What is calcium? It is a chemical element, an alkaline earth metal, number twenty in the periodic table.
It is the fifth most abundant element in the earth’s crust and in the human body. It is not found in its metallic form on the earth’s surface, but is found associated with other elements and molecular species as solids in minerals, and in ionized form complexed with a variety of other compounds. A mineral is a naturally formed substance that has a specific chemical composition and atomic structure with characteristic physical properties. Examples of calcium containing minerals are calcite and aragonite which have the same chemical composition, CaCO3, but different crystal structure (called polymorphs) and hydroxyapatite, Ca5(PO4)3 (OH).
These minerals are the most important ones for the purpose of this unit.
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Calcite and aragonite are the minerals produced by sea creatures, invertebrates, to make their shells, and by birds and reptiles, vertebrates, to contain their eggs. Limestone and marble can have the same chemical composition and their connection to shells will be discussed later. Hydroxyapatite is the mineral in bones and is also common in many varieties of rocks. As a mineral, calcium is locked up or sequestered in relatively insoluble compounds.
Calcium is “active” and relatively mobile in its other form as a positively charged particle, or cation. An ion is an atom or group of atoms which has a net charge because the number of negatively charged electrons present is different from the number of positively charged protons in the nucleus. Calcium is a relatively large atom with only two electrons in the outer orbit. These tend to be lost creating the calcium cation, Ca
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, which is attracted to and loosely held by molecules or substances which have negatively charged sites. In these situations calcium cations are said to form complexes which vary in the strength with which the calcium is held. Ionic Calcium can be found in fresh and salt water, held by certain proteins in the blood and extracellular fluid of animals and adsorbed onto clay and other colloidal particles in the soil. (Water is a dynamic polar molecule with a partial negative charge on the oxygen.) In the human body less than 1% of the calcium is in the active, ionized form, but it is vitally important.
In the human body and in the earth’s crust, the vast majority of calcium is present in the sequestered form. In the body the mineral in bones can be solubilized creating ionized calcium to maintain the critical level in the blood. In the earth’s crust the calcium containing minerals in rocks can be slowly dissolved to provide the Ca
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in solution in lakes, rivers and oceans and adsorbed in the soil. Most of this calcium is then stored again as a mineral. For example, it is estimated that of the apatite, calcium phosphate, minerals that have been dissolved in the sea, 99.8% have been reprecipitated in some way.
Where the calcium is and how it moves from one form to another is the next step in understanding this fascinating and essential element.