Editing Slate (section)

==Description==
[[File:Slate with pyrite.JPG|thumb|Slate with [[pyrite]]]]
Slate is a fine-grained, metamorphic rock that shows no obvious compositional layering but can easily be split into thin slabs and plates.<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 |archive-url=https://web.archive.org/web/20180403144017/http://nora.nerc.ac.uk/id/eprint/3226/1/RR99002.pdf |archive-date=2018-04-03 |url-status=live |access-date=27 February 2021}}</ref><ref>{{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=4th |chapter=Slate}}</ref> It is usually formed by low-grade [[regional metamorphism]] of [[mudrock]].<ref name=Jackson>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of Geology |date=1997 |publisher=American Geological Institute |location=Alexandria, VA |isbn=0922152349 |edition=4th |chapter=Slate}}</ref><ref name=BlattTracy>{{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, NY |isbn=0716724383 |edition=2nd |page=365}}</ref> This mild degree of metamorphism produces a rock in which the individual mineral crystals remain microscopic in size,<ref name=BlattTracy/> producing a characteristic [[slaty cleavage]] in which fresh cleavage surfaces appear dull. This is in contrast to the silky cleaved surfaces of [[phyllite]], which is the next-higher grade of metamorphic rock derived from mudstone.<ref>{{cite book |last1=Yardley |first1=B. W. D. |title=An Introduction to Metamorphic Petrology |date=1989 |publisher=Longman Scientific & Technical |location=Harlow, Essex |isbn=0582300967 |page=22}}</ref> The direction of cleavage is independent of any [[sedimentary structures]] in the original mudrock, reflecting instead the direction of regional compression.<ref>{{cite book |last1=Potter |first1=Paul Edwin |last2=Maynard |first2=J. Barry |last3=Pryor |first3=Wayne A. |title=Sedimentology of Shale |date=1980 |publisher=Springer-Verlag |location=New York |isbn=0387904301 |page=17}}</ref>

Slaty cleavage is continuous, meaning that the individual cleavage planes are too closely spaced to be discernible in hand samples. The [[Texture (geology)|texture]] of the slate is totally dominated by these pervasive cleavage planes. Under a microscope, the slate is found to consist of very thin lenses of [[quartz]] and [[feldspar]] (QF-domains) separated by layers of [[mica]] (M-domains).<ref name=Fossen>{{cite book |last1=Fossen |first1=Haakon |title=Structural Geology |date=2016 |publisher=Cambridge University Press |location=Cambridge |isbn=9781107057647 |edition=2nd |pages=287–289}}</ref> These are typically less than 100 μm (micron) thick.<ref name=Jackson/> Because slate was formed in low heat and pressure, compared to most other metamorphic rocks, some [[fossils]] can be found in slate; sometimes even microscopic remains of delicate organisms can be found in slate.<ref>BBC Video: ''[[David Attenborough]]: Lost Worlds, Vanished Lives''</ref><ref>{{cite journal |last1=Glass |first1=Alexander |last2=Blake |first2=Daniel B. |date=April 2004 |title=Preservation of tube feet in an ophiuroid (Echinodermata) from the Lower Devonian Hunsrück Slate of Germany and a redescription of ''Bundenbachia beneckei'' and ''Palaeophiomyxa grandis'' |journal=Paläontologische Zeitschrift |volume=78 |issue=1 |pages=73–95 |bibcode=2004PalZ...78...73G |doi=10.1007/BF03009131 |s2cid=140689763}}</ref>

The process of conversion of mudrock to slate involves a loss of up to 50% of the volume of the mudrock as it is compacted. Grains of platy minerals, such as [[clay minerals]], are rotated to form parallel layers perpendicular to the direction of compaction, which begin to impart cleavage to the rock. Slaty cleavage is fully developed as the clay minerals begin to be converted to [[chlorite group|chlorite]] and mica. Organic carbon in the rock is converted to [[graphite]].{{sfn|Yardley|1989|pp=64, 170}}

Slate is mainly composed of the minerals quartz, [[illite]], and chlorite, which account for up to 95% of its composition. The most important accessory minerals are iron oxides (such as [[hematite]] and [[magnetite]]), iron sulfides (such as [[pyrite]]), and carbonate minerals. Feldspar may be present as [[albite]] or, less commonly, [[orthoclase]].<ref>{{cite journal |last1=Walsh |first1=Joan A. |title=The use of the scanning electron microscope in the determination of the mineral composition of Ballachulish slate |journal=Materials Characterization |date=November 2007 |volume=58 |issue=11–12 |pages=1095–1103 |doi=10.1016/j.matchar.2007.04.013}}</ref> Occasionally, as in the purple slates of [[Slate industry in Wales|North Wales]], ferrous ([[iron(II)]]) reduction spheres form around iron nuclei, leaving a light-green, spotted texture. These spheres are sometimes deformed by a subsequent applied stress field into ovoids, which appear as ellipses when viewed on a cleavage plane of the specimen. However, some evidence shows that reduced spots may also form after deformation and acquire an elliptical shape from preferential infiltration along the cleavage direction, so caution is required in using reduction ellipsoids to estimate deformation.{{sfn|Fossen|2016|p=61}}