HR: 1340h
AN: V23A-1223 [Abstracts]
TI: Thermodynamics of SiO2-H2O at the upper critical end point
AU: * Newton, R C
EM: rcnewton@ucla.edu
AF: Dept. of Earth and Space Sciences, University of California, Los Angeles, CA 90095, United
States
AU: Manning, C E
EM: manning@ess.ucla.edu
AF: Dept. of Earth and Space Sciences, University of California, Los Angeles, CA 90095, United
States
AB:
The hydrous melting curve of quartz terminates at a critical end point (CEP) near 10 kbar and 1080°C
(Kennedy et al, AJS 260, 501, 1962), where the compositions of coexisting aqueous fluid and hydrous melt
become identical near mol fractions (SiO2: XS; H2O: XH) of 0.5. To explore this behavior we
investigated the thermodynamic mixing properties of SiO2-H2O near the CEP. We measured quartz
solubility at 10 kbar and 700°-1050°C by weight loss of single crystals encased with H2O in
welded Pt capsules in a piston-cylinder apparatus. The SiO2 mol fractions are, at each T(°C): (700,
0.0123); (800, 0.0217); (850, 0.0295); (900, 0.0398); (950, 0.0575); (1000, 0.0824)
(1035, 0.138); (1050, 0.199); (1060, 0.320). The last datum is from Kennedy et al (1962; 9.5 kbar). These data
along with solubility measurements on enstatite-forsterite (Newton and Manning, GCA 66, 4165, 2002) were used
to retrieve activity coefficients (γS) of SiO2 (reference state Si(OH)4): ln γS =
-0.0554(ln XS)2 - 1.064(ln XS) - 4.057, which is valid in the range of the measurements.
The activity coefficient of H2O (γH) in quartz-saturated solutions determined from the Gibbs-
Duhem equation varies from 1.003 at 700°C to 1.378 at 1060°C, with a projected value of 1.653 at
1080°C. The strong T dependence of XS above 1000°C and the γH near the
theoretical regular solution value of e1/2 = 1.649 at a critical point verify that a CEP must exist near 10 kbar,
1080°C and XS = 0.5. Furthermore, the activity of the monomer Si(OH)4 and that of the dimer
SiO(OH)6 are so low at the CEP that other more highly polymerized species must dominate. We propose a
change of reference state at high T and P in the system SiO2-H2O from Si(OH)4 to
SiO(OH)2 as XS approaches 0.5. For compositions near 1:1, the mixture is virtually a regular solution.
The reference state may consist of charge-balanced rings and/or long chains of stoichiometry near
H2SiO3 or a random mixture of charged or uncharged groups of near 1:1 composition.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1043 Fluid and melt inclusion geochemistry
DE: 3600 MINERALOGY AND PETROLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting