Thermodynamics of excess silicon in nepheline and kalsilite crystalline solutions

Enthalpies of solution have been measured in 20.1 weight % HF at 50 degrees C on synthetic nepheline and kalsilite samples having 0.125 excess silicon atoms per four oxygen atoms via (Na,K)AI = []Si substitution (where [ ] = vacancies), then compared with data for two series of nepheline-kalsilite samples having less excess silicon. Differences in the enthalpies of solution between the new samples and those having no excess silicon are 8.4 and 14.4 kJ/mol, respectively, for potassium-free nepheline and sodium-free kalsilite, energy rates that are comparable to those of K-Na substitution. Enthalpies of K-Na mixing, as well as the critical temperature of the solvus, also are increased by the higher Si content. Considering both the tendency of these solid solutions to contain excess silicon and the energetic consequences of that substitution, we suggest that they routinely be treated as ternary. This study documents the significant thermodynamic effects of relatively minor chemical substitution on both end-member data and mixing properties, and speaks to the need for thorough chemical characterization of phases on which thermodynamic and phase equilibrium studies are conducted. The application of end-member thermodynamic data to natural phases may be inappropriate, even if chemical differences between the two are small.

Title	Thermodynamics of excess silicon in nepheline and kalsilite crystalline solutions
Creator	Roux, J.
Creator	Hovis, Guy L.
Publisher	European Journal of Mineralogy
Academic Department	Geology & Environmental Geosciences
Division	Natural Sciences
Organization	Lafayette College
Date Issued	October 1999
Date Available	2023-06-05
Type	Article
Language	English
Bibliographic Citation	Hovis, G. L. and J. Roux (1999 Oct) "Thermodynamics of excess silicon in nepheline and kalsilite crystalline solutions." European Journal of Mineralogy 11 (5): 815-827.
Standard Identifier	DOI 10.1127/ejm/11/5/0815
Standard Identifier	Handle 10385/2f75r9639
Permalink	http://hdl.handle.net/10385/2f75r9639
Rights Statement	In Copyright
Rights Holders	E. Schweizerbart'sche Verlagsbuchhandlung.