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Journal of Petrology 2004 45(7):1467-1479; doi:10.1093/petrology/egh022
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Journal of Petrology 45(7) © Oxford University Press 2004; all rights reserved

Towards a Better Understanding of the Fibrolite Problem: the Effect of Reaction Overstepping and Surface Energy Anisotropy

R. SASSI1,*, C. MAZZOLI1,2, R. SPIESS1,2 and T. CESTER1

1 DEPARTMENT OF MINERALOGY AND PETROLOGY, UNIVERSITY OF PADOVA, C.SO GARIBALDI 37, 35137 PADOVA, ITALY
2 IST. GEOSCIENZE GEORISORSE, CNR, C.SO GARIBALDI 37, 35137 PADOVA, ITALY

* Corresponding author. Telephone: +39-049-8272019. Fax: +39-049-8272010. E-mail: raffaele.sassi{at}unipd.it.

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 750°C solvus that reflects near-equilibrium conditions of regional metamorphism. Plagioclase containing fibrolite plots closer towards the 950°C solvus, reflecting the tendency of plagioclase to re-equilibrate at high temperature during pyrometamorphism by a fibrolite-forming reaction:

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.

KEY WORDS: andalusite breakdown; fibrolite; pelitic xenoliths; reaction overstepping; sillimanite


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R. Sassi, G. Cruciani, C. Mazzoli, L. Nodari, and J. Craven
Multiple titanium substitutions in biotites from high-grade metapelitic xenoliths (Euganean Hills, Italy): Complete crystal chemistry and appraisal of petrologic control
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