Granite
(porphyritic); Shap, Cumbria
In terms of its mineral content this is a typical granite,
consisting of pink potassium feldspar, cream sodium feldspar
(plagioclase), grey quartz and black biotite mica. The most
striking feature of its texture is the large pink angular
potassium feldspar crystals (one of which is indicated by
the yellow pointer). This texture, of large well-formed
crystals in a finer matrix, is called porphyritic. It
indicates that some crystals formed early in the magma, and
had quite a long time to grow large before the magma was
brought into the place where it finally cooled and
crystallized. Porphyritic texture is quite common in
granites, as well as in lavas erupted from volcanoes.