Overview

Calcium phosphate refers to several inorganic salts composed of calcium cations and phosphate anions. The term most commonly denotes tricalcium phosphate (Ca3(PO4)2), but related materials span a range of stoichiometries and acidities. These compounds appear as white or off-white crystalline solids with low solubility in neutral water and important roles in geology, biology and industry. For a concise chemical summary see tricalcium phosphate.

Chemical forms and structure

Different calcium phosphate salts form depending on the relative amounts of calcium and phosphate and the solution pH. Common forms include tricalcium phosphate, dicalcium phosphate (often written as CaHPO4) and monocalcium phosphate (Ca(H2PO4)2). Some important related minerals are hydroxyapatite (the mineral phase of bone and teeth) and other apatite-group phosphates. These materials can be crystalline or partially amorphous and may incorporate water or other ions.

Properties

Calcium phosphates are typically hard, brittle, and sparingly soluble in water; their solubility increases in acidic conditions. They exhibit ionic bonding between Ca2+ and phosphate species and often crystallize in well-defined lattices. Variations in composition and microstructure alter physical properties, which is why different calcium phosphate types are chosen for specific applications. Read more on calcium ions and phosphate ions.

Occurrence and production

In nature, calcium phosphates occur as phosphate rock (apatite minerals) and as the principal inorganic component of vertebrate bone and tooth enamel. Industrially they are recovered from mined phosphate ores or produced by treating phosphate sources with calcium compounds. Processed calcium phosphates are then refined for uses ranging from agriculture to biomedical materials; see industrial notes at reference.

Uses and importance

  • Fertilizers: Phosphate salts are a principal source of phosphorus for crop nutrition and are a major agricultural commodity.
  • Food and nutrition: Certain calcium phosphates are used as dietary supplements, leavening agents or anti-caking additives in food processing.
  • Medical and dental: Hydroxyapatite and other biocompatible calcium phosphates serve as bone graft substitutes, coatings for implants and in toothpaste formulations to support remineralization.
  • Industrial: They are used in ceramics, polishing agents and some water-treatment processes.

Notable distinctions and facts

Biologically, calcium phosphate balance is tightly regulated by hormones and vitamins to maintain bone health and serum mineral levels. Environmentally, phosphate runoff from fertilizers can contribute to eutrophication of freshwater systems. Different calcium phosphate compounds are chosen for stability, solubility or biological response depending on the intended use; for instance, highly crystalline hydroxyapatite is preferred for long-term implants while more soluble forms may be used where gradual dissolution and ion release are desired. For more on specific salts see dicalcium phosphate and monocalcium phosphate.