As its name indicates, a quartzite is a rock composed essentially of quartz. It is usually light in color. Quartzite has been formed from sandstone by the forces of intense metamorphism. It is a common and widely distributed rock. Quartzite is a firm, compact rock which breaks with an uneven, splintery or conchoidal fracture, and is composed of grains of quartz-sand united by a cement consisting of the same material, that is, of deposited quartz. They are in general metamorphosed sandstones, and while no hard and fast line can be drawn between the two rocks, since all degrees of transition can be found between them, the quartzites are much harder and firmer than the sandstones; the latter have a more or less sugary or granular feeling and appearance; the individual grains are distinctly visible to the eye or lens, while in the quartzites the fractured surface is uneven, splintery or conchoidal; the luster vitreous or greasy, like that of quartz, and the grains are imperceptible or nearly so. This difference arises chiefly from the fact that in breaking the sandstone the fracture takes place in the cement, leaving the grains unaltered and outstanding, while in quartzite the grains are so firmly cemented, that there is nearly a homogeneous substance formed and the fracture takes place through cement and grains alike. This difference will serve as a practical distinction between the two rocks.
Minerals and General Properties. While some quartzites are very pure in mineral composition, others carry in greater or less abundance other minerals, which may be in part remains of original mineral grains, such as feldspar mixed with those of quartz, or new ones which have resulted from the metamorphism of the clay or lime cement, which formerly filled the interstices between the grains of the sandstone. Such are muscovite, chlorite, kyanite, epidote, etc. Iron hydroxides may be converted into magnetite or hematite, and carbonaceous substance into graphite. These resultant minerals are usually of microscopic size, and may give the rock a distinct color green, blue, purple, black, etc.; sometimes they are large enough to be clearly seen with the lens. The most important of them is muscovite, which, as it increases in amount, gives the rock a more schistose character, through which it attains a capacity for cleavage along the planes of the mica. Eventually this produces a transition into mica-schist, as previously explained under that rock.
The normal color of quartzite is white, light gray or yellowish into brown, but these are often modified by included material acting as a pigment, as explained above. The jointing of quartzite is usually platy, but sometimes very massive, and such rocks are in some places quarried and furnish good material for structural purposesThe chemical composition of a pure quartzite is nearly that of silica alone, but as more or less clay or calcareous material was mixed with the sand, small amounts of alumina, iron, lime, and alkalies appear.
Varieties. The different varieties of quartzite are chiefly those which are occasioned by the presence of some included substance. Thus we have epidotic quartzite, graphitic quartzite, sillimanite quartzite, and many others. Micaceous quartzite is also called quartz-schist. In very strongly folded and compressed mountain regions even pure quartzite may suffer such shearing as to break and crush the original grains and impose a more or less schistose structure. Such rocks are called stretched quartzites. In some places these rocks contain pebbles, of varying sizes, which retain their original shape and are sometimes by pressure and shearing reduced to lenticular, ovoid, or cylindrical bodies. These are called conglomerate-quartzite and were formed from gravels, like conglomerate-gneisses and conglomerate-mica-schists.
Oolitic Quartzite This a variety consisting of rounded grains, composed of chalcedony, a slightly hydrated form of silica, deposited around fragments of quartz which serve as nuclei. It resembles the roe of a fish, and if the globules are sufficiently large, their concentric structure can be plainly seen with a lens on the broken or polished surface of the rock. Such quartzites have been found at State College, Pa., and in Sumatra. Buhrstone is a name given to a variety of quartzite which is full of long, drawn out hollows or pores. Notwithstanding the porosity, it is quite firm and its hardness and toughness have caused its use as a millstone. It is thought to have been originally more or less of a limestone filled with fossils, which, by the action of solutions containing silica, has been converted into a quartzite, consisting mostly of chalcedony, whose cavities represent the leached out fossils. It occurs in western Massachusetts, Georgia, South Carolina and in the Paris Basin in France. It is chiefly of Tertiary age.
Occurrences. Quartzite is a widely distributed rock, mostly among the older metamorphosed strata. Thus, this rock is common in eastern North America, in the Rocky Mountains Cordillera and in various localities in Europe and other parts of the world. The occurrence of some special varieties has been already mentioned.
Alteration. On account of the insoluble, unyielding nature of its constituent grains and their cement, quartzite resists erosion and the atmospheric agencies well, and, where it is prominent in mountain regions and areas undergoing denudation, it forms prominent features of the landscape, bold ledges, cliffs, castellated crags, spires, etc. Eventually the rock breaks down into sandy soil of poor quality.
Distinction from Other Rocks. Quartzites, which are very homogeneous appearing rocks, may be confused, in the outcrop or hand specimen, with some limestones or felsites of a similar color and texture. From the former they are easily told by a test of the hardness, or by lack of effervescence with acid; from the latter, in the field by the different mode of geological occurrence, by the cleavage of the feldspar if visible under the lens, or by blowpipe test. It should be remembered that the chief minerals composing these three rocks are quartz, calcite, and feldspar respectively, and they should be tested accordingly.
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