Barbertonite
Encyclopedia
Barbertonite is a magnesium
chromium
carbonate mineral with formula of [Mg6Cr2(OH)16CO3·4H2O]. It is a hexagonal
polymorph
of 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).
[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).
-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).
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).
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
carbonate mineral with formula of [Mg6Cr2(OH)16CO3·4H2O]. It is a hexagonal
Hexagonal crystal system
In crystallography, the hexagonal crystal system is one of the 7 crystal systems, the hexagonal lattice system is one of the 7 lattice systems, and the hexagonal crystal family is one of the 6 crystal families...
polymorph
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
of the mineral stichtite
Stichtite
Stichtite is a mineral, a carbonate of chromium and magnesium; formula Mg6Cr2CO316·4H2O. Its colour ranges from pink through lilac to a rich purple colour. It is formed as an alteration product from chromium containing serpentine....
, and along with stichtite, is an alteration product of chromite
Chromite
Chromite is an iron chromium oxide: FeCr2O4. It is an oxide mineral belonging to the spinel group. Magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite ; substitution of aluminium occurs leading to hercynite .-Occurrence:Chromite is found in...
in serpentinite
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
. Barbertonite has a close association with stichtite, chromite, and antigorite (Taylor et al., 1973).
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: stichtiteStichtite
Stichtite is a mineral, a carbonate of chromium and magnesium; formula Mg6Cr2CO316·4H2O. Its colour ranges from pink through lilac to a rich purple colour. It is formed as an alteration product from chromium containing serpentine....
[3(Mg6Cr2(OH)16CO3.4H2O)], hydrotalcite
Hydrotalcite
Hydrotalcite is a layered double hydroxide of general formula Hydrotalcite is a layered double hydroxide of general formula Hydrotalcite is a layered double hydroxide of general formula (Mg6Al2(CO3)(OH)16·4(H2O) whose name is derived from its resemblance with talc and its high water content. It is...
[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 bruciteBrucite
Brucite is the mineral form of magnesium hydroxide, with the chemical formula Mg2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists; and formed during serpentinization of dunites...
-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 BarbertonBarberton, Mpumalanga
Barberton is a town in the Mpumalanga province of South Africa, which has its origin in the 1880s gold rush in the region. It is situated in the De Kaap Valley and is fringed by the Mkhonjwa Mountains...
district in Transvaal
Transvaal Province
Transvaal Province was a province of the Union of South Africa from 1910 to 1961, and of its successor, the Republic of South Africa, from 1961 until the end of apartheid in 1994 when a new constitution subdivided it.-History:...
, South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
. It can also be found in the Ag-Pb mine in Dumas
Dumas
Dumas is a Southern French topographic surname, with fused preposition and definite article du, for someone who lived in an isolated dwelling in the country rather than in a village, from Occitan mas 'farmstead' .- People :...
, Tasmania
Tasmania
Tasmania is an Australian island and state. It is south of the continent, separated by Bass Strait. The state includes the island of Tasmania—the 26th largest island in the world—and the surrounding islands. The state has a population of 507,626 , of whom almost half reside in the greater Hobart...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
(Anthony et al., 2003). Read and Dixon (et al. 1933) stated that the mineral that was found in Cunningsburgh
Cunningsburgh
Cunningsburgh, formerly also known as Coningsburgh , is a hamlet and ancient parish in the south of Mainland, Shetland. The hamlet is on the coast, nine miles south south west of Lerwick, about half way between there and Sumburgh Head. The parish was merged with Dunrossness and Sandwick...
, Shetland Islands
Shetland Islands
Shetland is a subarctic archipelago of Scotland that lies north and east of mainland Great Britain. The islands lie some to the northeast of Orkney and southeast of the Faroe Islands and form part of the division between the Atlantic Ocean to the west and the North Sea to the east. The total...
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
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
(Anthony et al. 2003).