Quartz syenite, Loch Borralan Igneous Complex

From scree material at the roadside on the north shore of Loch Borralan.

Hand specimen

Quartz syenite, Loch Borralan Igneous Complex
This pale creamy-pink rock is made up mostly of potassium feldspar, with a little quartz. If it had a somewhat higher proportion of quartz, it would be a granite. It has the typical texture of a slowly-cooled igneous rock, with interlocking regular-shaped feldspar crystals.

Quartz syenite, Loch Borralan Igneous Complex
This view of the cut surface of the sample shows the igneous texture of blocky interlocking tablets of pink feldspar. Greyish-white quartz fiills the more irregular spaces between feldspar crystals.

Thin section

Quartz syenite, Loch Borralan Igneous Complex
The potassium feldspar appears dusty in thin section (this dust is what imparts the pink colour in the specimen). Clearer quartz occupies the spaces betwen the feldspars.

Plane polarized light, field of view 6 mm across

Quartz syenite, Loch Borralan Igneous Complex
Between crossed polars the individual feldspar crystals are easily distinguished. The texture also gives the impression of a sequence of crystallization from the melt as the magma cooled. The larger feldspars (e.g. at left and right) formed first, and the magma in the spaces betwen them finally crystallized to an interlocking aggregate of smaller crystals of feldspar and quartz (centre).

Crossed polars, field of view 6 mm across

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