+enstatite+quartz. Compositional reversals have been determined for the
following three equilibria:
(a) 2 pargasite+9 quartz=tremolite+4 plagioclase (An50)+1.5 enstatite+H2O, (b) 2 pargasite+10 quartz=tremolite+4 plagioclase (An50)+talc, and (c) pargasite+diopside+5 quartz=tremolite+2 plagioclase (An50). These experiments indicate a continuous change of amphibole composition from pargasite to tremolite with increasing temperature,
and an opposite effect with increasing pressure. The third equilibria is used to constrain a site-mixing model for the pargasitic
amphiboles, which favor a single-coupled NaA-AlT1 site mixing. The thermochemical data for pargasite estimated from the reversal data of the three equilibrium reactions is
estimated as for ΔG
0
f
,Pg=−12022.11±5.2 kJ mole-1, and S
0
Pg=591.7 ±7.9 JK-1 mole-1.
Received: 31 July 1995/Accepted: 3 June 1996 |
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