sabato 17 settembre 2016

Quartzite - Quarzite

Quartzite
Quartzite (from German Quarzit) is a hard, non-foliated 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 gray, though quartzites often occur in various shades of pink and red due to varying amounts of iron oxide (Fe2O3). Other colors, such as yellow and orange, are due to other mineral impurities.

When sandstone is metamorphosed to quartzite, the individual quartz grains recrystallize along with the former cementing material to form an interlocking mosaic of quartz crystals. Most or all of the original texture and sedimentary structures of the sandstone are erased by the metamorphism. Minor amounts of former cementing materials, iron oxide, silica, carbonate and clay, often migrate during recrystallization and metamorphosis. This causes streaks and lenses to form within the quartzite.

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. The term is often misused, and should be used for only tightly-cemented metamorphic quartzites, not quartz-cemented quartz arenites. The typical distinction between the two (since each is a gradation into the other) is a proper quartzite is so highly cemented, diagentically altered, and metamorphosed that it will fracture and break across grain boundaries, not around them. 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 layer of soil and little vegetation.

La quarzite è un tipo di roccia metamorfica composta in netta prevalenza da quarzo. Deriva dallo smantellamento e successivo passaggio metamorfico di quarzoareniti. Le varietà più incolori, trasparenti e dall'aspetto vitreo sono composte quasi esclusivamente di quarzo.
Al quarzo possono essere associati altri minerali quali miche, feldspato potassico, plagioclasi, carbonati, ecc.. Data la grande stabilità del quarzo (non altera), le informazioni sul grado metamorfico sono date proprio dai minerali accessori. La tessitura è prevalentemente massiva, ma la presenza di miche può trasformala in scistosa.
Le quarziti derivano da rocce sedimentarie come arenarie, selci ma anche da apliti e pegmatiti.
La selce, roccia sedimentaria composta quasi esclusivamente di silice, non va confusa con la quarzite.
Hanno impiego in edilizia ed anche nell'industria dei refrattari e della ceramica, nonché in quella del vetro.
Abandoned quartzite mine in Kakwa Provincial Park and Protected Area near Wishaw Lake

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