A review of past terminology and previous petrological studies suggests that quartzite should be classified descriptively as both a sedimentary and a metamorphic rock. Quartzite is identified in the field as a quartz‐rich rock (exclusive of chert and vein quartz) that is exceptionally hard and, when broken by a rock hammer, fractures irregularly through both grains and cement (where present) to form an irregular or conchoidal fracture surface. Quartzite is differentiated from quartzose sandstone (arenite), which is softer and fractures around individual grains, and from chert and vein quartz by a bright vitreous luster. Quartzite is classified further on the basis of microscopic features into orthoquartzite and metaquartzite by the presence of clastic and metamorphic microtextures, respectively. Low‐grade, medium‐grade, and high‐grade subtypes of metaquartzite are differentiated by mortar, foam, and porphyroblastic microtextures, respectively. Composition is not used as a criterion for classification; hence, quartzite may contain a significant proportion (>10%) of nonquartz minerals. As defined here, quartzite is readily identified by megascopic features in outcrop, and subtypes of quartzite can be distinguished microscopically, even when the geologic context of the sample is unknown.
Environmental Sciences | Geology
Howard, J. L. (2005). The Quartzite Problem Revisited. Journal of Geology 113(6), 707-13. https://doi.org/10.1086/449328.