Category Carbonate mineral Crystal system Hexagonal | Strunz classification 5.DA.45 Space group P63/mmc | |
Formula
(repeating unit) Mg6(Cr,Al)2[(OH)16|CO3]·4H2O Crystal class Dihexagonal dipyramidal (6/mmm)
H-M symbol: (6/m 2/m 2/m) | ||
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.
Contents
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).