Nepheline syenite (borolanite), Loch Borralan Igneous Complex

Outcrop

Nepheline syenite (borolanite), Loch Borralan Igneous Complex
The alkaline igneous rocks mostly contain no quartz, as all the spare silica is used up in making sodium- and potassium-rich minerals, such as feldspar and other rarer minerals like nepheline. This rock, with its distinctive round white patches, contains an unusual combination of minerals and textures that has caused geologists to give it its own name, borolanite. This photograph is of the borolanite in its "type locality", i.e. the place it was first discovered and described. This is in a small quarry at Aultivullin, near Loch Borralan.

Hand specimen

Nepheline syenite (borolanite), Loch Borralan Igneous Complex
The rock is grey overall, consisting of white patches (mostly potassium feldspar), grey material (feldspar and nepheline) and black minerals. It is intermediate in overall colour between a granite and a gabbro.

Thin section

Nepheline syenite (borolanite), Loch Borralan Igneous Complex
The most distinctive dark mineral in borolanite is a brown garnet, seen at the top and left. Some crystals are darker-coloured than others. The smaller olive-coloured patches are a dark mica, and the colourless material (right) is mostly large crystals of potassium feldspar, making up one of the white spots seen in the hand specimen.

Plane polarized light. Field of view 7 mm across.

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