STABILITY OF ANDALUSITE AND SILLIMANITE AND THE AL2SIO5 TRIPLE POINT - CONSTRAINTS FROM THE BALLACHULISH AUREOLE, SCOTLAND

Andalusite (And) and sillimanite (Sil) occur in well-defined zones in the Ballachulish aureole. In most of the aureole's sillimanite-bearing rocks, sillimanite grain diameters range from less-than-or-equal-to 2-mu-m to greater-than-or-equal-to 10-mu-m. The boundary between prograde andalusite and prograde sillimanite is constrained to a zone of max. 30 m width upgrade of the muscovite + quartz = andalusite + K-feldspar + H2O reaction isograd. This boundary is interpreted to correspond to the And=Sil equilibrium. The common occurrence of andalusite well upgrade of the first occurrence of sillimanite is ascribed to sluggish kinetics of the polymorphic inversion. The P-T position of the And=Sil equilibrium in the aureole is estimated as 645 +/- 20-degrees-C and 3.0 +/- 0.4 kbar, based on a combination of (1) location of the Ms + Qtz = And + Kfs + H2O reaction, corrected for non-end member fluid and mineral compositions, and (2) independent pressure estimates obtained from equilibria in metapelites, marbles, calcsilcates, and granitoids, all in excellent agreement. The effects of grain size and minor element content of andalusite and sillimanite on the equilibrium appear negligible. From this P-T constraint, the And=Sil equilibrium was extrapolated, using entropy and volume data, to intersect the overlapping uncertainty envelopes for the Ky = And and Ky = Sil equilibria, giving an estimate of the triple point of 4.5 +/- 0.5 kbar, 550 +/- 35-degrees-C, midway between Richardson's and Holdaway's determinations, and satisfying the petrological criteria used to support either.