{{Short description|Mineral}} {{Infobox mineral | name = Shigaite | category = Sulfate mineral | boxwidth = | boxbgcolor = | image = Rhodochrosite-Shigaite-cktsr-4a.jpg | imagesize = | caption = Reddish-brown shigaite crystal (2 cm across) with pink rhodochrosite from South Africa | formula = NaAl<sub>3</sub>(Mn<sup>2+</sup>)<sub>6</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>18</sub>·12H<sub>2</sub>O<ref name=CNMNC_list>{{cite web|last=Nickel|first=Ernest H.|title=IMA/CNMNC List of Mineral Names|url=http://pubsites.uws.edu.au/ima-cnmnc/IMA2009-01%20UPDATE%20160309.pdf|publisher=Materials Data, Inc.|access-date=May 6, 2012}}</ref> | IMAsymbol = Sga<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|url=https://www.cambridge.org/core/journals/mineralogical-magazine/article/imacnmnc-approved-mineral-symbols/62311F45ED37831D78603C6E6B25EE0A|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free|url-access=subscription}}</ref> | molweight = | strunz = 7.DD.35<ref name=mindat>{{cite web|title=Shigaite|url=http://www.mindat.org/min-3638.html|publisher=Mindat|access-date=April 20, 2012}}</ref> | dana = 31.1.2.1<ref name=mindat/> | system = Trigonal | class = Rhombohedral ({{overline|3}}) <br/>H-M symbol: ({{overline|3}})<ref name=mindat/> | symmetry = ''R''{{overline|3}}<ref name=webmineral/> | unit cell = a = 9.51 Å, c = 32.83 Å,<ref name=mindat/> Z = 3<ref name=handbook/> | color = Yellow, burnt orange, brown, black<ref name=handbook/> | habit = | twinning = On {0001}<ref name=handbook>{{cite web|title=Shigaite|url=http://www.handbookofmineralogy.com/pdfs/shigaite.pdf|work=Handbook of Mineralogy|publisher=Mineral Data Publishing|access-date=April 20, 2012}}</ref> | cleavage = Perfect on {0001}<ref name=handbook/> | fracture = | tenacity = Moderately flexible<ref name=handbook/> | mohs = 2<ref name=handbook/> | luster = Vitreous to dull<ref name=mindat/> | polish = | refractive = n = 1.546<ref name=webmineral/> | opticalprop = Uniaxial (−)<ref name=mindat/> | birefringence = | 2V = | dispersion = | pleochroism = Distinct; O = yellow; E = very pale yellow<ref name=handbook/> | fluorescence= Non-fluorescent<ref name=webmineral/> | absorption = | streak = Very pale yellow to white<ref name=handbook/> | gravity = 2.32<ref name=webmineral>{{cite web|title=Shigaite Mineral Data|url=http://webmineral.com/data/Shigaite.shtml|publisher=Webmineral|access-date=May 20, 2012}}</ref> | density = | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = Transparent<ref name=mindat/> | other = | references = }} '''Shigaite''' is a mineral with formula NaAl<sub>3</sub>(Mn<sup>2+</sup>)<sub>6</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>18</sub>·12H<sub>2</sub>O that typically occurs as small, hexagonal crystals or thin coatings. It is named for Shiga Prefecture, Japan, where it was discovered in 1985.<ref name=mindat/> The formula was significantly revised in 1996, identifying sodium as a previously unknown constituent.
==Description== thumb|left|Dark-red and yellow shigaite on pink rhodochrosite Shigaite occurs as hexagonal tabular crystals up to {{convert|2|cm|in|abbr=on}} in size or as thin films and coatings. The mineral can be yellow, burnt orange, brown or black in color.<ref name=handbook/> Shigaite occurs in metamorphosed deposits of manganese ore<ref name=handbook/> and is the Mn<sup>2+</sup> analogue of motukoreaite.<ref name=Cooper91>Cooper, p. 91.</ref>
==Structure== Shigaite consists of oxycation sheets of [AlMn<sup>2+</sup><sub>2</sub>(OH)<sub>6</sub>]<sup>1+</sup> intercalated with oxyanion sheets of [Na(H<sub>2</sub>O)<sub>6</sub>{H<sub>2</sub>O}<sub>6</sub>(SO<sub>4</sub>)<sub>2</sub>]<sup>3−</sup>. Linkage between the sheets and within the oxyanion sheet results largely through hydrogen bonding.<ref name=Cooper91/>
==History== Shigaite was discovered in 1985 in the Ioi Mine,{{#tag:ref|Some sources incorrectly list it as the Loi Mine,<ref name=NewNames>{{cite journal|last1=Hawthorne|first1=Frank C.|title=New Mineral Names|journal=American Mineralogist|date=November–December 1986|volume=71|issue=11 & 12|page=1546|url=http://www.minsocam.org/ammin/AM71/AM71_1543.pdf|access-date=May 17, 2012|display-authors=etal}}</ref> presumably because of a mistaken reading of uppercase "i" as lowercase "L".|group=lower-alpha|name=location}} Shiga Prefecture, Japan.<ref name=mindat/> The original study, published in the journal ''Neues Jahrbuch für Mineralogie, Monatshefte'',<ref name=CNMNC_list/> identified the formula as Al<sub>4</sub>Mn<sub>7</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>22</sub>·8H<sub>2</sub>O.<ref name=NewNames/> The formula was significantly revised in 1996 using a sample from the N'Chwaning Mine, South Africa.<ref name=Cooper91>Cooper, p. 91.</ref> Sodium, discovered to be a component of shigaite, was not identified in the original study. However, an unidentified volatile had been noted that presumably was a sodium-containing complex.<ref name=Cooper96>Cooper, p. 96.</ref>
==Distribution== {{As of|2012}}, shigaite is known from the following sites:<ref name=mindat/> *Iron Monarch open cut, South Australia, Australia *Poudrette quarry, Quebec, Canada *Ioi mine, Shiga Prefecture, Japan *Wessels Mine, Northern Cape Province, South Africa *N'Chwaning Mine, Northern Cape Province, South Africa *Homer Mine, Michigan, United States *Bengal Mine, Michigan, United States
The type material is housed in the National Museum of Natural History in Washington, D.C. as sample 122089.<ref name=handbook/>
==Association== Shigaite has been found associated with the following minerals:<ref name=handbook/> {{col-begin}} {{col-break}} Ioi mine, Japan *rhodochrosite *sonolite *manganosite *pyrochroite *jacobsite *hausmannite *galaxite
{{col-break}} Wessels Mine, South Africa *rhodochrosite *leucophoenicite *gageite *caryopilite
{{col-break}} Iron Monarch, South Australia *arsenoclasite *gatehouseite *hematite *hausmannite *manganoan ferroan calcite *barite *gypsum {{col-end}}
==Notes== {{Reflist|group=lower-alpha}}
==References== {{commons category|Shigaite}} {{Reflist|colwidth=33em}}
===Bibliography=== *{{cite journal|last1=Cooper|first1=Mark A.|last2=Hawthorne|first2=Frank C.|title=The crystal structure of shigaite, (AlMn (super 2+)<sub>2</sub> (OH)<sub>6</sub>)<sub>3</sub> (SO<sub>4</sub>)<sub>2</sub> Na(H<sub>2</sub>O)<sub>6</sub> {H<sub>2</sub>O}<sub>6</sub>, hydrotalcite-group mineral|journal=The Canadian Mineralogist|date=February 1996|volume=34|issue=1|pages=91–97|url=http://rruff.geo.arizona.edu/doclib/cm/vol34/CM34_91.pdf|access-date=May 23, 2012|issn=0008-4476}}
==Further reading== *{{cite journal|last1=Pring|first1=A.|last2=Slade|first2=P. G.|last3=Birch|first3=W. D.|title=Shigaite from Iron Monarch, South Australia|journal=Mineralogical Magazine|date=September 1992|volume=56|issue=384|pages=417–419|url=http://rruff.geo.arizona.edu/doclib/MinMag/Volume_56/56-384-417.pdf|access-date=April 20, 2012|doi=10.1180/minmag.1992.056.384.15|bibcode=1992MinM...56..417P|s2cid=140697083 }}
Category:Sulfate minerals Category:Manganese(II) minerals Category:Aluminium minerals Category:Trigonal minerals Category:Minerals in space group 148