Barbertonite

{{Short description|Magnesium chromium carbonate mineral}} {{Infobox mineral | name = Barbertonite | boxwidth = | boxbgcolor = | image = | imagesize = | alt = | caption = | category = Carbonate mineral | formula = {{chem2|Mg6(Cr,Al)2(OH)16CO3*4H2O}} | molweight = | strunz = 5.DA.45 | dana = | system = Hexagonal | class = Dihexagonal dipyramidal (6/mmm) <br/>H-M symbol: (6/m 2/m 2/m) | symmetry = ''P''6₃/mmc | unit cell = a = 6.17&nbsp;Å, c = 15.52&nbsp;Å; Z&nbsp;=&nbsp;1 | color = Intense violet to rose-pink | colour = | habit = flattened plates, in fibrous matted masses, and as cross-fiber veinlets | twinning = | cleavage = Perfect on {0001} | fracture = | tenacity = Flexible but not elastic | mohs = {{frac|1|1|2}}–2 | luster = Waxy, pearly | streak = Very pale violet to white | diaphaneity = Transparent | gravity = 2.05–2.15 | density = | polish = | opticalprop = Uniaxial (−) may appear biaxial due to strain | refractive = n_ω = 1.557 n_ε = 1.529 | birefringence = δ = 0.028 | pleochroism = Weak, O = dark rose-pink to violet; E = pale rose-pink to violet | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = <!-- Please, do not insert a template {{chem2}} in the references citation code as this does not support it --> | references = <ref name="HBM">{{cite web | title=Barbertonite Mg₆Cr₂(CO₃)(OH)₁₆·4H₂O |website=RRUFF |url=http://rruff.geo.arizona.edu/doclib/hom/barbertonite.pdf}}</ref><ref name="Mindat">[http://www.mindat.org/min-514.html Barbertonite: mindat.org]</ref><ref name="Webmin">[http://www.webmineral.com/data/Barbertonite.shtml Barbertonite: webmineral.com]</ref> }}

'''Barbertonite''' is a magnesium chromium carbonate mineral with formula of {{chem2|Mg6Cr2(OH)16CO3*4H2O}}. It is polymorphous with the mineral stichtite and, along with stichtite, is an alteration product of chromite in serpentinite. Barbertonite has a close association with stichtite, chromite, and antigorite (Taylor, 1973).<ref name="Taylor1973">{{Cite journal |last=Taylor |first=H. F. W. |date=1973 |title=Crystal structures of some double hydroxide minerals |journal=Mineralogical Magazine |volume=39 |issue=304 |pages=377–389 |doi=10.1180/minmag.1973.039.304.01 |issn=0026-461X}}</ref> Mills et al. (2011) presented evidence that barbertonite is a polytype of stichtite and should be discredited as a mineral species.<ref name="Mills2011">{{cite journal|last1=Mills|first1=S. J.|author2=Whitfield P. S.|author3=Wilson S. A.|author4=Woodhouse J. N.|author5=Dipple G. M.|author6=Raudsepp M.|author7=Francis C. A.|year=2011|title=The crystal structure of stichtite, re-examination of barbertonite, and the nature of polytypism in MgCr hydrotalcites|journal=American Mineralogist|volume=96|issue=1|pages=179–187|doi=10.2138/am.2011.3531|bibcode=2011AmMin..96..179M|s2cid=55810652|url=https://nrc-publications.canada.ca/eng/view/accepted/?id=0efef0c0-56a3-4494-8ed1-07a7c83ac280}}</ref>

== Barbertonite family group == Barbertonite is a member of the hexagonal sjogrenite group along with manasseite {{chem2|Mg6Al2(OH)16CO3*4H2O}} and sjogrenite {{chem2|Mg6Fe2(OH)16CO3*4H2O}} (Palache et al., 1944).<ref name="Palache1944">{{Cite book |last1=Palache |first1=C. |last2=Berman |first2=H. |last3=Frondel |first3=C. |year=1944 |title=The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana, Yale University, 1837-1892: Halides, nitrates, borates, carbonates, sulfates, phosphates, arsenates, tungstates, molybdates, etc (Vol. 2) |edition=7th |pages=659 |publisher=John Wiley and Sons, Inc. |language=En |ref=Dana1941}}</ref>

The rhombohedral hydrotalcite group consists of the three minerals:<br /> – stichtite {{Spaces|5}} with 3 units of {{chem2|Mg6Cr2(OH)16CO3*4H2O}};<br /> – hydrotalcite with 3 units of {{chem2|Mg6Al2(OH)16CO3*4H2O}}, and;<br /> – pyroaurite {{Spaces|1}} with 3 units of {{chem2|Mg6Fe2(OH)16CO3*4H2O}}.

These two isostructural groups are polymorphous in relation to each other (Palache et al., 1944).<ref name="Palache1944" />

== Structure == The structure of barbertonite has brucite-like layers alternating with interlayers. Neighboring brucite layers are stacked so that the hydroxyl ions ({{chem2|OH-}}) are directly above one another (Taylor, 1973).<ref name="Taylor1973" /> In between brucite layers are interlayers containing {{chem2|CO3(2-)}} ions and {{H2O}} molecules (Taylor, 1973).<ref name="Taylor1973" /> Oxygen atoms are accommodated in a single set of sites distributed close to the axes that pass through the hydroxyl ions of adjacent brucite layers (Taylor, 1973).<ref name="Taylor1973" />

== Geologic occurrence == Barbertonite was first found in the Barberton district in Transvaal, South Africa. It can also be found in the Ag-Pb mine in Dumas, Tasmania, Australia (Anthony et al., 2003).<ref name="HBM" /> Read and Dixon (1933)<ref name="ReadDixon1933">{{Cite journal |last=Read |first=H. H. |last2=Dixon |first2=B. E. |date=1933 |title=On stichtite from Cunningsburgh, Shetland Islands |journal=Mineralogical Magazine and Journal of the Mineralogical Society |volume=23 |issue=140 |pages=309–316 |doi=10.1180/minmag.1933.023.140.03 |issn=0369-0148}}</ref> stated that the mineral that was found in Cunningsburgh, Shetland Islands was stichtite but it is now thought to be barbertonite because of the very similar indices of the minerals (Frondel et al. 1941).<ref name="Frondel1941">{{Cite journal |last=Frondel |first=C. |date=1941 |title=Constitution and polymorphism of the pyroaurite and sjogrenite groups |url=http://dx.doi.org/10.2138/am-2019-686 |journal=American Mineralogist |volume=26 |issue= |pages=295–315 |doi= |issn=0003-004X |ref=Frondel1941}}</ref> Barbertonite frequently occurs admixed with its rhombohedral analogue and as an alteration product of chromite in serpentinite (Anthony et al. 2003).<ref name="HBM" />

== References == <!-- Please, do not insert a template {{chem2}} in the references citation code as this does not support it --> {{Reflist}}

== Further reading == <!-- Please, do not insert a template {{chem2}} in the references citation code as this does not support it --> {{Refbegin}} * Mondel, S. K., Baidya, T.K. (1996). Stichtite [Mg₆Cr₂(OH)₁₆CO₃·4H₂O] in Nausahi ultramafites, Orissa, India – Its transformation at elevated temperatures. ''Mineralogical Magazine'', 60, 836–840. * Palache, C., Berman H., and Frondel C. (1944). ''Dana's System of Mineralogy'', (7th Edition), v. 1, 659. {{Refend}}

Category:Magnesium minerals Category:Chromium minerals Category:Carbonate minerals Category:Hexagonal minerals Category:Minerals in space group 194 Category:Polymorphism (materials science)