Thermodynamic mixing properties of disordered alkali feldspar solid-solution from Na–K partitioning and low-temperature calorimetry

Autor(en)

David Andreas Heuser, Elena Petrishcheva, Flora Ingegneri, Christian Leopold Lengauer, Edgar Dachs, Christoph Hauzenberger, Rainer Abart

Abstrakt

The equilibrium partitioning of Na and K between alkali feldspar and NaCl–KCl salt melt was determined at 800

^∘C, 850

^∘C, 900

^∘C, 950

^∘C and 1000

^∘C and close to ambient pressure. Four different natural gem-quality alkali feldspars with low degree of Al–Si ordering covering the range from orthoclase to high sanidine and with slightly different minor element concentrations were used as starting materials. The partitioning curves obtained for the four feldspars are indistinguishable indicating that Na–K partitioning independent of the differences of Al–Si ordering state and minor element concentrations existing amongst these feldspars. A sub-regular two parameter Margules type solution model was fitted to the partitioning data, and the excess Gibbs energy describing the thermodynamic non-ideality of the alkali feldspar solid-solution and the respective Margules parameters W

_gK and W

_g including their temperature dependence expressed as W

_g=W

_h-TW

_s were determined: (Formula presented.) The corresponding solvus has a critical temperature slightly above 650

^∘C and is well comparable with earlier direct experimental determinations of the low-sanidine-albite solvus curve. Comparison of the vibrational excess entropy determined from low-temperature heat capacity measurements with the total excess entropy derived from the temperature dependence of the excess Gibbs energy yields a negative configurational contribution to the excess entropy pointing towards short-range Na–K ordering on the alkali site.

Organisation(en)

Department für Lithosphärenforschung, Institut für Mineralogie und Kristallographie

Externe Organisation(en)

Paris-Lodron Universität Salzburg, Karl-Franzens-Universität Graz

Journal

Physics and Chemistry of Minerals

Band

51

ISSN

0342-1791

DOI

https://doi.org/10.1007/s00269-024-01270-z

Publikationsdatum

02-2024

Peer-reviewed

Ja

ÖFOS 2012

105120 Petrologie

Schlagwörter

ASJC Scopus Sachgebiete

Geochemistry and Petrology, Allgemeine Materialwissenschaften

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/6b6219ce-5c58-47dc-b295-615bce43aa16