Barbertonite

Barbertonite
General
CategoryCarbonate mineral
Formula
(repeating unit)		Mg6(Cr,Al)2(OH)16CO3·4H2O
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, 1973).[4] Mills et al. (2011) presented evidence that barbertonite is a polytype of stichtite and should be discredited as a mineral species.[5]

  1. ^ "Barbertonite Mg6Cr2(CO3)(OH)16·4H2O" (PDF). RRUFF.
  2. ^ Barbertonite: mindat.org
  3. ^ Barbertonite: webmineral.com
  4. ^ 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. Bibcode:2011AmMin..96..179M. doi:10.2138/am.2011.3531. S2CID 55810652.