Towards a better understanding of the fibrolite problem: the effect of reaction overstepping and surface energy anisotropy

Trachytes from the Euganean Hills District (Italy) contain metapelitic xenoliths that have been pyrometamorphosed during incorporation in the melt. In xenoliths containing sillimanite crystallized during a previous regional HT/LP metamorphism, fibrolite systematically nucleates at the grain boundaries of sillimanite prisms and within plagioclase crystals. Ternary feldspar thermometry shows that plagioclase in contact with sillimanite plots along the 750degreesC solvus that reflects near-equilibrium conditions of regional metamorphism. Plagioclase containing fibrolite plots closer towards the 950degreesC solvus, reflecting the tendency of plagioclase to re-equilibrate at high temperature during pyrometamorphism by a fibrolite-forming reaction: K-feldspar (1) + plagioclase (1) -->K-feldspar (2) + plagioclase(2) + fibrolite. In sillimanite-free xenoliths, fibrolite did not form during pyrometamorphism, because these xenoliths do not contain plagioclase. In these xenoliths, andalusite prisms are replaced by oriented fibrous corundum needles and K-feldspar. The petrographic evidence suggests that when reaction rate is high as a result of reaction temperature overstepping, sillimanite grows as fibrolite because the surface energy of {110} faces is low and their growth rate is rapid. The same explanation may hold also for the growth of acicular corundum pseudomorphing andalusite prisms in sillimanite-free xenoliths.