This rock was deposited as either a siltstone or as banded silt and mudstone in the late Proterozoic period, before being subject to regional metamorphism. It now forms part of the Dalradian succession within the ‘staurolite’ Barrovian metamorphic zone (Barrow zones include chlorite-biotite-garnet-staurolite-kyanite-sillimanite). The sample comes from an area between Letterfrack and Dawros, Connemara, County Galway, Ireland, part of the Irish Dalradian sequence that has been correlated with similar rocks in Scotland.
The thin section contains many small, inclusion poor grains of staurolite, which is a high relief mineral pleochroic with colours ranging from pale yellow to almost colourless in plane polarised light, and with low birefringence colours in crossed polars. The tectonic fabric of the rock is defined by muscovite and biotite micas, which alternate with layers dominated by quartz and feldspar. The Barrow indicator minerals garnet (in the bottom left corner of the section) and biotite are present, but the grade of the rock is defined by the presence of staurolite since it is the highest indicator mineral in Barrow’s classification.
