Compositional controls on the cell dimensions of 2M1 muscovite and paragonite
Guidotti, Charles V.; Mazzoli, Claudio; Sassi, Francesco P.; Blencoe, James G.
European Journal of Mineralogy Volume 4 Number 2 (1992), p. 283 - 298
published: Apr 21, 1992
manuscript accepted: Jun 26, 1991
manuscript received: May 8, 1991
ArtNo. ESP147050402008, Price: 29.00 €
Abstract Multivariant, second order regression equations have been developed to quantify the effects of Na* [= 100 Na/(Na + K)] and Fm [= (Feτ + Mg + Mn)] on the α, b, 1/2 c.sin β, 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 β and Γand 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 β 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 ΔFmix 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.