Barbertonite

Barbertonite is a hexagonal polymorph of 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). Closely intermixed with its polymorph, analysis of absolute material is therefore non-existent. This has long been a topic of confusion. The main factors contributing to this are lack of sufficient crystallographic data and the complex composition of the several species coupled with the inadequate nature of the chemical analysis available (Frondel et al. 1941). A further hurtle has been the lack of correlation of the publications on the subject.

Barbertonite family group

Barbertonite [Mg₆Cr₂(OH)₁₆CO₃.4H₂O] 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

Found closely associated with stichtite, chromite, and antigote, 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).

Where in the world

Barbertonite was first found in the Barberton district in the South African providence of Mpumalanga, known as “the land of the rising sun” to its Siswati and Zulu speaking inhabitants. Barberton was originated in the 1880s gold rush in the region, located in the De Kaap Valley and bordered by the Mkhonjwa Mountains. First flourishing, Barberton then diminished when its inhabitants moved away to the newly discovered gold fields on the reef.

References

* 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).
* 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.



List of minerals