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.

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

MAZZOLI, CLAUDIO;SASSI, FRANCESCO PAOLO;
1992

Abstract

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.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11577/2454882
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