Porphyry (geology)

(For other meanings of Porphyr, see Porphyry)

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

Porphyry is a groundmass, which is a rock "structure". Its chief characteristic is a large difference between the size of the tiny matrix crystals and other much larger crystals. 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. Many types of igneous rocks may display porphyry.

Formation

Porphyry deposits are formed when a column of rising magma is cooled in two stages. In the first stage, the magma is cooled slowly by contact with overlying rocks, creating the large crystal grains, with a diameter of 2 mm or more. In the final stage, the magma is cooled rapidly as it erupts from a volcano, creating small grains that are usually invisible to the unaided eye. The cooling also leads to a separation of dissolved metals into distinct zones. This process is one of the main reasons for the existence of rich, localised metal ore deposits such as those of gold, copper, molybdenum, lead, tin, zinc and tungsten.

Porphyry in history

In the ancient world, porphyry was quarried only in Egypt and was reserved for the use of the Pharaoh. In the Byzantine Empire, the palace room reserved for royal births was lined with porphyry, and the emperors born in this room were referred to as porphyrogenitus ('born in the purple').

Porphyry was used extensively for decoration in the south eastern portions of Germany, Poland, and Czechloslovakia . This can be seen in the Mannerist style sculpted portal outside the chapel entrance in Colditz Castle.

Louis XIV King of France obtained the largest collection of porphyry by acquiring the Borghèse collection.

Example Porphyries

• Dacite porphyry
• Diorite porphyry
• Granite porphyry
• Rhyolite porphyry