The Fibrolite Problem: Implications for Reaction Overstep Studies

Abstract

Despite several experimental studies the phase relations in aluminosilicates, especially involving the high-temperature polymorph, are not well understood. The widespread occurrence of fibrolite before sillimanite in rocks is described as the ‘fibrolite problem’. Fibrolite is considered to have higher free energy than sillimanite because of the excess surface energy contribution due to the higher surface/volume ratio. Equilibrium thermodynamics fails to reconcile the stability of fibrolite, with higher free energy, at lower temperatures than sillimanite. The existing literature invokes reaction kinetics to suggest that the low energy of {110} faces leads to acicular fibrolite growth rather than prismatic sillimanite. Here the ‘fibrolite problem’ is discussed from a perspective of high nucleation rates and hindered growth rates. An analog to fibrolite-sillimanite crystallization is suggested in the crystallization of mullite, a member of the Al₂O₃-SiO₂ system. The crystallization of fibrolite in rocks shifts the position of the sillimanite producing reactions from the equilibrium PT depending upon the DS and DV of the reaction. Thermodynamic datasets calibrated using such natural samples tend to produce accurate predictions for sillimanite stability in geological terranes but inherently possess a reaction overstep and hence should be used carefully while quantifying reaction oversteps.