Apatite can be mistaken to be a single mineral, but, it can be subdivided into three types depending on the amount of concentrations of OH−, F−, or Cl− ions, respectively, in the crystal. These ions can be freely substituted in the crystal lattice and mostly all three are present in every type. In some types, 100% of a single ion only is present.
Apatite minerals with high concentrations of fluoride ions in their crystal lattice are called Fluorapatite and have a composition of calcium fluoro-phosphate. Apatite minerals with high concentrations of chlorine ions in their crystal lattice are termed as Chlorapatite and are composed of calcium chloro-phosphate.
The ones with hydroxyl ions are called Hydroxylapatite which are composed of basic calcium phosphate. It is very difficult to differentiate between these types and thus often commonly called Apatites. However, the Fluorapatite is the most common of the three.
Apatite mineral's group chemical formula is Ca5(PO4)3(F, Cl, OH). The individual formulas are Ca5(PO4)3Cl for Chlorapatite, Ca5(PO4)3F for Fluorapatite and Ca5(PO4)3OH for Hydroxylapatite.
The mineral is found in the form of large crystals in some metamorphic rocks, otherwise they are found only in the form of cryptocrystalline fragments or tiny disseminated grains in other rocks.
Apatite crystals have a hexagonal crystal system (6/m) and are generally formed as well-shaped, elongated or stubby hexagonal crystals. They are usually transparent or translucent, vitreous with an indistinct cleavage in a single basal direction. The tenacity happens to be brittle and the fracture is conchoidal.
The hardness of Apatite is 5 in the Mohs scale (main test for apatite) and the specific gravity is 3.1-3.2. Apatites can be easily distinguished by their color, hardness, crystal form and can be scratched by a steel blade. Apatite dissolves in hydrochloric acid and attains a fluorescent orangish-yellow color after heating in long wave UV light.
It is used to fertilize tobacco in the US and is seen to partially starve the plant of nitrogen, thereby imparting a unique taste to American cigarettes. Earlier crystalline Apatite was extracted for phosphorous, however, nowadays massive deposits of Apatite-rich rock are used to extract phosphorous.
Calcium hydroxylapatite along with some other phosphate minerals forms the inorganic constituent of the bone and is the chief constituent of the tooth enamel. The enchanting colors and forms of Apatite has helped it gained popularity among gem collectors. It can be occasionally cut as a gemstone. However, it's a gem that is seldom found in jewelry stores.
However, rings studded with Apatite must be handled very delicately. Their use must be limited to special occasions. As Apatite is heat and shock sensitive, one must make sure they are not brought in contact with ultrasonics and streamers. It is also used as an ore mineral, as occasionally significant amounts of rare metals are found in them.
Also as Apatite is non-radioactive, it's preferred over rare earth metal ores, thereby not posing as an environmental hazard during mine tailings. At the Hoidas Lake Rare Earth Project, It is used as an ore mineral.
Apatite is a significant mineral, though often not well-recognized by us today. The wide range of arenas where Apatite is used tells us what kind of role it plays in our lives. Not every stone is suited to be a gem, but that does not mean Apatite is not precious. Each mineral is unique in its own way!