Editing Talc (section)

==Formation==
[[File:Talc block.jpg|thumb|right|A block of talc]]

Talc dominantly forms from the metamorphism of magnesian minerals such as [[Serpentine subgroup|serpentine]], [[pyroxene]], [[amphibole]], and [[olivine]], in the presence of carbon dioxide and water. This is known as "talc carbonation" or "steatization" and produces a suite of rocks known as [[talc carbonate]]s.

Talc is primarily formed by hydration and carbonation by this reaction:

:{{overset|[[Serpentine subgroup|serpentine]]|2 Mg<sub>3</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub>}} + {{overset|[[carbon dioxide]]|3 CO<sub>2</sub>}} → {{overset|talc|Mg<sub>3</sub>Si<sub>4</sub>O<sub>10</sub>(OH)<sub>2</sub>}} + {{overset|[[magnesite]]|3 MgCO<sub>3</sub>}} + {{overset|[[water]]|3 H<sub>2</sub>O}}

Talc can also be formed via a reaction between dolomite and silica, which is typical of [[skarn]]ification of dolomites by silica-flooding in contact metamorphic aureoles:

:{{overset|[[Dolomite (mineral)|dolomite]]|3 CaMg(CO<sub>3</sub>)<sub>2</sub>}} + {{overset|[[Silicon dioxide|silica]]|4 SiO<sub>2</sub>}} + {{overset|water|H<sub>2</sub>O}} → {{overset|talc|Mg<sub>3</sub>Si<sub>4</sub>O<sub>10</sub>(OH)<sub>2</sub>}} + {{overset|[[calcite]]|3 CaCO<sub>3</sub>}} + {{overset|carbon dioxide|3 CO<sub>2</sub>}}

Talc can also be formed from magnesium chlorite and quartz in [[blueschist]] and [[eclogite]] metamorphism by the following [[metamorphic reaction]]:

:[[Chlorite group|chlorite]] + [[quartz]] → [[kyanite]] + talc + water

Talc is also found as a diagenetic mineral in sedimentary rocks where it can form from the transformation of metastable hydrated magnesium-clay precursors such as [[kerolite]], [[sepiolite]], or [[stevensite]] that can precipitate from marine and lake water in certain conditions.<ref name="Strauss MacDonald Halverson Tosca 2015 pp. 881–944">{{cite journal | last1=Strauss | first1=Justin V. | last2=MacDonald | first2=Francis A. | last3=Halverson | first3=Galen P. | last4=Tosca | first4=Nicholas J. | last5=Schrag | first5=Daniel P. | last6=Knoll | first6=Andrew H. | title=Stratigraphic evolution of the Neoproterozoic Callison Lake Formation: Linking the break-up of Rodinia to the Islay carbon isotope excursion | journal=American Journal of Science | publisher=American Journal of Science (AJS) | volume=315 | issue=10 | year=2015 | issn=0002-9599 | doi=10.2475/10.2015.01 | pages=881–944| bibcode=2015AmJS..315..881S | s2cid=130671089 | url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:30367434 | doi-access=free }}</ref>

In this reaction, the ratio of talc and kyanite depends on [[aluminium]] content, with more aluminous rocks favoring production of kyanite. This is typically associated with high-pressure, low-temperature minerals such as [[phengite]], [[garnet]], and [[glaucophane]] within the lower [[Blueschist#Blueschist facies|blueschist facies]]. Such rocks are typically white, friable, and fibrous, and are known as
[[whiteschist]].

Talc is a trioctahedral layered mineral; its structure is similar to [[pyrophyllite]], but with magnesium in the octahedral sites of the composite layers.<ref name="DHZ" /> The crystal structure of talc is described as ''TOT'', meaning that it is composed of parallel ''TOT'' layers weakly bonded to each other by weak [[van der Waals force]]s. The ''TOT'' layers in turn consist of two tetrahedral sheets (''T'') strongly bonded to the two faces of a single trioctahedral sheet (''O''). It is the weak bonding between ''TOT'' layers that gives talc its perfect basal cleavage and softness.<ref>{{cite book |last=Nesse |first=William D. |date=2000 |title=Introduction to mineralogy |location=New York |publisher=Oxford University Press |isbn=9780195106916 |page=238}}</ref>

The tetrahedral sheets consist of silica tetrahedra, which are silicon ions surrounded by four oxygen ions. The tetrahedra each share three of their four oxygen ions with neighboring tetrahedra to produce a hexagonal sheet. The remaining oxygen ion (the ''apical'' oxygen ion) is available to bond with the trioctahedral sheet.{{sfn|Nesse|2000|p=235}}

The trioctahedral sheet has the structure of a sheet of the mineral [[brucite]]. Apical oxygens take the place of some of the hydroxyl ions that would be present in a brucite sheet, bonding the tetrahedral sheets tightly to the trioctahedral sheet.{{sfn|Nesse|2000|pp=235–237}}

Tetrahedral sheets have a negative charge, since their bulk composition is {{chem2|Si4O10(4-)}}. The trioctahedral sheet has an equal positive charge, since its bulk composition is {{chem2|Mg3(OH)2(4+)}} The combined TOT layer thus is electrically neutral.{{sfn|Nesse|2000|p=238}}

Because the hexagons in the T and O sheets are slightly different in size, the sheets are slightly distorted when they bond into a TOT layer. This breaks the hexagonal symmetry and reduces it to monoclinic or triclinic symmetry.{{sfn|Nesse|2000|pp=239, 242}} However, the original hexahedral symmetry is discernible in the pseudotrigonal character of talc crystals.<ref name=Handbook/>

<gallery>
File:Mica T.png|View of tetrahedral sheet structure of talc. The apical oxygen ions are tinted pink.
File:Talc to.jpg|View of trioctahedral sheet of talc. Yellow spheres are hydroxyl; blue are magnesium. Apical oxygen binding sites are white.
File:Talc structure.jpg|Talc crystal viewed along the [100] axis, looking along the layers of the crystal
</gallery>