Quartzite - Encyclopedia

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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 sandstones, 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 Banffshire 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 intrusive granite 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.)

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