Summry: A series of calc-silicate skarn veins is described; these occur in a limestone mass surrounded by granite in the Dalradian of western Donegal.
The skarn which borders these veins exhibits a zonal arrangement of the constituent minerals, idocrase and garnet, wollastonite, diopside and tremolite, which are enveloped by a zone of decolorized limestone. A central quartz-felspar dikelet is present in some cases.
The processes envisaged are the removal of graphite to an outer zone by means of a reversible reaction involving carbon dioxide or water; the addition of silica to yield wollastonite in an inner zone and diopside and tremolite in outer cooler zones, followed by an influx chiefly of alumina to produce idocrase and grossular by replacement of the wollastonite.
The sequence of zones is that of the established order of silicate formation with increasing temperature in a dolomite with excess quartz. The presence of wollastonite and calcite in equilibrium with quartz is taken to indicate a temperature lower than 750° C., and a temperature higher than 550° C. is probable for reactions involving the removal of graphite.