Compositional controls on the cell dimensions of 2M1 muscovite and paragonite.

Multivariant, second order regression equations have been developed to quantify the effects of Na* [= 100 Na/(Na + K)] and Fm [= (Fe(T) + Mg + Mn)] on the a, b, 1/2 c.sin beta, and V cell parameters of muscovite (143 specimens) and paragonite (22 specimens). Na* substitutions occur on the XII sites of these micas, and it is assumed that all Fm substitutions occur on the VI sites. The Na* substitution has a strong effect on 1/2 c.sin beta and V and only a small effect on a and b. Fm substitution is largely restricted to muscovite and causes the a and b dimensions to increase. However, the associated increase of the a and b dimensions of the octahedral sheet necessitates rotations in the associated tetrahedral sheet such that the 1/2 c.sin beta dimension decreases concomitantly. The net effect of Fm substitution on cell volume is thus only a small increase. Our regression equations for cell volume indicate that DELTA-V(mix) is positive for Na-bearing muscovites, but is either zero or slightly negative for K-bearing paragonites. A crystallochemical model is proposed to explain the latter behavior. These results may provide some insights on the effects of pressure on the solvus limbs, and a possible explanation of the low sodium-contents typical of phengitic muscovites, and the low phengite-contents typical of paragonites. These interrelations provide evidence which should enable formulation of better activity models for both paragonite and muscovite. Previous attempts to use the muscovite-paragonite solvus as a geothermometer have yielded only semi-quantitative estimates of final equilibration temperatures. Applying the results obtained in this report should make it possible to significantly improve the accuracy of these temperature estimates.