Editing Sandstone (section)

==Types of sandstone==
Sandstones are typically classified by point-counting a [[thin section]] using a method like the [[Gazzi-Dickinson Method]]. This yields the relative percentages of quartz, feldspar, and lithic grains and the amount of clay matrix. The composition of a sandstone can provide important information on the genesis of the sediments when used with a triangular quartz, feldspar, lithic chart ([[QFL diagrams]]). However, geologists have not been able to agree on a set of boundaries separating regions of the QFL triangle.<ref name="boggs-2006-119-135"/>
[[File:QFL Ternary Plot.jpg|thumb|Ternary plot showing the relative abundance of quartz, feldspar, and lithic grains in a sandstone]]

Visual aids are diagrams that allow geologists to interpret different characteristics of a sandstone. For example, a QFL chart can be marked with a [[Provenance (geology)|provenance]] model that shows the likely tectonic origin of sandstones with various compositions of framework grains. Likewise, the stage of textural maturity chart illustrates the different stages that a sandstone goes through as the degree of kinetic processing of the sediments increases.{{sfn|Boggs|2006|pp=130-131}}
[[File:QFLtriangle.svg|thumb|Schematic QFL diagram showing tectonic provinces and sandstone provenance]]
* A QFL chart is a representation of the framework grains and matrix that is present in a sandstone. This chart is similar to those used in igneous petrology. When plotted correctly, this model of analysis creates a meaningful quantitative classification of sandstones.<ref name="Carozzi">Carozzi, A. (1993). Sedimentary petrography. Englewood Cliffs, NJ: Prentice-Hall {{ISBN|0-13-799438-9}}</ref>
* A sandstone provenance chart is typically based on a QFL chart but allows geologists to visually interpret the different types of places from which sandstones can originate.
* A stage of textural maturity chart shows the differences between immature, submature, mature, and supermature sandstones. As the sandstone becomes more mature, grains become more rounded, and there is less clay in the matrix of the rock.<ref name="boggs-2006-119-135"/>

===Dott's classification scheme===
[[File:Sandstone Classification modified from Dott, 1964.jpg|thumb|Diagram showing a slightly modified version of the Dott (1964) classification scheme]]
Dott's (1964) sandstone classification scheme is one of many such schemes used by geologists for classifying sandstones. Dott's scheme is a modification of Gilbert's classification of silicate sandstones, and it incorporates R.L. Folk's dual textural and compositional maturity concepts into one classification system.<ref name="Dott">{{cite journal|title=Wacke, greywacke and matrix; what approach to immature sandstone classification?|author=Robert H. Dott|url=http://jsedres.geoscienceworld.org/content/34/3/625.abstract|year=1964 |volume=34|issue=3 |pages=625–32|doi=10.1306/74D71109-2B21-11D7-8648000102C1865D|journal=SEPM Journal of Sedimentary Research}}</ref> The philosophy behind combining Gilbert's and R. L. Folk's schemes is that it is better able to "portray the continuous nature of textural variation from mudstone to arenite and from stable to unstable grain composition".<ref name="Dott"/> Dott's classification scheme is based on the mineralogy of framework grains, and on the type of matrix present in between the framework grains.{{Citation needed|date=January 2024}}

In this specific classification scheme, Dott has set the boundary between arenite and wackes at 15% matrix. In addition, Dott also breaks up the different types of framework grains that can be present in a sandstone into three major categories: quartz, feldspar, and lithic grains.<ref name="boggs-2006-119-135"/>
* Arenites are types of sandstone that have less than 15% clay matrix in between the framework grains.
** Quartz arenites are sandstones that contain more than 90% of siliceous grains. Grains can include [[quartz]] or [[chert]] rock fragments.<ref name="boggs-2006-119-135"/> Quartz arenites are texturally mature to supermature sandstones. These pure quartz sands result from extensive [[weathering]] that occurred before and during transport. This weathering removed everything but quartz grains, the most stable [[mineral]]. They are commonly affiliated with rocks that are deposited in a stable cratonic environment, such as aeolian [[beach]]es or shelf environments.<ref name="boggs-2006-119-135"/> Quartz arenites emanate from multiple recycling of quartz grains, generally as sedimentary source rocks and less regularly as first-cycle deposits derived from primary [[igneous]] or [[metamorphic rock]]s.<ref name="boggs-2006-119-135"/>
** Feldspathic arenites are sandstones that contain less than 90% quartz, and more feldspar than unstable lithic fragments, and minor accessory minerals.<ref name="boggs-2006-119-135"/> Feldspathic sandstones are commonly immature or sub-mature.<ref name="boggs-2006-119-135"/> These sandstones occur in association with cratonic or stable shelf settings.<ref name="boggs-2006-119-135"/> Feldspathic sandstones are derived from granitic-type, primary crystalline, rocks.<ref name="boggs-2006-119-135"/> If the sandstone is dominantly plagioclase, then it is igneous in origin.<ref name="boggs-2006-119-135"/>
** Lithic arenites are characterised by generally high content of unstable lithic fragments. Examples include volcanic and metamorphic clasts, though stable clasts such as chert are common in lithic arenites.<ref name="boggs-2006-119-135"/> This type of rock contains less than 90% quartz grains and more unstable rock fragments than feldspars.<ref name="boggs-2006-119-135"/> They are commonly immature to submature texturally.<ref name="boggs-2006-119-135"/> They are associated with fluvial conglomerates and other fluvial deposits, or in deeper water marine conglomerates.<ref name="boggs-2006-119-135"/> They are formed under conditions that produce large volumes of unstable material, derived from fine-grained rocks, mostly [[shale]]s, [[volcanic rock]]s, and [[metamorphic rock]].<ref name="boggs-2006-119-135"/>
* Wackes are sandstones that contain more than 15% clay matrix between framework grains.
** Quartz wackes are uncommon because quartz arenites are texturally mature to supermature.<ref name="boggs-2006-119-135"/>
** Felspathic wackes are feldspathic sandstone that contain a matrix that is greater than 15%.<ref name="boggs-2006-119-135"/>
** Lithic wacke is a sandstone in which the matrix greater than 15%.<ref name="boggs-2006-119-135"/>
* [[Arkose]] sandstones are more than 25 percent [[feldspar]].<ref name=LF/> The grains tend to be poorly rounded and less well sorted than those of pure quartz sandstones. These feldspar-rich sandstones come from rapidly eroding [[granitic]] and metamorphic [[terrain]]s where [[chemical weathering]] is [[subordinate]] to [[physical weathering]].
* [[Greywacke]] sandstones are a [[heterogeneous]] mixture of [[Lithic Fragment (geology)|lithic fragments]] and angular grains of quartz and feldspar or grains surrounded by a fine-grained [[Clay mineral|clay]] matrix. Much of this [[Matrix (geology)|matrix]] is formed by relatively soft fragments, such as [[shale]] and some volcanic rocks, that are chemically altered and physically compacted after deep burial of the sandstone formation.

===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/>