Quartzite is a nonfoliated metamorphic rock that consists mostly of quartz. It’s usually a white to pale gray rock, but occurs in other colors, including red and pink (from iron oxide), yellow, blue, green, and orange. The rock has a grainy surface with a sandpaper texture, but polishes to a glassy shine.
Key Takeaways: Quartzite Rock
Quartzite is a hard, nonfoliated metamorphic rock formed by the action of heat and pressure upon sandstone.
Usually, the rock is white or gray, but it occurs in other pale colors. It has a grainy, rough surface. Magnification reveals a mosaic of quartz crystals.
Pure quartzite consists entirely of silicon dioxide, but usually iron oxide and trace minerals are present.
Quartzite occurs in folded mountain ranges at convergent plate boundaries worldwide.
How Quartzite Forms
Quartzite forms when pure or nearly pure quartz sandstone undergoes heating and pressure. Usually this is caused by tectonic compression. The sand grains of sandstone melt and recrystallize, cemented together by silica.
Quartzite arenite is the intermediate stage between sandstone and quartzite. Arenite is still considered to be a sedimentary rock, but it has an extremely high quartz content. However, it’s difficult to identify the transition from sandstone to quartzite. Some geologists use the term “quartzite” to refer to metamorphic rocks consisting almost exclusively of quartz. Here, quartzite is identified by the way it fractures across grain boundaries, while arenite breaks around them. Other geologists simply identify “quartzite” as a tightly-cemented rock found above or below a band of sedimentary quartz rock.
Quartzite consists almost entirely of silicon dioxide, SiO2. If the purity is about 99% SiO2, the rock is called orthquartzite. Otherwise, quartzite commonly contains iron oxide and may contain trace amounts of the minerals rutile, zircon, and magnetite. Quartzite may contain fossils.
Quartzite has a Mohs hardness of 7, which is comparable to that of quartz and considerably harder than sandstone. Like glass and obsidian, it breaks with a conchoidal fracture. Its coarse texture makes it difficult to hone to a fine edge. Under magnification, quartzite’s interlocking crystal structure becomes apparent.
Where to Find Quartzite
Quartzite forms at convergent tectonic plate boundaries. Converging plates bury sandstone and exert compression. As the boundary folds, mountains arise. Thus, quartzite is found in folded mountain ranges worldwide. While erosion weathers softer rock away, quartzite remains, forming peaks and cliffs. The rock also litters mountain sides as scree.
In the United States, you can find quartzite in eastern South Dakota, southwestern Minnesota, the Wasatch Range of Utah, the Baraboo Range of Wisconsin, Central Texas, near Washington, D.C., portions of Pennsylvania, and the mountains of Arizona and California. The town of Quartzite in Arizona takes its name from the rock in the nearby mountains.
Quartzite occurs throughout the United Kingdom, the La Cloche Mountains in Canada, the Rhenish Massif in Continental Europe, Brazil, Poland, and the Chimanimani Plateau of Mozambique.
Quartzite’s strength and toughness lends itself to many uses. Crushed quartzite is used in road construction and for railway ballast. It is used to make roofing tiles, stairs, and flooring. When cut and polished, the rock is quite beautiful, as well as durable. It is used to make kitchen countertops and decorative walls. High-purity quartzite is used to make silica sand, ferrosilicon, silicon carbide, and silicon. Paleolithic humans sometimes made stone tools out of quartzite, although it was harder to work than flint or obsidian.
Quartzite Versus Quartz and Marble
Quartzite is a metamorphic rock, while quartz is an igneous rock that crystallizes from magma or precipiates around hydrothermal vents. Sandstone under pressure becomes quartz arenite and quartzite, but quartzite does not become quartz. The construction industry further complicates the matter. If you buy “quartz” for countertops, it is actually an engineered material made from crushed quartz, resin, and pigments and not the natural rock.
Another rock commonly confused with quartzite is marble. Both quartzite and marble tend to be pale-colored, non-foliated rock. Despite having a similar appearance, marble is a metamorphic rock made from recrystallized carbonate minerals, not silicates. Marble is softer than quartzite. An excellent test to distinguish the two is to apply a bit of vinegar or lemon juice to the rock. Quartzite is impervious to weak acid etching, but marble will bubble and retain a mark.