Rhomb porphyry thin section exhibiting the textbook porphyritic texture of this rare igneous rock. The thin section shows a fine grained alkali feldspar groundmass with large phenocrysts of augite. Rhomb porphyry is actually classed as an alkaline syenite. It formed when a column of rising magma cooled in two stages. The first deep in the crust creating the larger crystals with a diameter of 2mm or more. In the final stage at much shallower depths or during ejection from a volcano the cooling is rapid creating the much finer matrix. This material was collected near Oslo in Norway. Each section is made to 30 microns thickness and the glass is 46 x 27 mm in size with a cover slip. Supplied in a protective card sleeve with a label, ideal for microscopy and educational purposes.