Mafic gneiss, Lewisian Gneiss Complex

The photographs and samples come from outcrops of mafic gneiss on the Scouriemore peninsula, SW of Scourie village.

Outcrop

Mafic gneiss, Lewisian Gneiss Complex, Scouriemore
This outcrop of the mafic gneiss shows a banding of minerals. Thin dark layers are rich in pyroxene. The rest of the rock is quite rich in feldspar and contains rounded brownish-red garnets. The hammer is 30 cm long.

Mafic gneiss, Lewisian Gneiss Complex, Scouriemore
This variety of mafic gneiss is much richer in dark pyroxene. Notice that the large red garnets have a halo of pale feldspar around them. This formed by a chemical reaction between garnet and pyroxene during the metamorphism. Chemical reactions like this are what cause the transformation of sedimentary and igneous rocks into metamorphic rocks, and they are driven by changes in temperature or pressure.

Hand specimen

Mafic gneiss, Lewisian Gneiss Complex, Scouriemore
This sample comes from a thick band of dark pyroxene-rich mafic gneiss. There is no noticeable mineral alignment in the hand specimen. Red garnet crystals 2 to 3 mm across are scattered evenly through the rock, and most of them have a narrow rim of white feldspar.

Thin section

Mafic gneiss, Lewisian Gneiss Complex, Scouriemore
The mineral in the centre is garnet, surrounded by a halo of clear feldspar. The other grey-green and grey-brown mineral grains are two varieties of pyroxene. What distinguishes this metamorphic rock from an igneous rock of the same composition is: a) the mineral grains do not show their own crystal shapes, but instead fit together rather like crazy paving, because they grew up against one another in the solid state rather than crystallizing from a liquid; b) the minerals are arranged into concentric patterns, because they were formed by chemical reactions between neighbouring minerals in the solid state.

Plane polarized light, field of view 7 mm across

Mafic gneiss, Lewisian Gneiss Complex, Scouriemore
Garnet, in the centre, is always black between crossed polars. Feldspar is striped in shades of white, grey and black. Pyroxene shows brighter colours.

Crossed polars, field of view 7 mm across

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