Felsic gneiss, Lewisian Gneiss Complex

Scouriemore peninsula, SW of Scourie village

Outcrop

Felsic gneiss, Lewisian Gneiss Complex
This outcrop photograph shows the mineral banding, technically called a gneissic foliation, typically shown by Lewisian felsic gneisses. It is not a perfectly regular layering, but it pinches and swells (as shown here) or is bent into tight folds. During high grade metamorphism rocks are hot enough, and deformed slowly enough, to flow somewhat like plasticine. The hammer is 30 cm long.

Felsic gneiss, Lewisian Gneiss Complex
This photograph shows in detail the mineral banding in the felsic gneiss outcrop from which the hand specimen was taken. Dark pyroxene crystals are arranged into narrow, parallel bands in a rock mainly composed of light-coloured feldspar and quartz. The hand lens is just over 2 cm across.

Hand specimen

Felsic gneiss, Lewisian Gneiss Complex, Scouriemore
This cut surface of a felsic gneiss shows the alignment of dark mineral bands in the matrix of lighter felsic minerals. This is a typical gneissic texture. Like many very high grade metamorphic rocks, the fresh rock is quite dark coloured even though it is made mostly of quartz and feldspar.

Thin section

Felsic gneiss, Lewisian Gneiss Complex, Scouriemore
Under the microscope we see trains of pink and green pyroxene, following the gneissic banding, interspersed with clear feldspar and quartz. This is a very typical texture for a high grade metamorphic rock.

Plane polarized light, field of view 6 mm across

Felsic gneiss, Lewisian Gneiss Complex, Scouriemore
Between crossed polars quartz and feldspar appear in shades of white, grey and black. The narrow striped patterns result from changes in the orientation of the crystal structure known as twinning, and are seen only in feldspar, not in quartz. Pyroxene shows a variety of red, yellow and blue colours.

Crossed polars, field of view 6 mm across

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