AN EQUATION OF STATE FOR ROEDDERITES, (K,NA)(2)MG-5[SI12O30] BASED ONKNUDSEN CELL MASS-SPECTROMETRY AND SYNTHESIS OF EIFELITE-ROEDDERITE CRYSTALLINE SOLUTIONS

Knudsen cell mass spectrometry has been applied to obtain activity (a( i)) vs composition (X(i)) data for 18 synthetic roedderites. (K, Na)(2 )Mg5Si12O30, at temperatures between 800 degrees and 1100 degrees C. T he samples were synthesized at 800 degrees C and 1 kbar P-H2O and char acterized by X-ray powder diffractometry (XRD) and electron probe micr oanalysis (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 crys talline solution. Using the Margules formalism, and expressing the res ults in terms of one atom of K-Na mixing, the tentative equation of st ate is: W-G [J/mol] =-8704-0.0067.P, with P given in bar. The temperat ure-dependence of W-G could not be resolved due to its large uncertain ty 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, si ngle-phase eifelite-roedderite crystalline solutions have also been sn ythesized for the first time. They show a symmetric positive excess vo lume of mixing,with W-V [J/bar.mol]=0.1064+/-0.0021.