(Fr- porphyre; Ger- Porphyr; Nor- porfyr; Rus- :@DL4D@&J6)
PORPHYRY (See also LEOPARD ROCKS entry.)
A. Porphyry. Diverse porphyries (widths - ca. 3.5 cm) from Egypt, Italy and Rumania fashioned as cabochons. Smithsonian Institution collection. (photo by L. Bolton)
B. Porphyry simulant. Kikkaseki, "stone/rock of chrysanthemum flower" (height - 36 cm), a metamorphic rock that contains clusters of carbonate crystals (dolomite, calcite or aragonite) arranged in a radial manner from Gifu Prefecture, central Japan. (© photo supplied by Nobuo Ishihara)
DESCRIPTION: Porphyry is the term geologists apply to igneous rocks that consist of relatively large grains or crystals, called phenocrysts, surrounded by a finer grained material, usually called the groundmass. Porphyries may be of just about any composition known for igneous rocks. Colors of the phenocrysts, which may or many not contrast markedly with the overall color of their surrounding groundmasses, may be virtually colorless, white, black or just about any color. Effective hardnesses of most porphyries range from from 6 to 7. Specific gravities range from 2.5 to nearly 4.
OTHER NAMES: Names for porphyries given by geologists in the field usually include three elements: 1. a geographical name indicating where the rock occurs; 2. its igneous rock name on the basis of its overall compostion; and 3.the word porphyry -- e.g., the Mount Rogers rhyolite porphyry of Grayson County, southwestern Virginia. Unfortunately, however, many porphyries have been recorded in the literature with no indication as to their overall composition - e.g., the Uncle Sam Porphyry that crops out on Uncle Sam Hill about 2½ miles southwest of Tombstone, Arizona.
Other names given include the following:
USES: Cabochons, eggs, spheres, figurines, etc.
OCCURRENCES: Typically as relatively small igneous masses, such as dikes; less commonly in border zones of larger igneous masses.
NOTEWORTHY LOCALITIES: See OTHER NAMES and REMARKS.
REMARKS: The reddish purple porphyry first quarried in Egypt during the Ptolemaic era (323-30 B.C.) has been referred to variously as porfido rosso antico, porphyrites leptosephos, lapis porphyrites and "The Stone of Rome," It is an altered andesite porphyry that consists of a dark reddish purple aphanitic groundmass -- the color apparently due primarily to the presence of the mineral piemontite (of the epidote group and in some literature referred to as withamite) -- plus relatively small white and flesh colored andesine (plagioclase feldspar) phenocrysts. This porphyry, apparently rediscovered by ancient Romans, was actively quarried from a dike on Jebel Dhokan Mountain, central eastern Egypt during Roman times. It has been fashioned into many relatively famous objects, some sculpted as recently as the 18th Century A.D. -- e.g., the bust of Alexander the Great, now in the Louvre (see Saudi Aramco World, November/December, 2003, p.46) and the head of Hadrian, which is in the British Museum at Bloomsbury, London, England -- as well as into such things as vases.
A porphyry quarried near the ancient cities of Sparti and Marathonisi, southern Greece that consists of a green groundmass and lighter colored green feldspar phenocrysts was known variously as marmor lacedaemonium viride and perfidio serpentino during the period often referred to as classical Greek.
The name llanite was given by one of America's famous early petrologists, Joseph Paxton Outings.
Kikkaseki (stone/rock of chrysanthemum flower) is not a porphyry, as has been reported (e.g., Dietrich, 1989). Instead, it is a metamorphic rock that contains clusters of carbonate crystals (dolomite, calcite or aragonite) arranged in radial groups. This rock, a gemrock in its own right, is used as an ornamental rock in Japan, and I am told it has been considered "a natural monument since 1941" (Nobuo Ishihara, personal communication, 1987) - [phenocryst-like masses are largely some carbonate mineral that effervesces with dilute HCl and has an inferior hardness as compared to typical phenocrysts.].
***Porphyry ware - Wedgewood china colored and patterned to resemble porphyry - [china can be seen with the naked eye or aided only by a handlens to be a single phase material.].
REFERENCES: No general reference.
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R. V. Dietrich © 2004
Revised: January 2, 2004
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