The origin of fumarolitic andradite at Menoyre,France and Fant'Ale,Ethiopia

Recent discoveries of fumarolitic andradite in alkaline or peralkaline trachytes (Menoyre, Cantal, France, and Fant'Ale, Ethiopia) are interpreted genetically.The andradites were formed from the constituents of pyroxene, always calcic and iron-rich in these rocks. The elements Ca and Fe were fixed on the surface of fractures by late fumaroles of high oxidizing capacity.The following reaction can be established at Menoyre: hedenbergite+O₂andradite+hematite+cristobalite.In a hyperalkaline medium, which is the case for the trachyte of Fant'Ale, the reaction is: Hedenbergite+sodium metasilicate+O₂andradite+aegirinic pyroxene+cristobalite.