Molal volume data are presented for single-phase Na-rich and K-rich (Na,K) Аl2(А1Sі3О10)(ОН)2 micas synthesized at 2, 4, and 8 kbar pressure. Micas synthesized at 2 and 4 kbar pressure are 1M polytypes; but at 8 kbar pressure 1M micas crystallized at low temperatures, and with increased temperature, 1M-2M1 mixtures and finally just 2Mx polytypes were obtained. X-ray powder-diffraction data were used in the refinement of unit-cell parameters for each mica, and molal volumes calculated from refined unit-cell volumes suggest that: (1) for an Na-rich or К-rich mica of specified composition, the molai volume of the 2M1 polytype is slightly smaller than that of the 1M polytype; (2) change in molai volume with Xmu (dV/dXmu) is significantly larger for Na-rich micas than for K-rich micas; (3) dV/dXmu for Na-rich micas is essentially constant and the same for both 1M and 2M1 polytypes, and similarly, dV/dXmu for K-rich micas is essentially constant and the same for both 1M and 2M1 polytypes; and (4) there is a small, approximately linear decrease in the molai volumes of 1M micas (of given composition) with increased synthesis pressure. Furthermore, although there is significant scatter in the molal volume data, least-squares fit polynomial equations calculated from these data suggest small, positive excess volumes for synthetic (Na,K)Al2(AlSi3O10)(OH)2 micas over the entire compositional range.

Cubic least-squares fit polynomial equations calculated from the 2, 4, and 8 kbar molal volume data have been used to derive WVl (= WV, pa) and WV2 (= Wv,mu) Margules parameters for (Na,K)Al2(AlSi3O10)(OH)2 micas. These Wv values are significantly different from Wv’s for paragonite-muscovite micas listed in previous studies. Analysis of the available molai volume data for paragonite-muscovite micas suggests that relatively small inaccuracies in these data can have a significant effect on not only Wv values, but also thermodynamic or phase-equilibrium calculations based upon equations of state which contain these Wv values.

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