Porphyry

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Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass. The larger crystals are called phenocrysts. In its non-geologic, traditional use, the term 'porphyry' refers to the purple-red form of this stone, valued for its appearance.

The term 'porphyry' is from Greek and means 'purple'. Purple was the color of royalty, and the 'Imperial Porphyry' was a deep purple igneous rock with large crystals of plagioclase. This rock was prized for various monuments and building projects in Imperial Rome and later.


Subsequently the name was given to igneous rocks with large crystals. Porphyritic now refers to a texture of igneous rocks. Its chief characteristic is a large difference between the size of the tiny matrix crystals and other much larger phenocrysts. Porphyries may be aphanites or phanerites, that is, the groundmass may have invisibly small crystals, like basalt, or the individual crystals of the groundmass may be easily distinguished with the eye, as in granite. Most types of igneous rocks may display some degree of porphyritic texture.


Porphyry deposits are formed when a column of rising magma is cooled in two stages. In the first stage, the magma is cooled slowly deep in the crust, creating the large crystal grains, with a diameter of 2 mm or more. In the final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano, creating small grains that are usually invisible to the unaided eye.

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