Andesite thin section microscope slide exhibiting an intermediate, fine grained groundmass with occasional phenocrysts of plagioclase feldspar. Andesite is an extrusive rock, intermediate in composition between a rhyolite and basalt. It is the extrusive equivalent of diorite with a silica content of 52-63% plus pyroxene ( Augite ) & feldspars ( Plagioclase ). Andesite lava is of moderate viscosity and forms thick lava flows and domes. The name Andesite is derived from the Andes Mountains in South America, where it can be found in abundance.This material was collected from Borrowdale in the Lake District, UK & is of Late Ordovician age. The glass is 46 x 27 mm in size, supplied in a protective card sleeve with a label, ideal for microscopy, petrography and educational use.