Stoichiometric Defects

A carbonate is referred to as a polyatomic anion. The general chemical formula is  CO32- . In the structure of carbonate, a carbon atom is surrounded by three oxygen atoms, thereby forming a trigonal planar geometry. The formal charge is -2, and so it is a Lewis base and will attract protons that are present in aqueous solutions. Some of the common carbonates are sodium carbonate (or soda) and calcium carbonate (or calcite). Carbonate is also known as carbonate minerals and carbonate rock. Carbonate has wide applications in the chemical industry as it is used in the cement industry, ceramic glazes, iron smelting, etc.

Types of carbonate

The carbonates are classified into two main types: dolomite rock and limestone. Limestone is the important classification and is composed of aragonite or calcite, which is a different crystal from  CaCO3. Dolomite or dolostone is composed of the mineral dolomite.

• Limestone

Limestone is precipitated when minerals such as calcite and aragonite mix with water. It can also be formed through non-biological and biological processes, e.g., by the accumulation of corals and shells in the sea. Limestones are highly soluble in water, and the solubility of calcium carbonate is controlled with the addition of carbon dioxide in the water. Limestones are commonly found in the marine environment, but some minor amounts are also found in freshwater and the environment. The limestones found in freshwaters contain a greater proportion of silica and clay minerals. It is used as a raw material in many industrial sectors. Since it can be readily converted or processed into brick powder or crushed concrete, it is very easy to cut into blocks. It is also used for the production of lime, which is used for treating soils, smelting copper and purifying water. 

Steps to obtain pure calcium carbonate:

• Calcium carbonate produces carbon dioxide and calcium oxide upon calcination. CaCO3 CaO + CO2 

This is used for obtaining pure calcium carbonate from carbonate limestone.

• The obtained calcium oxide reacts with water and is called slaking. 
• CaO + H2O Ca(OH)2
• Then the obtained calcium hydroxide reacts with carbon dioxide to form pure calcium carbonate. 

Ca(OH)2 + CO2    CaCO3 + H2O  

• Dolomite

An anhydrous carbonate mineral which is condenser calcium magnesium carbonate is dolomite. The chemical formula of dolomite is CaMg (CO3)2. Dolomite has a peculiarity: it will crystallise out in trigonal rhombohedral systems. It can form fine powder, but it is not highly soluble in dilute hydrochloric acid. So, it can also be called rock-forming minerals. The formation of dolomite is commonly found in a saline environment. Some of the very common uses of dolomite include the ornamental industry and the Pidgeon process for the production of magnesium. Dolomite when calcined can be used as a catalyst for the destruction of tar and in the gasification of biomass.

Importance of the two types of carbonates

• Calcium carbonate has wide applications in the plastic, paint, coatings, and paper industries. 
• Calcium carbonates are widely used as a source of calcium in the form of a medical tablet. 
• Calcium carbonate is the main ingredient in the construction field.
• Dolomite is a source of metal magnesium and is the constituent of refractory bricks.
• Dolomite is also used as a raw material for the production of cement instead of limestone.
• Dolomite is also used in the form of medical tablets as a supplement for calcium. 

Applications of carbonates

Carbonates like Na2CO3 (also called washing soda) can be used for softening hard water since hard water consists of bicarbonates such as . Also, several alkali metals can be mined in the form Na2CO3. For treating manic depression, lithium carbonates are used. Sodium carbonates are also used in the manufacturing of glasses. Sodium carbonates can also be obtained from sodium bicarbonate by the simple heating process. 

Effect of pH

The increase in pH increases the concentration of carbonates, i.e., is by the addition of a base to it. The crystal growth of carbonates is well observed in high pH values. The effect of pH can also be observed when we raise the temperature when performing the reaction. While the addition of iron is shown to retard crystal growth, but it is low in high supersaturations.

Conclusion

Carbonates are mainly of two types: limestone and dolomite. Limestones have a varied range of applications. Dolomite can also be used in many applications instead of calcite or limestone. Many industries make use of these carbonates for manufacturing a wide variety of materials.