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Greek baros : weight : pressure.
Eng: carbonate of barium
Minerals; Inorganic; Column Two
Allen's Encyclop. Mat. Med. Vol. II, 49; X 372.
Description of the substance
Barium carbonate (BaCO3), also known as witherite, is a chemical compound used in rat poison, bricks, and cement.
Witherite crystallizes in the orthorhombic system. The crystals are invariably twinned together in groups of three, giving rise to pseudo-hexagonal forms somewhat resembling bipyrarnidal crystals of quartz, the faces are usually rough and striated horizontally.
The mineral is named after W. Withering, who in 1784 recognized it to be chemically distinct from barytes. It occurs in veins of lead ore at Hexham in Northumberland, Alston in Cumbria, Anglezarke, near Chorley in Lancashire, and a few other localities. Witherite is readily altered to barium sulfate by the action of water containing calcium sulfate in solution, and crystals are therefore frequently encrusted with harytes. It is the chief source of barium salts, and is mined in considerable amounts in Northumberland. It is used for the preparation of rat poison, in the manufacture of glass and porcelain, and formerly for refining sugar.
Barium carbonate reacts with many acids to soluble barium salts, for example barium chloride:
BaCO3(s) + 2 HCl(aq) → BaCl2(aq) + CO2(g) + H2O(l)
However the reaction with sulfuric acid is poor, because barium sulfate is highly insoluble (1).
Density and phase 4.2865 g/cm3, solid
Solubility insoluble in water, soluble in acid
Melting point 1555 °C
Refractive Index 1.676
Mohs hardness 3.5
Specific gravity 4.3
The Borate minerals are more complex in their structures than typical carbonates, but because of the scarcity and limited distribution of boron in the Earth's crust there are only a few borates that can be considered common. Because boron in the BO3 ionic group has a positive three (+3) charge, it only requires half of the bond strength of each negative two (-2) oxygen. This allows the oxygens to bond with other borons evenly and thus link boron groups together into compound groups, chains, sheets and even a framework structure using BO4 tetrahedrons linked to BO3 groups. This makes the borates similar to the many structural variations found in the Silicate Class of minerals. These structural variations explain the large size of this subclass in terms of numbers of species. The chemistry is certainly not the culprit. There is little variation in the chemistry of these minerals as a quick scan of the list below reveals mostly sodium, calcium and/or magnesium borates, many with hydroxides, many hydrated and some with chlorine; but little else in terms of chemical variation.(3)
Witherite is an uncommon carbonate mineral, although it is the second most common barium mineral next to the barium sulfate mineral, barite. All members of the aragonite group of minerals can form twins, but witherite is one member that always forms twins!
Twinning is the result of an error during the growth of the crystal. It occurs when the atomic layer stacking, ABCABCABCABC etc, makes a mistake and a C layer instead of a B layer is place next to an A layer. The result is an ABCABCACBACBACBA stacking. Where the mistake occurs, a mirror plane is produced. If this occurs another time, forming three twins, that are joined in a circle, then a trilling is created. The symmetry of the crystal will appear hexagonal but is still orthorhombic. These crystals can be thought of as a "triple siamese twin" where one crystal takes up one third (or 120 degrees) of the hexagon. Witherite's twins are typically capped with a six sided pyramid and often are dipyramidal.
Witherite is an interesting and valuable collection specimen that anyone, especially a collector of twinned minerals, would love to own.