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Encyclopedia Britannica



QUARTZITE

This article appears in Volume V22, Page 718 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: PYR-RAY
QUARTZITE , in petrology, a sandstone which by the deposit of crystalline quartz between its grains has been compacted into a solid quartz rock. As distinguished from sand-stones, quartzites are free from pores and have a smooth fracture, since when struck with the
hammer
  they break through the sand grains, while in sandstones the fracture passes through the cementing material and the rounded faces of the grains are exposed, giving the broken surface a rough or granular appearance. The conversion of sandstone into quartzite is sometimes the work of percolating water under ordinary conditions. In the Reading beds of England, which are for the most part loose sands, there are often many large blocks of quartzite which weather out and are exposed at the surface, being known as grey-wethers. The silicification of these rocks must have taken place at no great depth and under ordinary pressures. Most quartzites, however, are found among ancient rocks, such as the Cambrian or Pre-Cambrian. Instances are the Lickey quartzite of Shropshire, the Holyhead quartzite of Anglesey, the Durness quartzite of Sutherlandshire, the Banff-shire and Perthshire quartzites and the Cherbourg quartzite. As these rocks lie in regions where there has been a considerable amount of metamorphism we may infer that (in addition to time and pressure) folding and rise of temperature favour the production of rocks of this type.
A normal quartzite has in microscopic section its clastic structure well preserved; the rounded sand grains are seen with patches of new quartz in the interspaces, and the latter is often deposited in crystalline continuity, so that the optical properties of the grains are similar to those of the material which surrounds them: a line of iron oxides or other impurities often indicates the boundary of the
original
  sand grain. As might be expected, however, many of the oldest quartzites have been crushed by folding movements and the quartz consists in large part of a mosaic of small crystalline fragments of irregular shape with interlocking margins; these are called " sheared quartzites," and when they contain white mica in parallel crystalline flakes they become more fissile and pass into quartz-schists. Where sandstones are baked by intrusivegranite or diabase they are often converted into pure quartzite, the heat evidently occasioning the deposit of interstitial quartz. The commonest minerals in quartzite, in addition to quartz, are felspar (microcline, orthoclase, oligoclase), white mica, chlorite, iron oxides, rutile, zircon and tourmaline. Except felspar they are usually present only in small quantity; the less frequent accessories include hornblende, sillimanite, garnet, biotite, graphite, magnetite and epidote. In colour quartzites are often snowy white; they frequently have a fine angular jointing and break up into rubble under the action of frost. Quartzites are too hard and splintery to be used as building stones to any large extent: they furnish a thin and very barren soil, and because they weather slowly tend to project as hills or mountain masses. They are rarely fossiliferous (e.g. Gorran in
Cornwall
 ), though many of them contain worm casts which may be dragged out into long sinuous markings when the rock is much folded (Durness quartzite). Although much used as road stones, being very hard, they are readily crushed to powder unless well embedded in the road surface; the Cherbourg and Emborough (near Bristol) stones are employed for this purpose. Quartzite blocks may be used in tube mills for crushing and grinding ores, cements, &c.; rarely they have been adopted as a substitute for flint by Palaeolithic man for the fabrication of weapons and tools. (J. S. F.)
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 they are often seen to contain rounded enclosures of the ground-mass or fluid cavities, which are frequently negative crystals with regular outlines resembling those of perfect quartz crystals. 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 full and cloudy from the formation of secondary
kaolin
  and muscovite throughout their substance. Their crystals are larger than those of quartz and sometimes attain a length of two inches. Not uncommonly scales of biotite are visible in the specimens, being hexagonal plates, which may be weathered into a mixture of chlorite and epidote. Other porphyritic minerals are few, but hornblende, augite and bronzite are sometimes found, and garnet, cordierite and muscovite may also occur. The garnets are small, of rounded shape and red or brownish colour; in some cases they appear to have been corroded or absorbed. Cordierite forms six-sided prisms with flat ends; these divide, between crossed nicols, into six triangular areas radiating from a centre, as the crystals, which belong to the rhombic system, are not simple but consist of three twins interpenetrating and crossing. In the vast majority of cases the cordierite has weathered to an aggregate of scaly chlorite and muscovite; this is known as pinite and is or dark green colour and very soft. The quartz-porphyries or elvans which occur as dikes in
Cornwall
  and Devon frequently contain this
mineral
 . The augite and hornblende of these rocks 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.
The ground-mass is finely crystalline and to the unaided eye has usually a dull aspect resembling common earthenware; 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 microscopethe felsitic and the microcrystalline. In the former the ingredients are so fine-grained that in the thinnest slices they cannot be deter-mined by means of the microscope. Some of these rocks show perlitic or spherulitic structure, and such rocks were probably originally glassy (obsidians or pitchstones), but by lapse of time and processes of alteration have slowly passed into very finely crystal-line 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 of
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 contains small rounded or shapeless patches of quartz in which many rectangular felspars are embedded; this structure is called micropoikilitic, 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 tourmaline. Fluorite and
kaolin
  appear also in these rocks, and the whole of these minerals are due to pneumatolytic action by vapours permeating the
porphyry
  after it had consolidated but probably before it had entirely cooled.
Many ancient rhyolitic quartz-porphyries show on their weathered surfaces numerous globular projections. They may be several inches in diameter, and vary from this size down to a minute fraction of an inch. When struck with a
hammer
  they may detach readily from the matrix as if their margins were defined by a fissure. If they are broken across their inner portions are often seen to be filled with secondary quartz, chalcedony or agate: some of them have a central cavity, often with deposits of quartz crystals; they also frequently exhibit a succession of rounded cracks or dark lines occupied by secondary products. Rocks having these structures are common in N. Wales and Cumberland; they occur also in
Jersey
 , the Vosges and Hungary. It has been proposed to call them pyromerides. Much discussion has taken place regarding the origin of these spheroids, but it is generally admitted that most of them were originally spherulites, and that they have suffered ektensive changes through decomposition and silicification.
Many of the older quartz-porphyries which occur in Palaeozoic and Pre-Cambrian rocks have been affected by earth movements and have experienced crushing and shearing. In this way they become schistose, and from their felspar minute plates of sericitic white mica are developed, giving the rock in some cases very much of the appearance of mica-schists. If there have been no phenocrysts in the original rock, very perfect mica-schists may be produced, which can hardly be distinguished from sedimentary schists, though chemically somewhat different on account of the larger amounts of alkalis which igneous rocks contain. When phenocrysts were present they often remain, though rounded and dragged apart while the matrix flows around them. The glassy or felsitic en-closures in the quartz are then very suggestive of an igneous origin for the rock. Such porphyry-schists have been called porphyroids or porphyroid-schists, and in America the name aporhyolite has been used for them. They are well known in some parts of the Alps, Westphalia, Charnwood (England), and
Pennsylvania
 . The halleflintas of Sweden are also in part acid igneous rocks with a well-banded schistose or granulitic texture.
The quartz-porphyries are distinguished from the rhyolites by being either intrusive rocks or Palaeozoic lava's. All Tertiary acid lavas are included under rhyolites. The intrusive quartz-porphyries are equally well described as granite-porphyries. The palaeozoic effusive quartz-porphyries (or acid lavas) would be called rhyolites by many English petrologists, who regard geological age as of no importance in petrological classifications. But the name quartz-porphyry, though somewhat ambiguous, is so expressive and so firmly established by long-continued use that it cannot be discarded, especially as a descriptive name for the use of field geologists. (J. S. F.)


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