Felsic gneiss (Laxfordian), Lewisian Gneiss Complex

Locality: road-cut on north shore of Loch Laxford

Outcrop

Laxfordian felsic gneiss, Lewisian Complex, Loch Laxford
This is the typical appearance of the gneiss on the north side of Loch Laxford. The main feature of the gneiss at this scale is the banding of grey and white layers. This banding is also folded. The white bands are coarser grained and very poor in mafic minerals. The grey bands contain mafic minerals, principally biotite mica. Some of the sheets of white material look as if they have been intruded after the formation of the banding, as they cut across it. At the right hand side there is also a small sheet of pink granite. Gneissic rocks in Precambrian terrains are often as complex as this.

Laxfordian felsic gneiss, Lewisian Complex, Loch Laxford
This close-up view of a more uniform part of the gneiss reveals the streaky banding of dark minerals and light minerals. Even in this photo the banding is interrupted here and there by veins of pink granite. The boundaries betwen gneiss and granite are not at all sharp. Both rocks formed when they were very hot: the gneiss from solid rock near its melting point, the granite from the crystallisation of small amounts of melted rock, formed nearby and introduced in small veins.

Hand specimen

Laxfordian felsic gneiss, Lewisian Complex, Loch Laxford
This rock is a biotite gneiss. It is mostly composed of creamy-white feldspar and quartz, together with dark minerals, of which the most abundant is the dark mica biotite. The dark minerals are arranged in a streaky banding, giving the rock a gneissic texture. The flakes of mica are also aligned with their long dimensions parallel to the mineral banding.

Thin section

Laxfordian felsic gneiss, Lewisian Complex, Loch Laxford
This gneiss contains essentially the same minerals as a granite, but is distinguished at this scale from an unmetamorphosed igneous rock by the alignment of the dark minerals. The colourless minerals are quartz and feldspar, the feldspar appearing dusty with alteration. The elongate dark brownish minerals are biotite mica, and at lower left there is some greenish hornblende.

Plane polarized light, field of view 6 mm across

Laxfordian felsic gneiss, Lewisian Complex, Loch Laxford
In this view, feldspars and quartz appear in shades of grey to black, while hornblende and biotite mica are brown and greenish-brown. The banding of the gneiss is apparent from the elongate shapes of the minerals, from top right to bottom left.

Crossed polars, field of view 6 mm across

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