Rhomb Porphyry (1kg)
An igneous rock exhibiting a porphyritic texture containing large cubic crystals of feldspar within a fine grained matrix. Collected near Oslo, Norway. Each sample is supplied in a labelled card tray. Specimens are supplied in a polythene bag in lots of 1 kg with a label.
A porphyritic alkaline syenite composed of an alkali feldspar groundmass with augites having rhombohedral cross sections as the principal phenocryst minerals. It is formed when a column of rising magma is 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. Traditionally the term "Porphyry" refers to the purple- red form of this stone prized in antiquity for it's hardness & excellent building qualities particularly in Ancient Rome.