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The Fibrolite Problem: Implications for Reaction Overstep Studies

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Journal of the Geological Society of India

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 ΔS and ΔV 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.

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Acknowledgments

The authors sincerely thank the anonymous reviewer whose suggestions helped improve this manuscript. JJS acknowledges the University Grants Commission for providing UGC-SRF scholarship (UGC-Ref. No. 414/(CSIR-UGC NET DEC. 2016)). The present work is a contribution under the MoES project “Age constraints on metamorphic evolution of the TransHimalayas” (MoES/P.O. (Geol/101[b]/2017) to NCP&AS and the financial assistance is thankfully acknowledged.

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Correspondence to Naresh Chandra Pant.

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Sharma, J.J., Pant, N.C. & Saikia, A. The Fibrolite Problem: Implications for Reaction Overstep Studies. J Geol Soc India 98, 1691–1696 (2022). https://doi.org/10.1007/s12594-022-2239-z

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  • DOI: https://doi.org/10.1007/s12594-022-2239-z

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