Felsic gneiss, Lewisian Gneiss Complex

North shore of Loch Roe, near Achmelvich

Outcrop

Felsic gneiss, Lewisian Gneiss Complex, Achmelvich
Much of the Lewisian gneiss is made up of interleaved bands of darker and lighter rock, so that in detail felsic, mafic and ultramafic gneiss can be found in the same small outcrop. The colour depends on the proportion of mafic minerals. The white and grey felsic varieties are the most abundant. This view is about 2 metres across.

Hand specimen

Felsic gneiss, Lewisian Gneiss Complex, Achmelvich
This sample was taken from the palest coloured layers at this locality. It consists of white feldspar and glassy grey quartz, and has very few mafic minerals. The arrangement of the quartz crystals into long streaks gives the rock a very strong gneissic texture. This would originally have been a sheet-like intrusion of granitic igneous rock, and it was very strongly deformed and recrystallized during high grade metamorphism.

Thin section

Felsic gneiss, Lewisian Gneiss Complex, Achmelvich
This view shows that at the microscopic scale the banding consists of alternating layers of clear quartz and dusty feldspar. One of the rare bands of mafic minerals is seen at the bottom right. At Scourie this would have contained pyroxene, but here at Achmelvich the high-grade mafic minerals have broken down and been replaced by medium-grade ones, and this dark mineral is blue-green hornblende.

Plane polarized light, field of view 5 mm across

Felsic gneiss, Lewisian Gneiss Complex, Achmelvich
The individual crystals of quartz and feldspar are made visible between crossed polars. Although the mineral bands are long and narrow they are made up of crystals that are roughly equidimensional. Feldspar and quartz are in shades of grey, hornblende mostly shows as yellow and orange.

Crossed polars, field of view 5 mm across

D.J. Waters, Department of Earth Sciences, May 2003