quartzite

A rock produced by the recrystallization of sandstone by metamorphism, and consisting of interlocking crystals of quartz (metaquartzite). It is also a sandstone with purely siliceous cement (orthoquartzite).

Through heating and pressure usually related to tectonic compression within orogenic belts, the original quartz sand grains and quartz silica cement were fused into one.

Orthoquartzite is a very pure quartz sandstone composed of usually well rounded quartz grains cemented by silica.

In true metamorphic quartzite, also called meta-quartzite, the individual quartz grains have recrystalized along with the former cementing material to form an interlocking mosaic of quartz crystals. Minor amounts of former cementing materials, iron oxide, carbonate and clay, are often recrystalized and have migrated under the pressure to form streaks and lenses within the quartzite.

Quartzite is very resistant to chemical weathering and often forms ridges and resistant hilltops. The nearly pure silica content of the rock provides little to form soil from and therefore the quartzite ridges are often bare or covered only with a very thin soil and little vegetation.

Because of its hardness (about 7 on Mohs' scale of mineral hardness), crushed quartzite is often used as railway ballast. In the United States, formations of quartzite can be found in eastern South Dakota, southwest Minnesota, the Wasatch Range in Utah, and as resistant ridges in the Appalachians and other mountain regions.