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What's porphyry?
Time:2011-9-18 9:44:39     Hits:20514     我要评论[0]

Porphyry has been used extensively for paving stones and building stones for its unique color and weatherproof character. China red porphyry is widely processed in to cobblestones, cube stone, paving slabs, kerbstones, palisades, block steps, flagstones, pool copings, and a seriers of urban furniture for garden decoration and urban landscaping.

Porphyry is a variety of from Greek and mean 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 fs "purple". Purple was the color of royalty, and the "Imperial Porphyry" was a deep brownish purple igneous rock with large crystals of plagioclase. This rock was prized for various monuments and building projects in Imperial Rome and later. Pliny's Natural History affirmed that the "Imperial Porphyry" had been discovered at an isolated site in Egypt in AD 18, by a Roman legionnaire named Caius Cominius Leugas (Werner 1998). It came from a single quarry in the Eastern Desert of Egypt, from 600 million year old andesite of the Arabian-Nubian Shield. The road from the quarry westward to Qena (Roman Maximianopolis) on the Nile, which Ptolemy put on his second-century map, was described first by Strabo, and it is to this day known as the Via Porphyrites, the Porphyry Road, its track marked by the hydreumata, or watering wells that made it viable in this utterly dry landscape. Porphyry was extensively used in Byzantine imperial monuments, for example in Hagia Sophia and in the "Porphyra", the official delivery room for use of pregnant Empresses in the Great Palace of Constantinople.

After the fourth century the quarry was lost to sight for many centuries. The scientific members of the French Expedition under Napoleon sought for it in vain, and it was only when the Eastern Desert was reopened for study under Muhammad Ali that the site was rediscovered by Burton and Wilkinson in 1823.

Subsequently the name was given to igneous rocks with large crystals. Porphyry 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 crystals, called 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. Many types of igneous rocks may display porphyrytic 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. 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, localized metal ore deposits such as those of gold, copper, molybdenum, lead, tin, zinc and tungsten.

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