Chemistry:Barbertonite

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Short description: Hydrotalcite supergroup (barbertonite, discredited 2012)
Barbertonite
General
CategoryCarbonate mineral
Formula
(repeating unit)
Mg
6
(Cr,Al)
2
(OH)
16
CO
3
 · 4H2O
Strunz classification5.DA.45
Crystal systemHexagonal
Crystal classDihexagonal dipyramidal (6/mmm)
H-M symbol: (6/m 2/m 2/m)
Space groupP63/mmc
Unit cella = 6.17 Å, c = 15.52 Å; Z = 1
Identification
ColorIntense violet to rose-pink
Crystal habitflattened plates, in fibrous matted masses, and as cross-fiber veinlets
CleavagePerfect on {0001}
TenacityFlexible but not elastic
Mohs scale hardness1 12–2
|re|er}}Waxy, pearly
StreakVery pale violet to white
DiaphaneityTransparent
Specific gravity2.05–2.15
Optical propertiesUniaxial (−) may appear biaxial due to strain
Refractive indexnω = 1.557 nε = 1.529
Birefringenceδ = 0.028
PleochroismWeak, O = dark rose-pink to violet; E = pale rose-pink to violet
References[1][2][3]

Barbertonite is a magnesium chromium carbonate mineral with formula of Mg
6
Cr
2
(OH)
16
CO
3
 · 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).[4] Mills et al. (2011) presented evidence that barbertonite is a polytype of stichtite and should be discredited as a mineral species.[5]

Barbertonite family group

Barbertonite is a member of the hexagonal sjogrenite group along with manasseite Mg
6
Al
2
(OH)
16
CO
3
 · 4H2O
and sjogrenite Mg
6
Fe
2
(OH)
16
CO
3
 · 4H2O
(Palache et al., 1944).[6] The rhombohedral hydrotalcite group consists of the minerals: stichtite with 3 units of Mg
6
Cr
2
(OH)
16
CO
3
 · 4H2O
, hydrotalcite with 3 units of Mg
6
Al
2
(OH)
16
CO
3
 · 4H2O
, and pyroaurite with 3 units of Mg
6
Fe
2
(OH)
16
CO
3
 · 4H2O
. These two isostructural groups are polymorphous in relation to each other (Palache et al., 1944).[6]

Structure

The structure of barbertonite has brucite-like layers alternating with interlayers. Neighboring brucite layers are stacked so that the hydroxyl ions (OH
) are directly above one another (Taylor, 1973).[4] In between brucite layers are interlayers containing CO2−
3
ions and H
2
O
molecules (Taylor, 1973).[4] 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).[4]

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).[1] Read and Dixon (1933)[7] 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).[8] Barbertonite frequently occurs admixed with its rhombohedral analogue and as an alteration product of chromite in serpentinite (Anthony et al. 2003).[1]

References

  1. 1.0 1.1 1.2 "Barbertonite Mg6Cr2(CO3)(OH)16·4H2O". http://rruff.geo.arizona.edu/doclib/hom/barbertonite.pdf. 
  2. Barbertonite: mindat.org
  3. Barbertonite: webmineral.com
  4. 4.0 4.1 4.2 4.3 Taylor, H. F. W. (1973). "Crystal structures of some double hydroxide minerals". Mineralogical Magazine 39 (304): 377–389. doi:10.1180/minmag.1973.039.304.01. ISSN 0026-461X. 
  5. Mills, S. J.; Whitfield P. S.; Wilson S. A.; Woodhouse J. N.; Dipple G. M.; Raudsepp M.; Francis C. A. (2011). "The crystal structure of stichtite, re-examination of barbertonite, and the nature of polytypism in MgCr hydrotalcites". American Mineralogist 96 (1): 179–187. doi:10.2138/am.2011.3531. Bibcode2011AmMin..96..179M. https://nrc-publications.canada.ca/eng/view/accepted/?id=0efef0c0-56a3-4494-8ed1-07a7c83ac280. 
  6. 6.0 6.1 Palache, C.; Berman, H.; Frondel, C. (1944) (in En). 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) (7th ed.). John Wiley and Sons, Inc.. pp. 659. 
  7. Read, H. H.; Dixon, B. E. (1933). "On stichtite from Cunningsburgh, Shetland Islands". Mineralogical Magazine and Journal of the Mineralogical Society 23 (140): 309–316. doi:10.1180/minmag.1933.023.140.03. ISSN 0369-0148. 
  8. Frondel, C. (1941). "Constitution and polymorphism of the pyroaurite and sjogrenite groups". American Mineralogist 26: 295–315. ISSN 0003-004X. http://dx.doi.org/10.2138/am-2019-686. 

Further reading

  • 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.