ISSN:
1432-2021
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
Abstract Knudsen cell mass spectrometry has been applied to obtain activity (a i) vs composition (X i) data for 18 synthetic roedderites. (K, Na)2Mg5Si12O30, at temperatures between 900° and 1100° C. The samples were synthesized at 800° C and 1 kbar P H 2O and characterized by X-ray powder diffractometry (XRD) and electron probe microanalysis (EPMA) prior to mass spectrometric activity measurement. The experimental a i-X i data have been smoothed, assuming for now the simplest possible model of a two-site K-Na mixing for this binary crystalline solution. Using the Margules formalism, and expressing the results in terms of one atom of K-Na mixing, the tentative equation of state is: W G [J/mol] =-8704 -0.0067 · P, with P given in bar. The temperature-dependence of W G could not be resolved due to its large uncertainty on the order of 2.8 kJ/mol. A more rigorous thermodynamic model for roedderites will have to be deferred until information on temperature-dependent K-Na disordering becomes available. Besides roedderites, single-phase eifelite-roedderite crystalline solutions have also been snythesized for the first time. They show a symmetric positive excess volume of mixing, with W V [J/bar·mol]=0.1064±0.0021.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00215107
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