Pegmatite, Lewisian Gneiss Complex

Scouriemore peninsula, SW of Scourie village

Outcrop

Pegmatite, Lewisian Gneiss Complex, Scouriemore
Pegmatites are extremely coarse-grained igneous rocks. They crystallise from melts that are very rich in dissolved water. Material moves about very easily in these wet melts, and crystals can grow very large in a short time. This pegmatite forms a sheet-like intrusion cutting across mafic, ultramafic and felsic gneisses. It consists of pink potassium feldspar, in crystals up to one metre across, with some white sodium feldspar, white quartz, and occasional "books" of dark biotite mica.

Pegmatite, Lewisian Gneiss Complex, Scouriemore
A closer view of the outcrop shows huge feldspar and quartz crystals, and also reveals that the feldspars are intergrown in a regular pattern with crystals of quartz. This is sometimes called a graphic intergrowth, because it looks a bit like the writing on Babylonian clay tablets. The hammer head is 15 cm across.

Hand specimen

Pegmatite, Lewisian Gneiss Complex, Scouriemore
This specimen is a small fragment of a single orange-pink potassium feldspar crystal from the pegmatite. The streaks of intergrown quartz can also be seen.

Thin section

Pegmatite, Lewisian Gneiss Complex, Scouriemore
This view shows dusty feldspar and clearer quartz. The feldspar is all part of a single huge crystal.. The red-brown dustiness is iron-rich material that gives the feldspar its pink colour in hand specimen and outcrop.

Plane polarized light, field of view 6 mm across

Pegmatite, Lewisian Gneiss Complex, Scouriemore
The feldspar (dark) is a single crystal, whereas the quartz is made up of a number of different crystals. For example, there is a row of small quartz crystals along the margin of the feldspar.

Crossed polars, field of view 6 mm across

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