Chemistry:Barbertonite

Short description: Hydrotalcite supergroup (barbertonite, discredited 2012)

Barbertonite
General
Category	Carbonate mineral
Formula (repeating unit)	Mg 6(Cr,Al) 2(OH) 16CO 3 · 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
|re|er}}	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

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

Barbertonite family group

Barbertonite is a member of the hexagonal sjogrenite group along with manasseite Mg
6Al
2(OH)
16CO
3 · 4H₂O and sjogrenite Mg
6Fe
2(OH)
16CO
3 · 4H₂O (Palache et al., 1944). The rhombohedral hydrotalcite group consists of the minerals: stichtite with 3 units of Mg
6Cr
2(OH)
16CO
3 · 4H₂O, hydrotalcite with 3 units of Mg
6Al
2(OH)
16CO
3 · 4H₂O, and pyroaurite with 3 units of Mg
6Fe
2(OH)
16CO
3 · 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 interlayers. Neighboring brucite layers are stacked so that the hydroxyl ions (OH−
) are directly above one another (Taylor, 1973). In between brucite layers are interlayers containing CO2−
3 ions and H
2O molecules (Taylor, 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, 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 (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).

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.