Barbertonite

From Wikipedia, the free encyclopedia
Barbertonite
General
CategoryCarbonate mineral
Formula
(repeating unit)		Mg6(Cr,Al)2[(OH)16|CO3]·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+1⁄2-2
LusterWaxy, 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 [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 et al., 1973). A study published in 2011 presented evidence that barbertonite is a polytype of stichite and should be discredited as a mineral species.[4]

Barbertonite family group[]

Barbertonite is a member of the hexagonal Sjogrenite group along with manasseite [Mg6Al2(OH)16CO3.4H2O] and sjogrenite [Mg6Fe2(OH)16CO3.4H2O] (Palache et al., 2003). The rhombohedral hydrotalcite group consists of the minerals: stichtite [3(Mg6Cr2(OH)16CO3.4H2O)], hydrotalcite [3(Mg6Al2(OH)16CO3.4H2O)], and [3(Mg6Fe2(OH)16CO3.4H2O)]. These two isostructural groups are polymorphous in relation to each other (Palache et al., 1944).

Structure[]

The structure of barbertonite has brucite-like layers alternating with inter layers. Neighboring brucite layers are stacked so that the hydroxyl groups are directly above one another (Taylor et al., 1973). In between brucite layers are inter layers containing CO ions and H2O molecules (Taylor et al., 1973). 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 et al., 1973).

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 , Tasmania, Australia (Anthony et al., 2003). Read and Dixon (et al. 1933) 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). Barbertonite frequently occurs admixed with its rhombohedral analogue and as an alteration product of chromite in serpentinite (Anthony et al. 2003).

References[]

  1. ^ Handbook of Mineralogy
  2. ^ Mindat.org
  3. ^ Webmineral data
  4. ^ 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. Bibcode:2011AmMin..96..179M. doi:10.2138/am.2011.3531. S2CID 55810652.
  • Anthony, J.W., Bideaux, R., Bladh, K. and Nichols, M. (2003). Barbertonite Mg6Cr2(CO3)(OH)16·4H2O. Handbook of Mineralogy. Mineral Data Publishing (republished by the Mineralogical Society of America).
  • Barbertonite: webmineral.com
  • Barbertonite: mindat.org
  • Frondel, C. (1941). Constitution and polymorphism of the pyroaurite and sjogrenite groups. American Mineralogist, 26, 295–315.
  • 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.
  • Read and Dixon (1933). On stichtite from Cunningsburgh, Shetlands. Mineralogical Magazine, 23, 309–316.
  • Taylor, H.F.W. (1973). Crystal structures of some double hydroxide minerals. Mineralogical Magazine, 39, 377–389.
Retrieved from ""