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refers to a category of igneous rocks, which consist of, crystals with large grains like feldspars and quartz. These crystals are referred to as phenocrysts. The name "porphyry" is from a Greek word meaning "purple". Being that the colour purple depicted royalty, the "Imperial Porphyry", used in Imperial Rome for monuments and buildings, is a deep purple igneous rock consisting of large crystals.

An igneous rock is termed "porphyritic" depending on its texture. It's characterised by the size of the crystals. Porphyries can either be phanerites or aphanites, with the rocks having microscopic crystals such as basalt or they can have moderate sized crystals which can be seen with the naked eye like granite.

Deposits of porphyry are formed during the cooling stages of magma. During the first stage, the lava slowly cools, deep within the Earths crust, and this creates large-grained crystals. During the last stage, the lava cools rapidly either as it is erupted or at a low depth beneath the Earth crust. This forms small grains unseen to the naked eye.

Kinds of Porphyry

Porphyry copper: The division of dissolved metals into various regions is caused by the different cooling stages creating porphyritic textures in the hypabyssal as well as the intrusive rocks. During this process, fluids move away from the cooling lava, giving rise to the existence of some valuable metal ore deposits which include the deposits of copper, gold, lead, molybdenum, tungsten and zinc. These deposits are collectively referred to as Porphyry copper deposits.

Rhomb Porphyry: This is a large, intrusive, gray-white rock with its phenocrysts in a red-brown ground mass with find grains. The lavas can be found in only three areas: The Mount Kilimanjaro, Mount Erebus and the Oslo graben.

The Imperial Porphyry was formed in 18 AD in Egypt and porphyry itself has been in use since 1850 BC in Rome and the Renaissance. When the term Porphyry is used by Geologists, it is to describe texture, and not composition of a rock.

Additional Resources

Plagioclase   Potassium Oxide   Protolith

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