Quartzite is a metamorphic rock consisting largely or entirely of quartz.
In the vast majority of cases, it is a metamorphosed sandstone or rock formed when quartz-rich sandstone or chert has been exposed to high temperatures and pressures. Such conditions fuse the quartz grains together forming a dense, hard, equigranular rock. The name implies not only a high degree of induration (hardness), but also a high quartz content. It generally comprises greater than 90% percent quartz, and some examples, containing up to 99% quartz, and are the largest and purest concentrations of silica in the Earth’s crust. Although a quartz-rich sandstone can look similar to quartzite, a fresh broken surface of quartzite will show breakage across quartz grains, whereas the sandstone will break around quartz grains. It also tends to have a sugary appearance and glassy lustre. The variety of colours displayed by quartzite are a consequence of minor amounts of impurities being incorporated with the quartz during metamorphism. Although it can sometimes appear superficially similar to marble, a piece of it will not be able to be scratched by a metal blade, and quartzite will not fizz on contact with dilute hydrochloric acid.
Texture – granular.
Grain size – medium grained; can see interlocking quartz crystals with the naked eye.
Hardness – hard.
Colour – variable – pure quartzite is white but quartzite exists in a wide variety of colours.
Mineralogy – quartz.
Other features – generally gritty to touch.
Uses – pure quartzite is a source of silica for metallurgical purposes, and for the manufacture of brick; as aggregate in the construction and roading industries; as armour rock for sea walls; dimension stone for building facings, paving etc. It is a decorative stone and may be used to cover walls, as roofing tiles, as flooring, and stair steps. Its use for countertops in kitchens is expanding rapidly.
source:southwest coarse path;sandatlas;NZ geology, rock and minerals