Calcium Magnesium Carbonate CaMg(CO3)2
In England, dolomite has become a useful source for the production of magnesite by reacting calcined dolomite with seawater.
Dolomite is a double carbonate rock with a composition of Calcium & Magnesium occurring as rhombohedral crystals, the proportion varying as per mining zone. Dolomite is harder and denser than the calcite form of calcium carbonate or limestone and is more chemically inert and more impervious to acid attacks. Chemically and structurally it is similar to calcite with half the calcium ions replaced by magnesium.
Theoretically, dolomite contains:
Analysis for Dolomite:
|Calcium Oxide||as CaO||32.11|
|Magnesium Oxide||as MgO||20.59|
|Ferric Oxide||as Fe2O3||0.5|
|Loss on Ignition||44.87|
|Calcium Carbonate||as CaCO3||50.94|
|Magnesium Carbonate||as MgCO3||43.0|
In nature, considerable variations in the composition of dolomite relating to lime and magnesia percentages are found. When the percentage of CaCO3 increases by 10% or more over the theoretical composition, the mineral is termed ‘calcitic dolomite’, ‘high-calcium dolomite’, or ‘lime-dolomite’. With the decrease in the percentage of MgCO3, it is called ‘dolomitic limestone. With the variations of MgCO3 between 5 to 10%, it is called ‘magnesian limestone’, and up to 5% MgCO3 or less it is taken to be limestone for all purposes in trade and commercial parlance.
Dolomite usually contains impurities, chiefly silica, alumina, and iron oxide. For commercial purposes, the percentage of combined impurities should not go beyond 7% above which, which becomes unsuitable for industrial use. It is then used only for road ballasts, building stones, flooring chips, etc.
|Associated Minerals||include calcite, sulfide ore minerals, fluorite, barite, quartz, and occasionally with gold.|
|Colour||Often pink or pinkish and can be colorless, white, yellow, gray or even brown or black when iron is present in the crystal|
|Characteristics||Unlike calcite, effervesces weakly with warm acid or when first powdered with cold HCl.|
|Luster||Pearly to vitreous to dull|
|Field Indicators||Typical pink color, crystal habit, hardness, slow reaction to acid, density and luster.|