English: Quartzite from the Precambrian of South Dakota, USA.

Quartzite is a common, crystalline-textured, intermediate- to high-grade metamorphic rock. It forms by metamorphism of quartzose sandstones or siltstones. Quartzite can be entirely composed of interlocking quartz crystals, or the original sand grains may still be visible. This rock is hard (H = 7), will not bubble in acid (unlike marble), and can be almost any color.

The term “quartzite” has been used in geology to refer to crystalline, quartzose metamorphic rocks and to hard, well-cemented quartzose sandstones that have not been subjected to metamorphism. It is difficult to not call hard, well-cemented sandstones “quartzite” - for example, the Clinch Quartzite in the Appalachian Mountains and the Eureka Quartzite of the Great Basin in western USA, but the Clinch and Eureka aren’t metamorphic rocks. The term "metaquartzite" has been used by some geologists to refer to crystalline-textured, quartzose rocks that have been metamorphosed. This implies that “quartzite” be restricted to well-cemented, non-metamorphosed sandstones. I don’t often see the term metaquartzite in the geologic literature.

The quartzite sample seen here is from the Precambrian-aged Sioux Quartzite, which outcrops in southeastern South Dakota and southwestern Minnesota. Despite being regionally metamorphed, the unit’s original sedimentary features, such as horizontal stratification, cross-bedding, and ripple marks, are still apparent in outcrop.

The Sioux Quartzite is a erosion-resistant unit in a long-lived paleotopographic high called the Sioux tectonic core. This high has existed since Precambrian times and is part of a northeast-southwest trending series of paleotopographic highs and depressions known as the Transcontinental Arch, which extends from Arizona to Minnesota (see Carlson, 1999).

Stratigraphy: Sioux Quartzite, upper Paleoproterozoic, 1.65 to 1.70 Ga

Locality: town of Sioux Falls, southeastern South Dakota, USA

---

Reference cited:

Carlson (1999) - Transcontinental Arch - a pattern formed by rejuvenation of local features across central North America. Tectonophysics 305: 225-233.