Granite, Lewisian Complex

Locality: road-cut on north shore of Loch Laxford

Outcrop

Granite, Lewisian Complex, Loch Laxford
Coarse-grained pink granite crystallized from acid magma intruded in sheet-like bodies into grey gneisses. Here we see the contact between pink granite and banded grey gneiss. The contact cuts across the banding in the gneiss at a small angle, and an offshoot of the granite forms a vein that cuts across the banding at a high angle.

Hand specimen

Granite, Lewisian Complex, Loch Laxford
This cut surface of a hand specimen shows the typical minerals of a granite. There are two varieties of feldspar: potassium feldspar is pink in colour, sodium feldspar is white. Quartz is glassy and transparent, appearing grey in hand specimen. These three minerals make up more than 95% of the rock Small amounts of black minerals are present: these are magnetite (iron oxide) and biotite (dark mica). The texture consists of interlocking crystals up to 1 cm across. Although in many granites the larger feldspars may show regular blocky shapes, in this one they are mostly more rounded.

Thin section

Granite, Lewisian Complex, Loch Laxford
In thin section the major minerals of the granite are colourless and show little contrast. Plagioclase (sodium feldspar) shows some alteration to dusty-looking material, and so can be distinguished in part from potassium feldspar and quartz. The opaque (black) mineral at top left is magnetite.

Plane polarized light, field of view 6 mm across

Granite, Lewisian Complex, Loch Laxford
The minerals of a granite are more easily distinguished with crossed polars. Quartz is clear and evenly illuminated, but the feldspars show striped and "tartan" patterns that result from twinning (systematic changes in orientation) in the crystal lattice. The "tartan" pattern is characteristic of potassium feldspar.

Crossed polars, field of view 6 mm across

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