gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1978.042.322.10 Hercynite as a Breakdown Product of Staurolite from Within the Aureole of the Ardara Pluton, Co. Donegal, Eire Atkin Brian P. Department of Geology, University of Nottingham, University Park, Nottingham NG7 2RD 6 1978 42 322 237 239 28 10 1977 13 12 1977 Copyright © 1978, The Mineralogical Society 1978 Summary

Zincian hercynite is present in pelites within the aureole of the Ardara Pluton. The hercynite is always surrounded by cordierite, garnet, or feldspar and is never in contact with quartz. From textural and chemical evidence it is apparent that the hercynite is produced by the reaction of zincian staurolite → zincian hercynite + cordierite + sillimanite. A temperature of about 725°C at pressure of about 2·6 kb and anφo2 below that defined by the Q.F.M. buffer is postulated for this reaction.