Gibbs energies and entropies of K-Na mixing for alkali feldspars from phase equilibrium data: Implications for feldspar solvi and short-range order

Delbove, F.; Bose, M. R.; Hovis, Guy L.

Gibbs energies and entropies of K-Na mixing for alkali feldspars from phase equilibrium data: Implications for feldspar solvi and short-range order

Abstract

Reversed brackets have been achieved at T = 800-degrees-C and P near 1 bar for K-Na partitioning between (K,Na)Br molten salts and members of two topochemically monoclinic alkali feldspar series that differ substantially in Al-Si distribution. The Gibbs energies of K-Na mixing derived from the phase equilibria for the two series are similar, but significantly less positive than those of a highly ordered topochemically triclinic series. These contrasting relations are consistent with higher critical temperatures for low microcline - low albite (Bachinski and Muller, 1971) and adularia-based feldspars, relative to highly disordered series.

Calculated entropies of mixing for highly disordered alkali feldspars, based both on this work and on solvus data, are somewhat uncertain; however, they are asymmetric with respect to composition and lower in magnitude than those from direct low-temperature heat capacity measurements. This, in turn, implies short-range order in such feldspars, consistent with recent observations in a more ordered low microcline-low albite series.

Recommended Margules parameters for internal energy, entropy, and volume for disordered alkali feldspars are W(E,Or) = 5450 and W(E,Ab) = 4670 cal/mol, W(S,Or) = 1.5 and W(S,Ab) = 2.5 cal/K.mol, W(V,Or) = 0.110 and W(V,Ab) = 0.078 cal/bar.mol. These parameters reproduce reversed low- and high-pressure solvi based on direct measurements by other investigators and are consistent with enthalpies of mixing from HF solution calorimetry as well as excess Gibbs energies from the present investigation.

Title	Gibbs energies and entropies of K-Na mixing for alkali feldspars from phase equilibrium data: Implications for feldspar solvi and short-range order
Creator	Delbove, F.
	Bose, M. R.
	Hovis, Guy L.
Publisher	American Mineralogist
Academic Department	Geology & Environmental Geosciences
Division	Natural Sciences
Organization	Lafayette College
Date Issued	1991
Date Available	2013-02-11T20:04:36Z
Type	Article
Language	English
Bibliographic Citation	Hovis, G. L., F. Delbove, and M. R. Bose. (1991) "Gibbs energies and entropies of K-Na mixing for alkali feldspars from phase equilibrium data: Implications for feldspar solvi and short-range order." American Mineralogist 76 (5/6): 913-927.
Standard Identifier	Handle 10385/1093
Permalink	http://hdl.handle.net/10385/1093
Rights Statement	In Copyright
Rights Holders	Mineralogical Society of America