The metamorphic rock composed almost entirely of quartz.
What is Quartzite?
Quartzite is a nonfoliated metamorphic rock composed almost entirely of quartz. It forms when a quartz-rich sandstone is altered by the heat, pressure, and chemical activity of metamorphism. These conditions recrystallize the sand grains and the silica cement that binds them together. The result is a network of interlocking quartz grains of incredible strength.
The interlocking crystalline structure of quartzite makes it a hard, tough, durable rock. It is so tough that it breaks through the quartz grains rather than breaking along the boundaries between them. This is a characteristic that separates true quartzite from sandstone.
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."
Hammer With Caution !
Wise geologists, who have memorable experiences with quartzites, hit them with a rock hammer only when necessary. If a freshly broken piece is needed for examination, they break off a small protrusion with a light tap. That small piece is usually more than enough.
Don't hit quartzite hard with a rock hammer. It's not a good idea. If you must, be sure that you are wearing impact-resistant goggles, gloves, long sleeves, long pants, and sturdy shoes. A sharp hammer blow usually bounces off. That bounce can cause injury. When the rock does break, the impact often yields sparks and sharp pieces of rock traveling at high velocity.
Be certain that nearby field partners are warned and safely away. Hold the base of your goggles with your free hand before striking the rock. That will protect the lower half of your face from sparks and sharp flakes of high velocity rock. You have been warned.
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.
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.
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.
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.
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.
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.
Author: Hobart M. King, Ph.D.
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