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Quartz-Porphyry

quartz, crystals, usually, rocks, minerals, rounded and quartz-porphyries

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QUARTZ-PORPHYRY, in petrology, the name given to a group of hemi-crystalline acid rocks containing porphyritic crystals of quartz in a more fine-grained matrix which is usually of micro-crystalline or felsitic structure. In the hand specimens the quartz appears as small rounded, clear, greyish, vitreous blebs, which are crystals (double hexagonal pyramids) with their edges and corners rounded by resorption or corrosion. Under the microscope rounded enclosures of the ground-mass or fluid cavities are often seen; these are frequently negative crystals with regular outlines resembling those of perfect quartz crystals, and many of the latter contain liquid carbonic acid and a bubble of gas which may exhibit vibratile motion under high magnifying powers. In addition to quartz there are usually phenocrysts of felspar, mostly orthoclase, though a varying amount of plagioclase is often present. The felspars are usually cloudy from the forma tion of secondary kaolin and muscovite throughout their sub stance; their crystals are larger than those of quartz and some times attain a length of two inches. Not uncommonly scales of mica are visible as hexagonal plates. Other porphyritic minerals are few, but hornblende, augite and bronzite are sometimes found; the augite and hornblende are in most cases green, and are frequently decomposed into chlorite, but even then can usually be identified by their shape. A colourless rhombic pyroxene (enstatite or bronzite) occurs in a limited number of the rocks of this group and readily weathers to bastite. Apatite, magnetite, and zircon, all in small but frequently perfect crystals, are almost universal minerals of the quartz-porphyries.

Structure.

The ground-mass is finely crystalline and to the unaided eye has usually a dull aspect resembling common earthen ware; it is grey, green, reddish or white. Often it is streaked or banded by fluxion during cooling, but as a rule these rocks are not vesicular. Two main types may be recognized by means of the microscope—the felsitic and the microcrystalline. In the former the ingredients are so fine-grained that in the thinnest slices they cannot be determined in this way. Some of the rocks

show perlitic or spherulitic structure; such were probably origi nally glassy (obsidians or pitchstones), but by lapse of time have slowly passed into a very finely crystalline state. This change is called devitrification; it is common in glasses, as these are essentially unstable.

A large number of the finer quartz-porphyries are also in some degree silicified or impregnated by quartz, chalcedony and opal, derived from the silica set free by decomposition (kaolinization) of the original felspar. This re-deposited silica forms veins and patches of indefinite shape or may bodily replace a considerable area of the rock by metasomatic substitution. The opal is amorphous, the chalcedony finely crystalline and often arranged in spherulitic growths which yield an excellent black cross in polarized light. The microcrystalline ground-masses are those which can be resolved into their component minerals in thin slices by use of the microscope. They prove to consist essentially of quartz and felspars, which are often in grains of quite irregular shape (microgranitic). In other cases these two minerals are in graphic intergrowth, often forming radiate growths of spherulites consisting of fibres of extreme tenuity ; this type is known as granophyric. There is another group in which the matrix con tains small rounded or shapeless patches of quartz in which many rectangular felspars are embedded ; this structure is called micro poikilitic, and though often primary is sometimes developed by secondary changes which involve the deposit of new quartz in the ground-mass. As a whole those quartz-porphyries which have microcrystalline ground-masses are rocks of intrusive origin. Elvan is a name given locally to the quartz-porphyries which occur as dikes in Cornwall; in many of them the matrix contains scales of colourless muscovite or minute needles of blue tourma line. Fluorite and kaolin appear also, and these minerals as a whole are due to pneumatolytic action by vapours permeating the porphyry after it had consolidated but probably before it had entirely cooled.

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