Calcite

The teeth and bones of mammals, the shells of marine organisms, even the sharp spines of sea urchins, are all made of the same material, calcite. Calcite is a carbonate mineral and is the most stable form of calcium carbonate.

Marine organisms use the most readily available elements in the sea water, that is calcium, oxygen and carbon, to produce their teeth, bones or shells. In seawater solutions, organisms can use the available calcium and carbonate ions to precipitate calcium carbonate.

Calcite is also commonly found in sedimentary rocks, and especially in limestone which is formed from the shells of dead marine organisms. Calcite is the mineral found in cave formations such as stalactites and stalagmites. Overlying limestones and marbles are dissolved away by slightly acidic groundwater which then percolates into caverns below. As the water evaporates, calcite is precipitated.


Calcite has some unusual properties.
It becomes less soluble in water as the temperature increases, a property known as retrograde solubility.
Single crystals of calcite display an optical property known as birefringence which causes objects viewed through a piece of calcite to appear to be doubled.

The sea urchin tooth is a biomineral composed of calcite crystals in both plate and fiber forms, arranged crosswise and cemented together with super-hard calcite nanocement. Between the crystals are layers of organic materials which are weaker than the calcite crystals. These weaker organic materials allow pieces of the tooth to break off in predefined places which means that the sea urchin tooth is self-sharpening!

Reference
Christopher E. Killian, Rebecca A. Metzler, Yutao Gong, Tyler H. Churchill, Ian C. Olson, Vasily Trubetskoy, Matthew B. Christensen, John H. Fournelle, Francesco De Carlo, Sidney Cohen, Julia Mahamid, Andreas Scholl, Anthony Young, Andrew Doran, Fred H. Wilt, Susan N. Coppersmith and Pupa U. P. A. Gilbert. Self-Sharpening Mechanism of the Sea Urchin Tooth. Advanced Functional Materials, 2010; DOI: 10.1002/adfm.2010015

Further Reading
Naming Ionic Compounds
Writing Ionic Formulae
Molecular Mass
Percentage Composition
Shapes of Molecules
Lewis Structures
Solubility Product

Study Questions

1. Give the chemical formula for calcium carbonate.
2. List the names and formulae of other compounds that can be made out of carbon, oxygen and calcium.
3. Calculate the percentage composition of calcium carbonate as well as the compounds you listed in question 2.
   
4. Write a balanced chemical equation to show the precipitation of calcium carbonate from ions in sea-water.
5. What shape do you expect the carbonate ion to take? Explain your answer.
6. Draw a Lewis Structure, electron dot diagram, for a "molecule" of calcium carbonate.
7. Why is it inaccurate to describe calcium carbonate as a molecule?
   
8. Describe the process by which calcium carbonate can dissolve in water.
9. Would calcium carbonate more readily precipitate from fresh water or sea-water at the same temperature? Explain your answer.