Many are found as minerals, the more important of such naturally occurring carbonates being cerussite (lead carbonate, PbC03), malachite and azurite (both basic copper carbonates), calamine (zinc carbonate, ZnCO 3), witherite (barium carbonate, BaCO 3), strontianite (strontium carbonate, SrC03), calcite (calcium carbonate, CaC03), dolomite (calcium magnesium carbonate, CaCO 3 MgCO 3), and sodium carbonate, Na 2 CO 3.
It also enters (as carbonates) into the composition of many minerals, such as chalk, dolomite, calcite, witherite, calamine and spathic iron ore.
It is a constituent of the minerals cerussite, malachite, azurite, spathic iron ore, calamine, strontianite, witherite, calcite aragonite, limestone, &c. It may be prepared by burning carbon in excess of air or oxygen, by the direct decomposition of many carbonates by heat, and by the decomposition of carbonates with mineral acids, M2C03+2HC1=2MCl-FH 2 O+CO 2.
Barium occurs chiefly in the form of barytes or heavy spar, BaS 04, and witherite, BaCO 3, and to a less extent in baryto-calcite, baryto-celestine, and various complex silicates.
Barium chloride, BaCl 2.2H 2 O, can be obtained by dissolving witherite in dilute hydrochloric acid, and also from heavy spar by ignition in a reverberatory furnace with a mixture of coal, limestone and calcium chloride, the barium chloride being extracted from the fused mass by water, leaving a residue of insoluble calcium sulphide.
Barium carbonate, BaCO 31 occurs rather widely distributed as witherite, and may be prepared by the addition of barium chloride to a hot solution of ammonium carbonate, when it is precipitated as a dense white powder of specific gravity 4.3; almost insoluble in water.
It crystallizes in the orthorhombic system and is isomorphous with aragonite and witherite.
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