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, 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 | |
|---|---|
| General | |
| Category | Carbonate mineral |
| Formula (repeating unit) | Mg6(Cr,Al)2(OH)16CO3·4H2O |
| 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., 1944).[6] The rhombohedral hydrotalcite group consists of the minerals: stichtite with 3 units of Mg6Cr2(OH)16CO3·4H2O, hydrotalcite with 3 units of Mg6Al2(OH)16CO3·4H2O, and pyroaurite with 3 units of Mg6Fe2(OH)16CO3·4H2O. 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 H2O 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
- "Barbertonite Mg6Cr2(CO3)(OH)16·4H2O" (PDF). RRUFF.
- Barbertonite: mindat.org
- Barbertonite: webmineral.com
- 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.
- 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.
- Palache, C.; Berman, H.; Frondel, C. (1944). 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. p. 659.
- 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.
- Frondel, C. (1941). "Constitution and polymorphism of the pyroaurite and sjogrenite groups". American Mineralogist. 26: 295–315. ISSN 0003-004X.
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.