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QUARTZITE
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QUARTZITE , in See also:petrology, a See also:sandstone which by the See also:deposit of crystalline See also:quartz between its grains has been compacted into a solid quartz See also:rock. As distinguished from See also:sand-stones, quartzites are See also:free from pores and have a smooth fracture, since when struck with the See also: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 See also:surface a rough or granular See also:appearance. The See also:conversion of sandstone into quartzite is sometimes the See also:work of percolating See also:water under See also:ordinary conditions. In the See also:Reading beds of See also:England, which are for the most See also:part loose sands, there are often many large blocks of quartzite which See also:weather out and are exposed at the surface, being known as See also:grey-wethers. The silicification of these rocks must have taken See also:place at no See also:great See also:depth and under ordinary pressures. Most quartzites, however, are found among See also:ancient rocks, such as the See also:Cambrian or Pre-Cambrian. Instances are the Lickey quartzite of See also:Shropshire, the See also:Holyhead quartzite of See also:Anglesey, the Durness quartzite of See also:Sutherlandshire, the See also:Banff-See also:shire and See also:Perthshire quartzites and the See also:Cherbourg quartzite. As these rocks See also:lie in regions where there has been a considerable amount of See also:metamorphism we may infer that (in addition to See also:time and pressure) folding and rise of temperature favour the See also:production of rocks of this type.
A normal quartzite has in microscopic See also: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 See also:optical properties of the grains are similar to those of the material which surrounds them: a See also:line of See also:iron oxides or other impurities often indicates the boundary of the See also:original sand See also:grain. As might be expected, however, many of the See also:oldest quartzites have been crushed by folding movements and the quartz consists in large part of a See also:mosaic of small crystalline fragments of irregular shape with interlocking margins; these are called " sheared quartzites," and when they contain See also: Except felspar they are usually See also:present only in small quantity; the less frequent accessories include See also:hornblende, See also:sillimanite, See also:garnet, See also:biotite, See also:graphite, See also:magnetite and See also:epidote. In See also:colour quartzites are often snowy white; they frequently have a See also:fine angular jointing and break up into See also:rubble under the See also:action of See also:frost. Quartzites are too hard and splintery to be used as See also:building stones to any large extent: they furnish a thin and very barren See also:soil, and because they weather slowly tend to project as hills or See also:mountain masses. They are rarely fossiliferous (e.g. Gorran in See also:Cornwall), though many of them contain See also:worm casts which may be dragged out into See also: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 See also:powder unless well embedded in the road surface; the Cherbourg and Emborough (near See also:Bristol) stones are employed for this purpose. Quartzite blocks may be used in See also:tube See also:mills for crushing and grinding ores, cements, &c.; rarely they have been adopted as a substitute for See also:flint by See also:Palaeolithic See also:man for the fabrication of weapons and tools. (J. S. F.) QUARTZ-See also:PORPHYRY, in petrology, the name given to a See also:group of hemi-crystalline See also:acid rocks containing porphyritic crystals of quartz in a more fine-grained See also:matrix which is usually of micro-crystalline or felsitic structure. In the See also:hand specimens the quartz appears as small rounded, clear, greyish, vitreous blebs, which are crystals (See also:double hexagonal pyramids) with their edges and corners rounded by resorption or corrosion. Under the See also:microscope they are often seen to contain rounded enclosures of the ground-See also:mass or fluid cavities, which are frequently negative crystals with See also:regular outlines resembling those of perfect quartz crystals. Many of the latter contain liquid carbonic acid and a bubble of See also:gas which may exhibit vibratile See also:motion under high magnifying See also:powers. In addition to quartz there are usually phenocrysts of felspar, mostly orthoclase, though a varying amount of See also:plagioclase is often present. The felspars are usually full and cloudy from the formation of secondary See also:kaolin and See also: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, See also:augite and See also: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 See also:flat ends; these See also:divide, between crossed nicols, into six triangular areas radiating from a centre, as the crystals, which belong to the rhombic See also:system, are not See also:simple but consist of three twins interpenetrating and See also:crossing. In the vast See also:majority of cases the cordierite has weathered to an aggregate of scaly chlorite and muscovite; this is known as pinite and is or dark See also:green colour and very soft. The quartz-porphyries or elvans which occur as dikes in Cornwall and See also:Devon frequently contain this See also: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 See also:pyroxene (See also:enstatite or bronzite) occurs in a limited number of the rocks of this group and readily weathers to bastite. See also: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 See also:eye has usually a dull aspect resembling See also:common earthenware; it is grey, green, reddish or white. Often it is streaked or banded by fluxion during cooling, but as a See also:rule these rocks are not vesicular. Two See also: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 deter-See also: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 See also:lapse of time and processes of alteration have slowly passed into very finely crystal-line See also:state. This See also: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, See also:chalcedony and See also:opal, derived from the See also:silica set free by decomposition (kaolinization) of the original felspar. This re-deposited silica forms See also:veins and patches of indefinite shape or may bodily replace a considerable See also: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 See also:black See also:cross in polarized See also: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 See also:spherulites consisting of See also: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 See also:primary is sometimes See also: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 See also:minute needles of See also: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 See also:diameter, and vary from this See also:size down to a minute fraction of an See also: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 See also:agate: some of them have a central cavity, often with deposits of quartz crystals; they also frequently exhibit a See also:succession of rounded cracks or dark lines occupied by secondary products. Rocks having these structures are common in N. See also:Wales and See also:Cumberland; they occur also in See also:Jersey, the See also:Vosges and See also:Hungary. It has been proposed to See also: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 See also: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 See also: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 See also:America the name aporhyolite has been used for them. They are well known in some parts of the See also:Alps, See also:Westphalia, Charnwood (England), and See also:Pennsylvania. The halleflintas of See also: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 See also:lava's. All See also:Tertiary acid lavas are included under rhyolites. The intrusive quartz-porphyries are equally well described as See also:granite-porphyries. The palaeozoic effusive quartz-porphyries (or acid lavas) would be called rhyolites by many See also:English petrologists, who regard See also:geological See also: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 See also: Additional information and CommentsThere are no comments yet for this article.
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