Quartzite is a sort of sandstone, and like all sandstones it consists of grains of rock, the sand and a sort of glue which glues them together. In the case of quartzites it all consists almost completely of quartz or silica (SiO2). It is formed either directly as a clastic sediment, or sandstone with a high quartz content may become quartzite during metamorphism, because the other materials are dissolved under temperature and pressure and are thus removed. It typically has a low porosity.
Quartz is probably the most resistant material in the crust, not soluble by most acids, so resistant we use it to make glas which is used for chemical factories, because it is so inert. Unfortunately it is rather brittle. But sandstone is far less brittle, because of its inner structure. A grain of sand might break, but its not that sure that the force will be big enough to break the next grain, or the cement between the grains. As a result quartzite, not pure quartz, is the most durable rock known.
That's pretty bad for the formation of caves, though. What we need for caves is a material which can be removed, and that requires dissolution or at least some kind of weathering.
|Cueva Ojos de Cristal||Venezuela||Roraima||16,100m||73m|
|Cueva Charles Brewer||Venezuela||Churi||7,300m||110m|
|Gruta do Centenário||Brazil||Minas Gerais||4,700m||481m|
|Gruta das Torras||Brazil||Bahia||3,800m||0m|
|Gruta da Bocaina||Brazil||Inficionado||3,200m||404m|
|Sima Auyan-tepuy Noroeste||Venezuela||Auyantepuy||2,900m||370m|
|Gruta das Bromelias||Brazil||Ibitipoca||2,700m||0m|
|Cueva de la Arana||Venezuela||Churi||2,500m||0m|