Overview

What is Calcium Carbonate

Calcium carbonate is a white powder commonly found in rocks and in shells of marine and land organism, pearls, and egg shell. The main applications of calcium carbonate are in paper, construction industry, paints, plastics, ceramics and food. It is also found in agricultural lime which adjusts soil properties and acts as a fertilizer. It is commonly used as a calcium supplement or antacid.
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite) and is the main component of pearls and the shells of marine organisms, snails, and eggs. Calcium carbonate is the active ingredient in agricultural lime and is created when calcium ions in hard water react with carbonate ions to create limescale. It is medicinally used as a calcium supplement or as an antacid, but excessive consumption can be hazardous.
Calcium carbonate causes a unique reaction with acids.  Upon contact with an acid – no matter the strength – it produces carbon dioxide.  This provides geologists with a reliable test to identify calcium carbonate.  This same phenomenon is important to the formation of caves.  Acidic rain water runs off and goes underground where it dissolves the calcium carbonate limestone.  The calcium carbonate water runs down and eventually reaches an air-filled cavity underground where the carbon dioxide can be released.  When it is released, the calcium carbonate crystallizes again.  Stalactite and stalagmite formations are created when water containing calcium carbonate drips, leaving some mineral at the source of the drip at the roof of the cave and some where it falls.  This is an extremely long process, and often takes place over many thousands of years.

Structure

The thermodynamically stable form of CaCO3 under normal conditions is hexagonal β-CaCO3 (the mineral calcite). Other forms can be prepared, the denser (2.83 g/cm3) orthorhombic λ-CaCO3 (the mineral aragonite) and hexagonal μ-CaCO3, occurring as the mineral vaterite. The aragonite form can be prepared by precipitation at temperatures above 85 °C, the vaterite form can be prepared by precipitation at 60 °C. Calcite contains calcium atoms coordinated by six oxygen atoms, in aragonite they are coordinated by nine oxygen atoms. The vaterite structure is not fully understood. Magnesium carbonate (MgCO3) has the calcite structure, whereas strontium carbonate and barium carbonate (SrCO3 and BaCO3) adopt the aragonite structure, reflecting their larger ionic radii.