Barbertonite

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
General
Category	Carbonate mineral
Formula (repeating unit)	Mg6(Cr,Al)2[(OH)16|CO3]·4H2O
Strunz classification	5.DA.45
Crystal system	Hexagonal
Crystal class	Dihexagonal dipyramidal (6/mmm) H-M symbol: (6/m 2/m 2/m)
Space group	P63/mmc
Unit cell	a = 6.17 Å, c = 15.52 Å; Z = 1
Identification
Color	Intense violet to rose-pink
Crystal habit	flattened plates, in fibrous matted masses, and as cross-fiber veinlets
Cleavage	Perfect on {0001}
Tenacity	Flexible but not elastic
Mohs scale hardness	​1 ^1⁄2-2
Luster	Waxy, pearly
Streak	Very pale violet to white
Diaphaneity	Transparent
Specific gravity	2.05 - 2.15
Optical properties	Uniaxial (-) may appear biaxial due to strain
Refractive index	nω = 1.557 nε = 1.529
Birefringence	δ = 0.028
Pleochroism	Weak, O = dark rose-pink to violet; E = pale rose-pink to violet
References	[1][2][3]

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 pyroaurite [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 Dumas, 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: 179–187. doi:10.2138/am.2011.3531.

• 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.W.F. (1973) Crystal Structures of some double Hydroxide Minerals. Mineralogical Magazine, 39, 377-389.