Calorimetric investigation of Na-K mixing and polymorphism in the alkali feldspars
Heat of solution measurements of the enthalpies of Na-K mixing, Al-Si ordering, and fusion for crystalline solutions and end-member phases in the system Na(AlSi3O8)–K(AlSi3O8) were obtained with a vacuum-jacketed, isoperibolic calorimeter using 20.1 percent hydrofluoric acid at temperatures ranging from 40 to 60°C. Heat of solution data for artificially prepared microcline (–low-albite) crystalline solutions at 49.7°C are represented by
ΔHsol = −149.408 + 5.230 N2 − 5.928 N1N22 − 8.457 N2N12 kcal/mole
based on 39 measurements of 14 different bulk compositions, where N1 and N2 denote the mole fractions of NaAlSi3O8 and KAlSi3O8 respectively. The thermochemical uncertainty of the above least-squares formulation is ± 0.032 kcal/mole.
Unit-cell parameters determined by least-squares refinement of diffraction data indicate that the excess volumes of Na-K mixing of these microcline solutions are positive over the entire composition range. Combining the calorimetric and crystallographic data leads to the following expression for the variation of molar enthalpy of mixing with pressure:
ΔHex = (5928 + 0.110 P) N1N22 + (8457 + 0.110 P) N2N12 cal/mole.
The sign and magnitude of ΔHex determined by calorimetry agree closely with ΔHex calculated from a third-order Margules formulation of high-temperature two-phase equilibrium data in the critical region.
Citation Information
Publication Year | 1971 |
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Title | Calorimetric investigation of Na-K mixing and polymorphism in the alkali feldspars |
DOI | 10.1524/zkri.1971.134.5-6.381 |
Authors | David Robert Waldbaum, Richard A. Robie |
Publication Type | Article |
Publication Subtype | Journal Article |
Series Title | Zeitschrift für Kristallographie — Crystalline Materials |
Index ID | 70227458 |
Record Source | USGS Publications Warehouse |