Porphyry (geology)
Porphyry is a very hard
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
Latin and means "
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 afirmed that the "Imperial Pophyry" 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.
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 Bruton 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, localised metal ore deposits such as those of
gold,
copper,
molybdenum,
lead,
tin,
zinc and
tungsten.
All the porphyry columns in Rome, the red porphyry togas on busts of emperors, the porphyry panels in the revetment of the
Pantheon, as well as the altars and vases and fountain basins reused in the Renaissance and dispersed as far as
Kiev, all came from the one quarry at
Mons Porpyritis ("Porphyry Mountain", the Arabic
Jabal Abu Dukhan), which seems to have been worked intermittently between
29 and
330, when
Constantine the Great celebrated the founding of his capital
Constantinople with a 30-meter (100') pillar, built of seven stacked porphyry drums, which still stands. A triumphant last use were the eight monolithic columns of porphyry that support
exedrae (semicircular niches) in
Hagia Sophia. Justinian's chronicler, Procopius, called the columns "a meadow with its flowers in full bloom, surely to make a man marvel at the purple of some and at those on which the crimson glows." (noted by Werner).
Byzantine historians distinguished emperors who got their power through a coup from those "
born to the purple" (
porphyrogenitus), from the imperial family in a room in
the palace veneered with purple porphyry that was described by
Anna Comnena, daughter of the eleventhth-century emperor Alexius I.
The imperial family were entombed in the purple as well, beginning with
Nero, the first to be immured in a porphyry sarcophagus. Roman sarcophagi were re-used for imperial burials in Sicily: the porphyry sarcophagi of Holy Roman Emperors
Frederick II and
Henry IV and king
William I of Sicily and the Empress
Constance, are preserved in the cathedrals of
Palermo and
Monreale.
The Romans used the Imperial porphyry for the monolithic pillars of
Baalbek's
Temple of Heliopolis in
Lebanon. Today there are at least 134 porphyry columns in buildings around Rome, all reused from imperial times, since the stone is not naturally present in Italy, and countless altars, basins and other objects.
Porphyry was used extensively for decoration in
Germany,
Poland, and
Czechoslovakia. 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.
In 1840,
Bonapartists recovered the body of
Napoleon I from Saint Helena and intended to bury in a porphyry sarcophagus in
Les Invalides, Paris.However, the Egyptian site was not available and a similar red
quartzite from Finland was chosen, in spite of buying it from the Russian Empire, an enemy of France.
*
Dacite porphyry
*
Trachyte/
latite porphyry
*
Diorite porphyry
*
Granite porphyry
*
Rhyolite porphyry
*
basalt porphyry
* see also
porphyritic texture* see also
list of rock texturesRhomb porphyry
Rhomb porphyry is a
volcanic rock with gray-white large
porphyritic rhomb shaped
phenocrysts enbedded in a very fine grained red-brown
matrix. The composition of rhomb porphyry place it in the
trachyte -
latite classification of the
QAPF diagram.
Rhomb porphyry
lavas are known only from three
rift areas: The
East African Rift (including
Mount Kilimanjaro),
Mount Erebus near the
Ross Sea in
Antarctica, and the
Oslo graben in
Norway.
*
Roman occupation of the Eastern Desert of Egypt and the Imperial Porphyry quarries.*
Mons Porphyrites quarries, Egypt*
A visit to the ancient Imperial Porphyry quarries in Egypt.*
Rhomb porphyry lavas*
Flash showing rhomb porphyry formation*
Saudi Aramco World Louis Werner, "Via Porphyrites" November/December 1998
