Ultramafic gneiss, Lewisian Gneiss Complex

Locality: Scouriemore peninsula, Scourie

Outcrop

Ultramafic gneiss, Lewisian Gneiss Complex, Scouriemore
The ultramafic gneiss is a rather uniform dark greenish-grey rock, with a layering that reveals itself as small ridges of harder and softer rock, seen here dipping steeply to the left. Ultramafic rocks are made up almost entirely of dark, mafic minerals, typically magnesium silicates such as olivine and pyroxene. Most of the Earth's mantle is made of ultramafic rock.

Ultramafic gneiss, Lewisian Gneiss Complex, Scouriemore
This photograph shows the layering in the ultramafic gneiss in more detail. Softer layers contain a greater proportion of olivine, while the harder layers contain mostly pyroxene. This was originally an igneous rock, formed when olivine and pyroxene crystals settled out of a magma. Later it was deformed and metamorphosed into a gneiss.

Hand specimen

Ultramafic gneiss, Lewisian Gneiss Complex, Scouriemore
This ultramafic rock consists entirely of mafic minerals. The hand specimen has a weathered surface (upper right) that shows an overall yellowish-brown colour with grey mineral grains standing out in relief. The yellowish colour comes from the weathering of olivine; the darker minerals are mostly pyroxene. The broken surface of the specimen (lower left) shows that all the minerals are dark coloured on a fresh surface. The rock is a gneiss because it shows alternating bands a few cm thick of different proportions of minerals - the banding is too coarse to be obvious in this hand specimen.

Thin section

Ultramafic gneiss, Lewisian Gneiss Complex, Scouriemore
The dominant mafic minerals can be seen in this view. Olivine is colourless, with black-outlined fractures. Pyroxene is of two kinds, showing here in pale pink and very pale green. Hornblende is pale olive-green. The black mineral is an opaque oxide.

Plane polarized light, field of view 6 mm across

Ultramafic gneiss, Lewisian Gneiss Complex, Scouriemore
The bright colours in this view are mostly shown by olivine (centre). The larger crystals are mostly pyroxene with some hornblende.

Crossed polars, field of view 6 mm across

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