£1.00 – £2.74
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. Each sample is supplied in a labelled card tray.
Rhomb porphyry is an 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. This rock specimen is available in various optional sizes as well as in a 3" size piece with a cut and lacquered face to detail the crystal structure of the rock in much clearer detail.