Sandstone that has been converted into a solid quartz rock. Quartzites are usually white; they fracture smoothly and break up into rubble under frost action. Sandstone may be converted to quartzite by precipitation of silica from waters below the Earth's surface; such rocks are called orthoquartzites, whereas those produced by recrystallization (metamorphism) are metaquartzites. Because they weather slowly, they tend to project as hills or mountain masses. Many prominent ridges in the Appalachian Mountains are composed of quartzite. Pure quartzites are a source of silica for metallurgical purposes and for the manufacture of silica brick. Quartzite is also quarried for paving and roofing materials.
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In true metamorphic quartzite, also called meta-quartzite, the individual quartz grains have recrystallized 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 recrystallized and have migrated under the pressure to form streaks and lenses within the quartzite. Virtually all original textures and structure have usually been erased by the metamorphism.
Orthoquartzite is a very pure quartz sandstone composed of usually well rounded quartz grains cemented by silica. Orthoquartzite is often 99% SiO2 with only very minor amounts of iron oxide and trace resistant minerals such as zircon, rutile and magnetite. Although few fossils are normally present, the original texture and sedimentary structures are preserved.
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.