Hercynite as a breakdown product of staurolite from within the aureole of the Ardara Pluton, Co. Donegal, Eire

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.