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Amazing World of Quartzite

What is Quartzite?

Quartzite is a hard, non-foliated metamorphic rock which was originally pure quartz 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, though quartzites often occur in various shades of pink and red due to varying amounts of iron oxide (Fe2O3). Other colors, such as yellow, green, blue and orange, are due to other minerals.

When sandstone is cemented 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. The grainy, sandpaper-like surface becomes glassy in appearance. 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 also traditionally used for quartz-cemented quartz arenites, and both usages are found in the literature. The typical distinction between the two (since each is a gradation into the other) is a metamorphic quartzite is so highly cemented, diagenetically altered, and metamorphosized so 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 material for soil; therefore, the quartzite ridges are often bare or covered only with a very thin layer of soil and little (if any) vegetation.

Physical Properties of Quartzite

Quartzite is usually white to gray in color. Some rock units that are stained by iron can be pink, red, or purple. Other impurities can cause quartzite to be yellow, orange, brown, green, or blue.

The quartz content of quartzite gives it a hardness of about seven on the Mohs Hardness Scale. Its extreme toughness made it a favorite rock for use as an impact tool by early people. Its conchoidal fracture allowed it to be shaped into large cutting tools such as ax heads and scrapers. Its coarse texture made it less suitable for producing tools with fine edges such as knife blades and projectile points.

Where Does Quartzite Form?

Most quartzite forms during mountain-building events at convergent plate boundaries. There, sandstone is metamorphosed into quartzite while deeply buried. Compressional forces at the plate boundary fold and fault the rocks and thicken the crust into a mountain range. Quartzite is an important rock type in folded mountain ranges throughout the world.

Quartzite as a Ridge-Former

Quartzite is one of the most physically durable and chemically resistant rocks found at Earth’s surface. When the mountain ranges are worn down by weathering and erosion, less-resistant and less-durable rocks are destroyed, but the quartzite remains. This is why quartzite is so often the rock found at the crests of mountain ranges and covering their flanks as a litter of scree.

Quartzite is also a poor soil-former. Unlike feldspars which break down to form clay minerals, the weathering debris of quartzite is quartz. It is therefore not a rock type that contributes well to soil formation. For that reason it is often found as exposed bedrock with little or no soil cover.

How the Name “Quartzite” Is Used

Geologists have used the name “quartzite” in a few different ways, each with a slightly different meaning. Today most geologists who use the word “quartzite” are referring to rocks that they believe are metamorphic and composed almost entirely of quartz.

A few geologists use the word “quartzite” for sedimentary rocks that have an exceptionally high quartz content. This usage is falling out of favor but remains in older textbooks and other older publications. The name “quartz arenite” is a more appropriate and less confusing name for these rocks.

It is often difficult or impossible to differentiate quartz arenite from quartzite. The transition of sandstone into quartzite is a gradual process. A single rock unit such as the Tuscarora Sandstone might fully fit the definition of quartzite in some parts of its extent and be better called “sandstone” in other areas. Between these areas, the names “quartzite” and “sandstone” are used inconsistently and often guided by habit. It is often called “quartzite” when rock units above and below it are clearly sedimentary. This contributes to the inconsistency in the ways that geologists use the word “quartzite.”

Uses of Quartzite

Quartzite has a diversity of uses in construction, manufacturing, architecture, and decorative arts. Although its properties are superior to many currently used materials, its consumption has always been low for various reasons. The uses of quartzite and some reasons that it is avoided are summarized below.

Architectural Use

In architecture, marble and granite have been the favorite materials for thousands of years. Quartzite, with a Mohs hardness of seven along with greater toughness, is superior to both in many uses. It stands up better to abrasion in stair treads, floor tiles, and countertops. It is more resistant to most chemicals and environmental conditions. It is available in a range of neutral colors that many people prefer. The use of quartzite in these uses is growing slowly as more people learn about it.

Construction Use

Quartzite is an extremely durable crushed stone that is suitable for use in the most demanding applications. Its soundness and abrasion resistance are superior to most other materials.

Unfortunately, the same durability that makes quartzite a superior construction material also limits its use. Its hardness and toughness cause heavy wear on crushers, screens, truck beds, cutting tools, loaders, tires, tracks, drill bits, and other equipment. As a result, the use of quartzite is mainly limited to geographic areas where other aggregates are not available.

Manufacturing Use

Quartzite is valued as a raw material because of its high silica content. A few unusual deposits have a silica content of over 98%. These are mined and used to manufacture glass, ferrosilicon, manganese ferrosilicon, silicon metal, silicon carbide, and other materials.

Decorative Use

Quartzite can be a very attractive stone when it is colored by inclusions. Inclusions of fuchsite (a green chromium-rich variety of muscovite mica) can give quartzite a pleasing green color. If the quartzite is semitransparent to translucent, the flat flakes of mica can reflect light to produce a glittering luster known as aventurescence.

Material that displays this property is known as “aventurine,” a popular material used to produce beads, cabochons, tumbled stones, and small ornaments. Aventurine can be pink or red when stained with iron. Included dumortierite produces a blue color. Other inclusions produce white, gray, orange, or yellow aventurine.

Stone Tools

Quartzite has been used by humans to make stone tools for over one million years. It was mainly used for impact tools, but its conchoidal fracture allowed it to be broken to form sharp edges. Broken pieces of quartzite were used for crude cutting and chopping tools.

Quartzite was not the preferred material for producing cutting tools. Flint, chert, jasper, agate, and obsidian all can be knapped to produce fine cutting edges, which are difficult to produce when working quartzite. Quartzite served as an inferior substitute for these preferred materials.

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