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Quartzite

Quartzite is sandstone that has been metamorposed. Unlike sandstone, quartzite breaks through, not around, the quartz grains, producing a smooth surface instead of a rough and granular one. Quartzites are snowy white, less often pink or gray. They yield a thin and very barren soil and, because they weather slowly, tend to project as hills or mountain masses. Many prominent ridges in the Appalachian Mountains are composed of highly resistant tilted beds of quartzite.

The term quartzite implies not only a high degree of hardening (induration), or "welding," but also a high content of quartz. Most quartzites contain 90 percent or more quartz, but some contain 99 percent and are the largest and purest concentrations of silica in the Earth's crust. Pure quartzites are a source of silica for metallurgical purposes and for the manufacture of brick. Quartzite is also quarried for paving blocks, riprap, crushed stone, railroad ballast, and roofing granules.

In microscopic section the clastic structure of some quartzites is well preserved; the rounded sand grains are seen with quartz overgrowths deposited in crystalline continuity, so that optical properties of the grains are similar to those of the material surrounding them. In some cases a line of iron oxides may indicate the boundary of the original sand grain. Many quartzites, however, have been crushed, and the quartz largely is a mosaic of small, irregularly shaped crystalline fragments with interlocking margins.

Sandstones turn into quartzite in two different ways. The first way involves low pressure and temperature, where circulating fluids fill the spaces between sand grains with silica cement. When this rock is broken, the fractures go right through the original grains, not around them. This kind of quartzite, orthoquartzite, is not strictly speaking a metamorphic rock because the original mineral grains are still there, and bedding planes and other sedimentary structures are still evident.

Under the high pressures and temperatures of deep burial, the mineral grains recrystallize and all traces of the original sediments are erased. The result is a true metamorphic rock, called metaquartzite. This boulder is probably a metaquartzite.

Quartzite is a very strong stone but is difficult to work. Because quartzite has a limited color range and is not often found in large bodies suited for quarrying, the building industry prefers granite for demanding applications.

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