Quartzite

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Quartzite (from German Quarzit), not to be confused with the mineral quartz, is a hard, metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. 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 commonly due to impurities of minor amounts of other minerals.

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.

Uses

Because of its hardness, about 7 on Mohs' scale of mineral hardness, crushed quartzite is often used as railway ballast.

Occurrences

In the United States, formations of quartzite can be found in eastern South Dakota, Central Texas, southwest Minnesota, Devil's Lake State Park in the Baraboo Hills in Wisconsin, the Wasatch Range in Utah, near Salt Lake City, Utah and as resistant ridges in the Appalachians and other mountain regions. Quartzite is also found in the Morenci Copper Mine in Arizona. The town of Quartzsite in western Arizona derives its name from the quartzites in the nearby mountains.

The La Cloche Mountains in Ontario are also composed primarily of white quartzite.

References

External links

• R. V. Dietrich's GemRocks: Quartzite
• CSU Pomona Geology: Quartzite
• Cowen's "The First Geologists" (chapter on Stone Age/Homo habilis use of quartzite)
• Minnesota Department of Natural Resources : Natural History: Minnesota's geology
• Wisconsin's Baraboo Syncline (map and aerial photos of Baraboo quartzite quarries)
• South Dakota 2002 Mineral Summary: Production, Exploration and Environmental Issues (including 2002 quartzite production)
• Big Sioux River: History of Sioux Falls and Quartzite Photos

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Last updated on Wednesday June 18, 2008 at 17:48:44 PDT (GMT -0700)
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