Photo on left is from a thin section of a coarse grained granite taken with crossed polars. It is a classical example of an alkali granite, viz. where the feldspars are mainly potassic consisting of orthoclase and microcline. The granite was found on the beach near the ferry terminal of North Uist, grid ref. NF 923 812. It is probably a glacial erratic.
Most of the field of view consists of potassic feldspars with generally darker grey interference colours due to their low birefringence, and slightly higher birefringence quartz with generally brighter grey and white colours depending on their orientation with regard to the crossed polars.

The biotite crystal near bottom left, its interference colours masked by its natural brown colour, is 1.5mm in length.
The large crystal in the centre of the field of view is a form of potassic feldspar known as microcline. Here it shows the

characteristic cross hatched or "tartan" twinning.




Photo to left is a higher magnification view taken in plane polarised light of one of the biotite crystals. It is 1.8mm in length. At the bottom of the crystal can be seen a small inclusion of what is either zircon or sphene, probably the former, surrounded by a "halo". It is known as a pleochroic halo which is created by the bombardment of the atoms of the biotite crystal lattice by alpha particles from the radioactive inclusion.


Photo to right is of the thin section taken with a scanner and using crossed polars, hence the grey and white interference colours of the feldspars and quartz. The biotite micas, light to dark brown colours, average 1.5mm in length.

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