Barbertonite Explained

Barbertonite
Category:Carbonate mineral
Strunz:5.DA.45
System:Hexagonal
Class:Dihexagonal dipyramidal (6/mmm)
H-M symbol: (6/m 2/m 2/m)
Symmetry:P63/mmc
Unit Cell:a = 6.17 Å, c = 15.52 Å; Z = 1
Color:Intense violet to rose-pink
Habit:flattened plates, in fibrous matted masses, and as cross-fiber veinlets
Cleavage:Perfect on
Tenacity:Flexible but not elastic
Mohs:–2
Luster:Waxy, pearly
Streak:Very pale violet to white
Diaphaneity:Transparent
Gravity:2.05–2.15
Opticalprop:Uniaxial (−) may appear biaxial due to strain
Refractive: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 is a magnesium chromium carbonate mineral with formula of . 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 family group

Barbertonite is a member of the hexagonal sjogrenite group along with manasseite and sjogrenite (Palache et al., 1944).[6]

The rhombohedral hydrotalcite group consists of the three minerals:
stichtite with 3 units of ;
hydrotalcite with 3 units of, and;
– pyroaurite with 3 units of .

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 are directly above one another (Taylor, 1973). In between brucite layers are interlayers containing ions and 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)[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).

Further reading

Notes and References

  1. Web site: Barbertonite Mg6Cr2(CO3)(OH)16·4H2O . RRUFF .
  2. http://www.mindat.org/min-514.html Barbertonite: mindat.org
  3. http://www.webmineral.com/data/Barbertonite.shtml Barbertonite: webmineral.com
  4. Taylor . H. F. W. . 1973 . Crystal structures of some double hydroxide minerals . Mineralogical Magazine . 39 . 304 . 377–389 . 10.1180/minmag.1973.039.304.01 . 0026-461X.
  5. 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. 10.2138/am.2011.3531. 2011AmMin..96..179M. 55810652.
  6. Book: 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 . 659 . John Wiley and Sons, Inc. . En . Dana1941.
  7. 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 . 10.1180/minmag.1933.023.140.03 . 0369-0148.
  8. Frondel . C. . 1941 . Constitution and polymorphism of the pyroaurite and sjogrenite groups . American Mineralogist . 26 . 295–315 . 0003-004X . Frondel1941.