Editing Sandstone (section)

===Quartzite===
{{Main|Quartzite}}
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, to form the [[metamorphic rock]] called [[quartzite]]. Most or all of the original texture and sedimentary structures of the sandstone are erased by the metamorphism.<ref name= EG>{{cite book |title=Essentials of Geology |edition=3rd |author-first1=Stephen |author-last1=Marshak |page= 182}}</ref> 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 }}</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 quartzite. 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>{{cite book |last1=Allaby |first1=Michael |title=A dictionary of geology and earth sciences |date=2013 |publisher=Oxford University Press |location=Oxford |isbn=9780199653065 |edition=Fourth}}</ref>

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>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |page=525}}</ref> 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 ([[porphyroblast]]s.)<ref name=Howard2005/>