1911 Encyclopædia Britannica/Porphyry (rock)

by John Smith Flett

PORPHYRY (Gr. Πορφύρεος, Lat. purpureus, purple), in petrology, a beautiful red volcanic rock 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 obtained on the west coast of the Red Sea, where it forms a dike 80 or 90 ft. thick. For a long time the knowledge of its source was lost, but the original locality, marked by many ancient quarries, 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 plagioclase felspars, black shining prisms of hornblende, and small plates of iron oxide. The red colour of the felspars and of the ground-mass is unusual in rocks of this group, and arises from the partial conversion of the plagioclase felspar into thulite and manganese-epidote. These minerals also occur in thin veins crossing the rock. Many specimens show effects of crushing and in extreme cases this has produced brecciation. Another famous porphyry, hardly less beautiful, is the verde antique, porfido verde antico, or marmor lacedaemonium viride of Pliny, which was obtained between Lebetsova and Marathonisi in Peloponnesus. It has the same structure as the red porphyry as it contains large white or green felspars in a fine ground-mass. The green colour arises from the abundant formation of chlorite and epidote in the large felspars and throughout the rock. In ancient times it was much used as an ornamental stone, these two varieties of porphyry making a fine contrast with one another. Green porphyries are not so rare as red. A similar rock is obtained at Lambay Island near Dublin. They are still used extensively, especially for small ornaments. Large pieces are difficult to obtain free from flaws, and marble is preferred for mural work, not only because of the greater variety of patterns but also because it is much softer and more easily cut and polished.

Many igneous rocks possess the structure which characterizes these porphyries (see Petrology, Plate III.): the presence of scattered crystals of larger size in a fine-grained ground-mass. Most lavas, and many of the rocks which occur as dikes and sills, have porphyritic structure. These may be called porphyries and this term has consequently been applied to a great variety of rocks, e.g. diorite-porphyry, granite-porphyry, greenstone-porphyry, augite-porphyry, liebenerite-porphyry, &c. More recently the use of the term has been restricted to a series of rocks which are of intrusive origin and contain much porphyritic felspar (with or without quartz or nepheline). The porphyritic intrusive rocks with large crystals of augite, olivine, biotite, and hornblende are for the most part grouped under the lamprophyres; while the term porphyry is rarely now applied to any of the effusive rocks or lavas. Furthermore, it has become usual to subdivide the intrusive porphyries 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,” while the second group is called “porphyrites.” There are porphyries which correspond chemically and mineralogically to granites, syenites, and nepheline-syenites; while the porphyrites form a parallel series 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); While granite, gabbro and the other holocrystalline non-porphyritic rocks belong to the plutonic or abyssal group which cooled very slowly at great depths and under enormous pressure.

The principal subdivisions of the group are the granite-porphyries, the syenite-porphyries and the elaeolite-porphyries. In all of them porphyritic orthoclase or alkali felspar is the characteristic mineral. The granite-porphyries and quartz-porphyries (q.v.) consist mainly of orthoclase, quartz and ferro-magnesian mineral, usually biotite but sometimes hornblende, augite or enstatite.

Granite-porphyries are exceedingly common in all regions where acid intrusive rocks occur. Many granite masses are surrounded 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, hornblende 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 rhomben-porphyry 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. Rhomben-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.

Elaeolite- and leucite- (syenite) porphyries form apophyses and dikes around nepheline- and leucite-syenite intrusions. The former contain porphyritic nepheline which is often weathered to soft, finely crystalline aggregates of white mica and other secondary products as in the well-known liebenerite-porphyry of Tirol and gieseckite-porphyry of Greenland. The felspars of these rocks are albite, orthoclase and anorthoclase, and they often contain soda-augite and amphiboles. Elaeolite-porphyries occur along with nepheline-syenites in such districts as the Serra de Monchique, south Norway, Kola, Montreal. Allied to them are the tinguaites (so called from the Serra de Tingua, Rio de Janeiro, Brazil), which are pale green rocks with abundant alkali felspar nepheline, needles of green aegirine, and sometimes biotite and cancrinite. As a rule, however, these are not porphyritic. Some authors group the tinguaites with the aplites rather than the porphyries. Grorudites are quartz-tinguaites free from nepheline, and sölvsbergites are tinguaitic rocks in which neither quartz nor nepheline occur. The two last varieties have been described from the Christiania district in Norway, but tinguaites are known with nepheline-syenites in many parts of the world, e.g. Norway, Brazil, Portugal, Canada, Sweden, Greenland.

The following analyses of porphyries of different types will show the chemical composition of a few selected examples:—

	SiO2	Al2O3	Fe2O3	FeO	MgO	CaO	K2O	Na2O	H2O
I.	72.51	13.31	tr.	3.87	1.50	0.60	6.65	0.43	0.60
II.	67.18	16.65	0.55	2.15	1.54	2.35	2.91	4.03	0.75
III.	71.60	13.60	2.40	—	0.21	2.30	3.53	5.55	0.70
IV.	58.82	21.06	3.26	0.70	1.38	3.03	3.70	6.83	1.26
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V.	45.18	23.31	6.11		1.45	4.62	11.16	5.94	1.14
VI.	54.46	19.96	2.34	3.33	0.61	2.12	8.68	2.76	5.20
VII.	75.20	12.65	1.53	0.28	0.26	0.60	4.14	5.67	0.12

I., Elvan or granite porphyry (with pinite after cordierite)—Prah sands, Cornwall. II., Granophyry—Armboth, Cumberland. III., Granophyre—Carrock Fell, Cumberland. IV., Rhomben-porphyry—Tönsterg, Norway. V., Elaeolite porphyry—Beemerville, New Jersey. VI., Tinguaite—Kola. VII., Grorudite—Assynt, Scotland.

Porphyrites.—The porphyrites as above mentioned are intrusive or hypabyssal rocks of porphyritic texture, with phenocrysts of plagioclase felspar and hornblende, biotite or augite (sometimes also quartz) in a fine ground-mass. The name has not always been used in this sense, but formerly signified rather decomposed andesitic and basaltic lavas of Carboniferous age and older. Both the red porphyry and the green porphyry of the ancients are more properly classified in this group than with the granite-porphyries, as their dominant felspar is plagioclase and they contain little or no primary quartz. Porphyrites occur as dikes which accompany masses of diorite, and are often called diorite-porphyrites; they differ from diorites in few respects except their porphyritic structure. The phenocrysts are plagioclase, often much zoned with central kernels of bytownite or labradorite and margins of oligoclase or even orthoclase. In a special group there are corroded blebs or porphyritic quartz: these rocks are called quartz-porphyrites, and are distinguished from the granite-porphyries by the scarcity or absence of orthoclase. The hornblende of the porphyrites is often green but sometimes brown, resembling that of the lamprophyres, a group from which the porphyrites are separated by their containing phenocrysts of felspar, which do not occur in normal lamprophyres. Augite, when present, is nearly always pale green; it is not so abundant as hornblende. Dark brown biotite is very common in large hexagonal plates. Muscovite and olivine are not represented in these rocks. The ground-mass is usually a crystalline aggregate of granular felspar in which plagioclase dominates, though orthoclase is rarely absent. The Alpine dike rocks known as ortlerites and suldenites are porphyrites containing much green or brown hornblende and augite; these, however, hardly require a distinctive designation. Diorite-porphyrites have almost as wide a distribution as granite-porphyries, and occur in all parts of the world where intrusions of granite and diorite have been injected; they are in fact among the commonest hypabyssal rocks.

To gabbros and norites certain types of porphyrite correspond which have the same mineral and chemical composition as the parent rocks but with a porphyritic instead of granitic structure. Gabbro-porphyrites are not numerous; or rather most of these rocks are described as porphyritic basalts and dolcrites. The beerbachites are finely granular dike rocks resembling gabbros in all respects except in their being less coarsely crystalline. Norite-porphyrites have porphyritic plagioclase (labradorite usually) with hypersthene or bronzite, often altered to bastite. They accompany norite masses in Nahe (Prussia) and Tirol. They have vitreous forms which are described as andesitic-pitchstones or hypersthene-andesites.

	SiO2	Al2O3	Fe2O3	FeO	MgO	CaO	K2O	Na2O	H2O
I.	64.94	17.50	0.69	3.94	2.59	2.83	3.11	3.44	1.36
II.	61.58	18.84	4.68	—	6.59	2.04	1.49	4.27	1.61
III.	56.85	16.70	5.92	7.13	5.97	3.25	1.91	2.78	0.54

I., Quartz-porphyrite—Lippenhof, Schwarzwald. II., Porphyrite—Esterel, France. III., Norite-porphyrite—Klausen, Tirol.

 (J. S. F.)