Quartzite
Quartzite is a hard,
metamorphic rock which was originally sandstone. 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. Pure quartzite is usually white to grey. Quartzites often occur in various shades of pink and red due to varying amounts of iron oxide. Other colors are due to impurities of minor amounts of other minerals.
Orthoquartzite is a very pure quartz sandstone composed of usually well rounded
quartz grains cemented by
silica. Orthoquartzite is often 99% SiO
2 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.
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. Virtually all original textures and structure have usually been erased by the
metamorphism.
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. The town of
Quartzsite in western
Arizona derives its name from the quartzites in the nearby mountains.
