QUARTZITE
(METAQUARZITE)
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 (see photos of
4 color variants below).
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 & 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 (see samples below). This implies that “quartzite” be restricted
to well-cemented, non-metamorphosed sandstones. I don't often see the
term metaquartzite used in the geologic literature.
Quartzite (metaquartzite), 3.1 cm across.
Quartzite (metaquartzite), 3.9 cm across.
Quartzite (metaquartzite), 2.6 cm across.
Quarzite
(metaquartzite), 3.0 cm across.