Carbonates, Natural

Carbonates, Natural 

a group of widely distributed minerals composed of salts of carbonic acid (H₂CO₃). Natural carbonates form more than 80 natural compounds (minerals) with the lithophilic (Na, Ca, Mg, Sr, Ba, TR) and chalcophilic (Zn, Cu, Pb, Bi) elements. One or two main cations with or without additional anions are included in the composition of natural carbonates.

The basic structure of natural carbonates is a plane triangle [CO₃]²⁻, where the carbon atoms are arranged in a triple coordination in relationship to the oxygen atoms. The [CO₃]²⁻ groups are isolated and are joined together by cations or by the supplementary anions (OH)⁻, F⁻, and Cl⁻. Natural carbonates exhibit a lamellar structure owing to the foliate arrangement of the [CO₃]²⁻ groups (calcite type) or a chain formation [bastnaesite Ce(CO₃)F type] when the [CO₃]²⁻ group dissociates along the axis. Planar [CO₃]²⁻ groups are arranged either in parallel layers and chains or according to a different symmetry. Most of the natural carbonates crystallize in the orthorhombic, monoclinic, or hexagonal (trigonal) system. These carbonates possess a hardness of 3–5 on the mineralogical scale, increased solubility in water (aqueous carbonates of alkali metals, in particular), rapid solubility in hydrochloric acid, and high birefringence. They dissociate upon heating. The color of carbonate minerals depends on the presence of chromophore ions. Copper carbonates are green and blue; uranium carbonates yellow; carbonates of iron and rare earth metals, cinnamon; and cobalt and manganese carbonates, pink. The remaining carbonates are either colorless or of very pale color.

Natural carbonates are formed under diverse conditions: insedimentary marine deposits (in marine deposits, calcium car-bonates form thick strata of limestone, partly of biogenic origin, and dolomite), in hydrothermal ore deposits (calcite, siderite, ankerite), in the erosion crust (magnesite), in metasomatic for-mations (magnesite, siderite), and in the oxidation zone of poly-metallic deposits (malachite, azurite, smithsonite, cerussite).Carbonatites are produced by the magmatogenic process, whichis related to the formation of apatite and rare earths. Manynatural carbonates (for example, smithsonite, malachite, cerus-site, strontianite, and siderite) are used as ores for Zn, Pb, Bi, Ba, Sr, Cu, Fe, Mn, rare earths, and other metals; as raw materialsin the cement and chemical industries (dolomite, magnesite); andas building materials (limestone, marble).