Editing Gibbsite

{{short description|Form of aluminium hydroxide, mineral}}
{{Infobox mineral
| name        = Gibbsite
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| image       = Gibbsite-fiu14a.jpg
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| alt         = 
| caption     = Gibbsite
| category    = [[Hydroxide minerals]]
| formula     = Al(OH)<sub>3</sub> 
| IMAsymbol   = Gbb<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|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}}</ref>
| molweight   = 
| strunz      = 4.FE.10
| dana        = 
| system      = [[Monoclinic]]
| class       = Prismatic (2/m) <br/><small>(same [[H-M symbol]])</small>
| symmetry    = ''P2''<sub>1</sub>/n
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| mohs        = 2.5–3
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| gravity     = 2.35
| density     = 
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[[Image:Gibbsite-crystal-3D-balls.png|thumb|right|[[Ball-and-stick model]] of the part of the [[crystal structure]] of gibbsite]]
<!-- [[File:Gibbsite-270400.jpg|thumb|upright|An unusual specimen of Gibbsite from [[Linwu County]], [[Hunan Province]], China. Size: 9.1 x 4.6 x 4.4 cm.]] -->
'''Gibbsite''', Al(OH)<sub>3</sub>, is one of the mineral forms of [[aluminium hydroxide]]. It is often designated as γ-Al(OH)<sub>3</sub><ref name=":0" />{{rp| 2}} (but sometimes as α-Al(OH)<sub>3</sub><ref>N.N. Greenwood and A. Earnshaw, "Chemistry of Elements", 2nd edition, Butterworth and Heinemann, 1997</ref>). It is also sometimes called '''hydrargillite''' (or '''hydrargyllite''').

Gibbsite is an important ore of [[aluminium]] in that it is one of three main [[phase (matter)|phases]] that make up the rock [[bauxite]].

Gibbsite has three named structural [[polymorphism (materials science)|polymorphs]] or [[polytype]]s: '''bayerite''' (designated often as α-Al(OH)<sub>3</sub>,<ref name=":0" />{{rp| 2}} but sometimes as β-Al(OH)<sub>3</sub>){{Citation needed|date=October 2020}}, ''[[doyleite]]'', and '''nordstrandite'''. Gibbsite can be [[monoclinic]] or [[Triclinic crystal system|triclinic]], while bayerite is monoclinic.<ref name=":0">{{Cite book|last1=Wefers|first1=Karl|url=http://worldcat.org/oclc/894928306|title=Oxides and hydroxides of aluminum|last2=Misra|first2=Chanakya|date=1987|publisher=Alcoa Research Laboratories|oclc=894928306}}</ref>{{rp| 13}} Doyleite and nordstrandite are triclinic forms.<ref name=":0" />{{rp|13}}

==Structure==
The structure of gibbsite is interesting and analogous to the basic structure of the [[mica]]s. The basic structure forms stacked sheets of linked [[octahedron|octahedra]]. Each octahedron is composed of an [[aluminium]] ion bonded to six hydroxide groups, and each [[hydroxide]] group is shared by two aluminium octahedra.<ref>{{cite journal |journal= Zeitschrift für Kristallographie |volume= 139 |year= 1974 |pages= 129–135 |title= Refinement of the crystal structure of gibbsite, Al(OH)<sub>3</sub> |first1= H. |last1= Saalfeld |first2= M. |last2= Wedde |issue= 1–2 |doi= 10.1524/zkri.1974.139.1-2.129 |bibcode= 1974ZK....139..129S |url= https://rruff.info/doclib/zk/vol139/ZK139_129.pdf }}</ref> One third of the potential octahedral spaces are missing a central aluminium. The result is a neutral sheet: with aluminium as a +3 ion and hydroxide a −1 ion, the net cationic charge of one aluminium per six hydroxides is (+3)/6 = +1/2, and likewise the net anionic charge of one hydroxide per two aluminium atoms is (−1)/2 = −1/2. The lack of a charge on the gibbsite sheets means that there is no charge to retain ions between the sheets and act as a "glue" to keep the sheets together. The sheets are only held together by weak residual bonds and this results in a very soft easily cleaved mineral.{{Citation needed|date=January 2013}}

Gibbsite's structure is closely related to the structure of [[brucite]], Mg(OH)<sub>2</sub>. However the lower charge in brucite's magnesium (+2) as opposed to gibbsite's aluminium (+3) does not require that one third of the octahedrons be vacant of a central ion in order to maintain a neutral sheet. The different symmetry of gibbsite and brucite is due to the different way that the layers are stacked.

It is the gibbsite layer that in a way forms the "floor plan" for the mineral [[corundum]], Al<sub>2</sub>O<sub>3</sub>. The basic structure of corundum is identical to gibbsite except the hydroxides are replaced by [[oxygen]]. Since oxygen has a charge of −2 the layers are not neutral and require that they must be bonded to other aluminiums above and below the initial layer producing the framework structure that is the structure of corundum.

Gibbsite is interesting for another reason because it is often found as a part of the structure of other minerals. The neutral aluminium hydroxide sheets are found sandwiched between silicate sheets in important clay groups: the [[illite]], [[kaolinite]], and [[montmorillonite]]/[[smectite]] groups. The individual aluminium hydroxide layers are identical to the individual layers of gibbsite and are referred to as the ''gibbsite layers''.<ref>[http://www.galleries.com/minerals/oxides/gibbsite/gibbsite.htm Gibbsite on Galleries.com]</ref>

The lattice parameters for gibbsite depending upon the particular method used to measure or calculate them and are therefore displayed as ranges below. An Al-Al interlayer spacing of 0.484 or 0.494&nbsp;nm has been reported.<ref>{{Cite book|url=https://www.worldcat.org/oclc/894277485|title=Advances in catalysis.|date=24 November 2014|isbn=978-0-12-800300-8|location=Amsterdam|pages=321–325|oclc=894277485}}</ref>
{| class="wikitable"
|+Gibbsite structural properties <!-- These values vary widely and ideally would be confirmed with additional sources. -->
! rowspan="2" |Crystal system
! rowspan="2" |Molecules per unit cell
! colspan="3" |Lattice parameters (nm)
! colspan="3" |Lattice angles (°)
|-
!a
!b
!c
!
!
!
|-
|Monoclinic<ref>{{Cite journal|last1=Gale|first1=Julian D.|last2=Rohl|first2=Andrew L.|last3=Milman|first3=Victor|last4=Warren|first4=Michele C.|date=2001|title=An ab Initio Study of the Structure and Properties of Aluminum Hydroxide: Gibbsite and Bayerite|url=https://pubs.acs.org/doi/10.1021/jp011795e|journal=The Journal of Physical Chemistry B|language=en|volume=105|issue=42|pages=10236–10242|doi=10.1021/jp011795e|issn=1520-6106|url-access=subscription}}</ref> 
|4
|0.850–0.868
|0.487–0.508
|0.922–0.974
|
|92.8–94.5
|
|-
|Triclinic <ref name=":0" />{{rp| 13}}
|16
|1.733
|1.008
|0.973
|94.2
|92.1
|90.0
|}

== Mineralogical properties ==
{| class="wikitable"
|+Index of Refraction <ref name=":0" />{{rp| 12}}
!α
!β
!γ
|-
|1.568
|1.568
|1.587
|}

==Etymology==
Gibbsite is named after [[George Gibbs (mineralogist)|George Gibbs]] (1776–1833), an American [[mineral collector]].<ref>[http://webmineral.com/data/Gibbsite.shtml Gibbsite Mineral Data on Webmineral.com]</ref>

==References==
{{reflist}}

==Further reading==
*Hurlbut, Cornelius S.; Klein, Cornelis, 1985, ''Manual of Mineralogy'', 20th ed., {{ISBN|0-471-80580-7}}

{{commons category|Gibbsite}}

[[Category:Aluminium minerals]]
[[Category:Hydroxide minerals]]
[[Category:Monoclinic minerals]]
[[Category:Minerals in space group 11]]
[[Category:Luminescent minerals]]