Halite-sylvite thermoconsolution

LDEO Publication: 
Publication Type  Journal Article
Year of Publication  2005
Authors  Walker, D.; Verma, P. K.; Cranswick, L. M. D.; Clark, S. M.; Jones, R. L.; Buhre, S.
Journal Title  American Mineralogist
Volume  90
Issue  1
Pages  229-239
Journal Date  Jan
ISBN Number  0003-004X
Accession Number  ISI:000226318500026
LDEO Publication Number  6670
Key Words  crystalline solutions; thermal-expansion; temperature; equations; halides; volumes; state; nacl; kcl

An asymmetric binary Margules formulation for excess Gibbs energy, enthalpy, and entropy is adequate to describe the 1 bar halite-sylvite solvus in NaCl-KCl (Thompson and Waldbaum 1969). However, a binary Margules formulation is not adequate to describe excess volumes of single-phase chlorides in P-V-T-X from ambient P-T to similar to20 kbar and 700degreesC. Excess volumes across NaCl-KCl increase with temperature, decrease with pressure, and show systematic deficits in the consolute region. These patterns can be explained by the importance of a third component- vacancy defects that relieve the lattice stresses of K-Na size mismatch. New, two-phase observations in P-V-T-X allow delineation of the excess Gibbs energies to high pressures where the excess enthalpies and entropies do not depend on Tat each P, but show significant variation between 1 bar and similar to20 kbar. The volume, entropy, and enthalpy of K-Na mixing become more ideal at high pressure. But the solvus expands with pressure because entropy approaches ideality faster than enthalpy and, therefore, Gibbs energy of K-Na mixing becomes less ideal with pressure. The consolute temperature rises about 80degreesC in 17 kbar, with little change in consolute composition. The binary Margules equation of state provides a prediction of the rise of the solvus that is impressively convergent with the new observations. This convergence is especially impressive given the clear inadequacies of the binary excess volume formulation on which the prediction is based.


887PRTimes Cited:2Cited References Count:16

URL  <Go to ISI>://000226318500026