Editing Chalk (section)

==Description==
[[File:Nitzana chalk curves (2), Western Negev, Israel.jpg|thumb|"Nitzana Chalk curves" situated at Western [[Negev]], [[Israel]], are chalk deposits formed in the [[Mesozoic]] era's [[Tethys Ocean]]]]
[[File:Seale Chalk Pit 02.JPG|thumb|right|Open chalk pit, Seale, Surrey, UK]]
Chalk is a fine-textured, earthy type of [[limestone]] distinguished by its light colour, softness, and high porosity.<ref name="jackson-1997">{{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 |chapter=Chalk}}</ref><ref>{{cite book |last1=Boggs |first1=Sam |title=Principles of sedimentology and stratigraphy |date=2006 |publisher=Pearson Prentice Hall |location=Upper Saddle River, N.J. |isbn=0131547283 |edition=4th}}</ref> It is composed mostly of tiny fragments of the [[calcite]] shells or skeletons of [[plankton]], such as [[foraminifera]] or [[coccolithophore]]s.<ref name="jackson-1997"/> These fragments mostly take the form of calcite plates ranging from 0.5 to 4 microns in size, though about 10% to 25% of a typical chalk is composed of fragments that are 10 to 100 microns in size. The larger fragments include intact plankton skeletons and skeletal fragments of larger organisms, such as [[mollusc]]s, [[echinoderm]]s, or [[bryozoans]].<ref name="hancock-1975">{{cite journal |last1=Hancock |first1=Jake M. |title=The petrology of the Chalk |journal=Proceedings of the Geologists' Association |date=January 1975 |volume=86 |issue=4 |pages=499–535 |doi=10.1016/S0016-7878(75)80061-7|bibcode=1975PrGA...86..499H }}</ref><ref name=craven>{{cite web|url=http://www.kabrna.com/cpgs/rocks/sedimentary/chalk.htm| archive-url=https://web.archive.org/web/20090620013255/http://www.kabrna.com/cpgs/rocks/sedimentary/chalk.htm|archive-date=20 June 2009|title=Chalk|website=Craven & Pendle Geological Society}}</ref><ref name=geology>{{cite web | title=Chalk: A biological limestone formed from shell debris | website=Geology.com|first=Hobart M. |last=King| url=https://geology.com/rocks/chalk.shtml | access-date=1 March 2021}}</ref>

Chalk is typically almost pure calcite, {{chem2|CaCO3}}, with just 2% to 4% of other minerals. These are usually [[quartz]] and [[clay mineral]]s, though collophane (cryptocrystalline [[apatite]], a [[phosphate]] mineral) is also sometimes present, as nodules or as small pellets interpreted as fecal pellets. In some chalk beds, the calcite has been converted to [[dolomite (mineral)|dolomite]], {{chem2|CaMg(CO3)2}}, and in a few cases the dolomitized chalk has been dedolomitized back to calcite.<ref name="hancock-1975"/>

Chalk is highly porous, with typical values of porosity ranging from 35 to 47 per cent.<ref name="hancock-1975"/> While it is similar in appearance to both [[gypsum]] and [[diatomite]], chalk is identifiable by its hardness, [[fossil]] content, and its reaction to [[acid]] (it produces [[effervescence]] on contact).<ref name=geology/>