The nature and significance of the mechanism of sillimanite growth in the Connemara schists,Ireland

During prograde metamorphism of the Connemara pelites, sillimanite first develops in biotite immediately adjacent to, or replacing, garnet. In some rocks, breakdown of garnet+muscovite and staurolite +muscovite+quartz leads to the development of fibrolite pseudomorphs after garnet. The textures indicate a constant volume replacement of garnet with movement of Al from staurolite and muscovite in the matrix towards the few, widely scattered, garnet sites. The complex ionic reaction patterns are the result of the strong preference of sillimanite to grow on biotite that is replacing garnet, and this pattern of preferred nucleations is taken to indicate that the equilibrium conditions for the reaction were only overstepped by the minimum required for initial sillimanite nucleation. Chemical movements were controlled by the heterogeneous nucleation pattern, not by intrinsic properties of the moving species. In order for extensive reaction to occur under near-equilibrium conditions, the rate at which the total thermal reaction proceeded must have been controlled by the supply of heat to the rocks rather than by diffusion or local reaction steps.