Barbertonite

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).

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).

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).