F-OH substitution in natural tremolite, talc, and phlogopite

Abstract

The distribution of F between tremolite and talc has been determined in metamorphosed siliceous carbonates from the Grenville Province, Ontario. Wavelength dispersive electron microprobe analyses of contiguous, texturally compatible tremolite-talc pairs indicate that the substitution of F for OH is the most significant deviation from end-member stoichiometry in the samples studied. Mixing of F and OH components has been represented by an ideal solution model for F in tremolite and an asymmetric model for F in talc. Both linear and nonlinear regression techniques have been used to derive activity coefficients for the exchange of one equivalent of OH and F components in talc. The following expressions are the result of nonlinear regression of 32 analyses from coexisting mineral pairs:

$$\begin{gathered} \ln \gamma _{TC(OH)} = X_{TC(F)}^2 [2.447 - 2.845X_{TC(OH)} ] \hfill \\ \ln \gamma _{TC(F)} = X_{TC(OH)}^2 [1.024 + 2.845X_{TC(F} ] \hfill \\ \end{gathered} $$

Isobaric $T - X_{CO_2 }$ sections constructed using these equations show an enhanced stability for the assemblages talc+calcite and phlogopite+quartz+calcite with F substituting for OH. Projection of isothermal invariant points into P-T space predicts a shift in the stability of the assemblage talc-calcite from lower grade into the sillimanite field with increasing substitution of F for OH in talc.