by F. J. Pettijohn

      QUARTZITE is a sandstone which has been converted into a solid quartz rock. Unlike sandstones, quartzites are free from pores and have a smooth fracture; if struck with a hammer they break through the grains, whereas in sandstones the fracture passes through the cementing material and the rounded surfaces of the grains are exposed, giving the broken surface a rough and granular appearance. Conversion of sandstone to quartzite may be accomplished by precipitation of silica from interstitual waters (hydrous metamorphism) at no great depth and under ordinary pressures. In contrast to these rocks, termed orthoquartzites, those produced by recrystallization under high pressure and elevated temperatures are called metaquartzites.

      In microscopic section the classic structure of some quartzites is well preserved; the rounded sand grains are seen with quartz overgrowths 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 may indicate the boundary of the original sand grain. Many quartzites, however, have been crushed and the quartz consists in large part of a mosaic of small crystalline fragments of irregular shape with interlocking margins; these are the "sheared quartzites"; and if they contain white mica in parallel crystalline flakes they become more fissle and pass into quartz schists.

      The term quartzite implies not only a high degree of induration or "welding" but also a hoigh content of quartz. Rocks of like induration but containing appreciable quantities of other minerals and rock particles are impure quartzites, more appropriately called greywacke, etc. Most quartzites contain 90% or more of quartz and in some the quartz content exceeds 99%. These constitute the largest and purest concentrations of silica in the Earth's crust. Quartzites are snowny white, less often pink or gray; they commonly have a fine angular jointing and break up into rubble under the action of frost. They yield a thin and very barren soil, and because they weather slowly tend to project as hills or as mountain masses. Many of the prominent ridges in the Appalachian mountains are the topographic expression of highly resistant tilted beds of Paleozoic quartzite. The Precambrian rocks of the Lake Huron and Lake Superior regions include thick and very pure quartzites. The Baraboo quartzite in Wisconsin is often deep red in color. In Scotland, the mountains of Schiehallion in Perth and the Paps of Jura form conspicuous conical mountains of quartzite in the Dalradian system of the highlands and Cambrian quartzites cap mountains in western Scotland. The pure quartzites are a source of silica for metallurgical purposes and for the manufacture of silica brick. Quartzite is also quarried for paving blocks, riprap, road metal (crushed stone), railroad ballast and roofing granules.

      This document is the QUARTZITE listing in the Encyclopedia Britannica, 1972 ed. Volume 18, pages 938-939.