Calcium carbonate, or CaCO3, comprises more than 4% of the earth’s crust and is found throughout the world. Its most common natural forms are chalk, limestone, and marble, produced by the sedimentation of the shells of small fossilized snails, shellfish, and coral over millions of years. Although all three forms are identical in chemical terms, they differ in many other respects, including purity, whiteness, thickness and homogeneity. Calcium carbonate is one of the most useful and versatile materials known to man.
Many of us encounter calcium carbonate for the first time in the school classroom, where we use blackboard chalk. Chalk has been used as a writing tool for over 10,000 years and is a fine, microcrystalline material. As limestone, calcium carbonate is a biogenic rock, and is more compacted than chalk. As marble, calcium carbonate is a coarse-crystalline, metamorphic rock, which is formed when chalk or limestone is recrystallised under conditions of high temperature and pressure. Large deposits of marble are found in North America and in Europe; e.g., in Carrara, Italy, the home of the pure white "statuario" from which Michelangelo created his sculptures.
Calcium carbonate, as it is used for industrial purposes, is extracted by mining or quarrying. Pure calcium carbonate can be produced from marble, or it can be prepared by passing carbon dioxide into a solution of calcium hydroxide. In the later case calcium carbonate is derived from the mixture, forming a grade of product called "precipitated calcium carbonate,” or PCC. PCC has a very fine and controlled particle size, on the order of 2 microns in diameter, particularly useful in production of paper. The other primary type of industrial product is "ground calcium carbonate,” or GCC. GCC, as the name implies, involves crushing and processing limestone to create a powdery-like form graded by size and other properties for many different industrial and pharmaceutical applications.
A study of calcium carbonate provides important lessons about the history of the earth, since chalk, limestone and marble trace their origin to shallow water. Thus, observation that large amounts of chalk deposits of the same age are found on many continents led to the discovery that there existed a period in which there was shallow water world-wide where shelled organisms thrived. Some offer this as proof for the Biblical flood. Nature returns the favor as calcium carbonate solutions from current deposits provide living organisms today with the material they need to grow their protective shells and skeletons. Eggshells, for example, are composed of approximately 95% calcium carbonate.
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.