Editing Quartzite (section)

==Characteristics and origin==
Quartzite is a very hard rock composed predominantly of an interlocking mosaic of quartz crystals. The grainy, sandpaper-like surface is glassy in appearance. Minor amounts of former cementing materials, iron oxide, silica, carbonate and clay, often migrate during recrystallization, causing streaks and lenses to form within the quartzite.<ref name= EG/> To be classified as a quartzite by the [[British Geological Survey]], a metamorphic rock must contain at least 80% quartz by volume.<ref name="BGS">{{cite journal |last1=Robertson |first1=S. |title=BGS Rock Classification Scheme, Volume 2: Classification of metamorphic rocks |journal=British Geological Survey Research Report |date=1999 |volume=RR 99-02 |url=http://nora.nerc.ac.uk/id/eprint/3226/1/RR99002.pdf |access-date=27 February 2021}}</ref>

Quartzite is commonly regarded as metamorphic in origin.<ref name=BlattTracy1996>{{cite book |last1=Blatt |first1=Harvey |last2=Tracy |first2=Robert J. |title=Petrology : igneous, sedimentary, and metamorphic. |date=1996 |publisher=W.H. Freeman |location=New York |isbn=0716724383 |edition=2nd |page=367}}</ref><ref name=oxford2013/> When sandstone is subjected to the great heat and pressure associated with regional metamorphism, the individual quartz grains recrystallize along with the former cementing material. Most or all of the original texture and sedimentary structures of the sandstone are erased by the metamorphism.<ref name= EG/> The recrystallized quartz grains are roughly equal in size, forming what is called a granoblastic texture, and they also show signs of metamorphic annealing, in which the grains become coarser and acquire a more polygonal texture.<ref name=BlattTracy1996/> The grains are so tightly interlocked that when the rock is broken, it fractures through the grains to form an  irregular or conchoidal fracture.<ref name=Howard2005>{{cite journal |last1=Howard |first1=Jeffrey L. |title=The Quartzite Problem Revisited |journal=The Journal of Geology |date=November 2005 |volume=113 |issue=6 |pages=707–713 |doi=10.1086/449328|bibcode=2005JG....113..707H |s2cid=128463511 |url=https://digitalcommons.wayne.edu/cgi/viewcontent.cgi?article=1077&context=geofrp |url-access=subscription }}</ref>

Geologists had recognized by 1941 that some rocks show the macroscopic characteristics of quartzite, even though they have not undergone metamorphism at high pressure and temperature. These rocks have been subject only to the much lower temperatures and pressures associated with [[diagenesis]] of sedimentary rock, but diagenesis has cemented the rock so thoroughly that microscopic examination is necessary to distinguish it from metamorphic quartize. The term ''orthoquartzite'' is used to distinguish such sedimentary rock from ''metaquartzite'' produced by metamorphism. By extension, the term ''orthoquartzite'' has occasionally been more generally applied to any quartz-cemented [[quartz arenite]]. Orthoquartzite (in the narrow sense) is often 99% SiO<sub>2</sub> with only very minor amounts of iron oxide and trace resistant minerals such as [[zircon]], [[rutile]] and [[magnetite]]. Although few [[fossil]]s are normally present, the original texture and [[sedimentary]] structures are preserved.<ref name="Ireland-1974">{{cite journal|doi=10.1306/74D729F0-2B21-11D7-8648000102C1865D|author=Ireland, H. A.|year= 1974|title= Query: Orthoquartzite????|journal= Journal of Sedimentary Petrology|volume= 44|issue=1|pages=264–265}}</ref><ref name=oxford2013/>

The typical distinction between a true orthoquartzite and an ordinary quartz sandstone is that an orthoquartzite is so highly cemented that it will fracture across grains, not around them.<ref name=jackson1997/> This is a distinction that can be recognized in the [[Field work|field]]. In turn, the distinction between an orthoquartzite and a metaquartzite is the onset of recrystallization of existing grains. The dividing line may be placed at the point where strained quartz grains begin to be replaced by new, unstrained, small quartz grains, producing a ''mortar texture'' that can be identified in thin sections under a polarizing microscope. With increasing grade of metamorphism, further recrystallization produces ''foam texture'', characterized by polygonal grains meeting at triple junctions, and then ''porphyroblastic texture'', characterized by coarse, irregular grains, including some larger grains ([[porphyroblasts]]).<ref name=Howard2005/>