Editing Shale (section)

==Composition and color==

[[File:Mudrock Colors.jpg|thumb|Color chart for shale based on [[oxidation state]] and organic carbon content]]
Shales are typically gray in color and are composed of clay minerals and quartz grains. The addition of variable amounts of minor constituents alters the color of the rock. Red, brown and green colors are indicative of [[ferric oxide]] ([[hematite]] – reds), [[iron hydroxide]] ([[goethite]] – browns and [[limonite]] – yellow), or [[micaceous]] minerals ([[Chlorite group|chlorite]], [[biotite]] and [[illite]] – greens).<ref name=Blatt/> The color shifts from reddish to greenish as [[iron]] in the [[Redox|oxidized]] ([[ferric]]) state is converted to iron in the reduced ([[ferrous]]) state.<ref>{{cite book |last1=Potter |first1=Paul Edwin |title=Sedimentology of shale : study guide and reference source |last2=Maynard |first2=J. Barry |last3=Pryor |first3=Wayne A. |date=1980 |publisher=[[Springer Science+Business Media|Springer-Verlag]] |isbn=0387904301 |location=[[New York (state)|New York]] |pages=54–56}}</ref> Black shale results from the presence of greater than one percent [[carbonaceous]] material and indicates a reducing environment.<ref name=Blatt/> Pale blue to blue-green shales typically are rich in [[Carbonate mineral|carbonate minerals]].{{sfn|Potter|Maynard|Pryor|1980|p=56}}

Clays are the major constituent of shales and other mudrocks. The clay minerals represented are largely [[kaolinite]], [[montmorillonite]] and illite. Clay minerals of Late [[Tertiary]] mudstones are expandable [[Smectite|smectites]], whereas in older rocks (especially in mid-to early [[Paleozoic]] shales) illites predominate. The transformation of smectite to illite produces [[silica]], [[sodium]], [[calcium]], [[magnesium]], iron and water. These released elements form [[authigenic]] [[quartz]], [[chert]], [[calcite]], [[Dolomite (mineral)|dolomite]], [[ankerite]], hematite and [[albite]], all trace to minor (except quartz) minerals found in shales and other mudrocks.<ref name=Blatt/> A typical shale is composed of about 58% clay minerals, 28% quartz, 6% [[feldspar]], 5% carbonate minerals, and 2% [[Iron oxide|iron oxides]].<ref name=":0">{{Cite journal |last1=Ferriday |first1=Tim |last2=Montenari |first2=Michael |date=2016 |title=Chemostratigraphy and Chemofacies of Source Rock Analogues: A High-Resolution Analysis of Black Shale Successions from the Lower Silurian Formigoso Formation (Cantabrian Mountains, NW Spain) |url=https://www.sciencedirect.com/science/article/abs/pii/S2468517816300053 |journal=Stratigraphy & Timescales |volume=1 |pages=123–255 |doi=10.1016/bs.sats.2016.10.004 |via=[[Elsevier Science Direct]]}}</ref> Most of the quartz is [[detrital]] (part of the original sediments that formed the shale) rather than authigenic (crystallized within the shale after deposition).{{sfn|Potter|Maynard|Pryor|1980|pp=47-49}}

Shales and other mudrocks contain roughly 95 percent of the organic matter in all sedimentary rocks. However, this amounts to less than one percent by mass in an average shale. Black shales, which form in [[Anoxic waters|anoxic]] conditions, contain reduced free [[carbon]] along with [[ferrous iron]] (Fe<sup>2+</sup>) and [[sulfur]] (S<sup>2−</sup>). [[Amorphous solid|Amorphous]] [[iron sulfide]], along with carbon, produce the black coloration.<ref name=Blatt/> Because amorphous iron sulfide gradually converts to [[pyrite]], which is not an important pigment, young shales may be quite dark from their iron sulfide content, in spite of a modest carbon content (less than 1%), while a black color in an ancient shale indicates a high carbon content.{{sfn|Potter|Maynard|Pryor|1980|p=56}}

Most shales are marine in origin,{{sfn|Potter|Maynard|Pryor|1980|p=72}} and the [[groundwater]] in shale formations is often highly [[Saline water|saline]]. There is evidence that shale acts as a semipermeable medium, allowing water to pass through while retaining dissolved salts.{{sfn|Potter|Maynard|Pryor|1980|p=59}}<ref>{{cite journal|last1=Berry |first1=F.A. |year=1960 |title=Geologic field evidence suggesting membrane properties of shales |journal=AAPG Bulletin |volume=44 |number=6 |pages=953–954 |url=http://archives.datapages.com/data/bulletns/1957-60/data/pg/0044/0006/0950/0953b.htm |access-date=13 April 2021}}</ref>