PORPHYRY, in petrology, a beautiful red volcanic rock (Gr. wopOpEos, Lat. purpureus, purple), which was much used by the Romans for ornamental purposes when cut and polished. The famous red porphyry (porfido rosso antico) came from Egypt, but its beauty and decorative value were first recognized by the Romans in the time of the emperor Claudius. It was ob tained on the W. coast of the Red Sea, where it forms a dike 8o or 90 f t. thick. For a long time the knowledge of its source was lost, but the original locality has been re-discovered at Jebel Dhokan, and the stone is again an article of commerce. In a dark red ground-mass it contains many small white or rose-red plagio clase felspars, black shining prisms of hornblende, and small plates of iron oxide. The red of the felspars and of the ground mass is unusual in rocks of this group, and arises from the par tial conversion of the plagioclase felspar into thulite and man ganese-epidote. These minerals also occur in thin veins crossing the rock. Many specimens show effects of crushing and in ex treme cases this has produced brecciation.
Many igneous rocks possess the scattered crystals of larger size in a fine-grained ground-mass which characterize the por phyries (see PETROLOGY), and most lavas and many of the rocks which occur as dikes and sills have porphyritic structure. The use of the term porphyry is now restricted to a series of rocks which are of intrusive origin and contain much porphyritic fel spar (with or without quartz or nepheline). The porphyritic in trusive rocks with large crystals of augite, olivine, biotite and hornblende are for the most part grouped under the lampro phyres. Furthermore, it has become usual to subdivide the por phyries into two classes ; in one of these the phenocrysts are mainly orthoclase, in the other mainly plagioclase felspar. The first series is known as the "porphyries," the second as the "porphy rites." There are porphyries which correspond chemically and mineralogically to granites, syenites and nepheline-syenites; while the porphyrites form a series parallel to the diorites, norites and gabbros. In each case the porphyritic type occurs generally
as dikes and thin sheets which consolidated beneath the surface but probably at no great depth (hypabyssal rocks).
The principal subdivisions of the group are the granite-por phyries, the syenite-porphyries and the elaeolite-porphyries. In all of them porphyritic orthoclase or alkali felspar is the char acteristic mineral. The granite-porphyries and quartz-porphyries consist mainly of orthoclase, quartz and ferro-magnesian mineral, usually biotite but sometimes hornblende, augite or en statite. Granite-porphyries are exceedingly common in all regions where acid intrusive rocks occur. Many granite masses are sur rounded by dikes of this kind, and in some cases the chilled margin of a granite consists of typical porphyry.
The syenite-porphyries, like the syenites, are less common than the granite-porphyries and granites. They are characterized by an abundance of orthoclase and a scarcity or absence of quartz. The phenocrysts are orthoclase (and oligoclase), biotite, horn blende or augite ; the ground-mass is principally alkali felspar with sometimes a little quartz. In many specimens the felspars of the second generation form a mosaic of ill-shaped grains, in others they are little rectangular crystals which may have a fluxion arrangement (orthophyric type of ground-mass). Some of the rocks formerly known as orthoclase-porphyries belong to this group; others are ancient trachytic lavas (orthophyres). Closely related to the syenite-porphyries is the rhomb-por phyry of south Norway and West Africa. In these the large felspars have rhomb-shaped sections owing to their peculiar crystalline development. Olivine, augite and biotite occur in these rocks, but there is no quartz or soda-lime felspar. The porphyritic felspars contain both soda and potash and belong to anorthoclase. Rhomb-porphyries occur as dikes connected with the syenites (laurvikites of southern Norway), and many ice-borne boulders of these rocks have been found among the drift deposits of the east of England.