The chemical formula of the Apatite is indicated by Ca5(PO4)3(OH,F,Cl), Calcium (Fluoro, Chloro, Hydroxyl) Phosphate. Apatite is a Phosphate mineral. Apatite is known to crystallize in the hexagonal system. In optical mineralogy, hexagonal system of crystallization comprises crystals having four axes. Three of which are positioned in a single plane with equal length and are symmetrically spaced. The fourth axis is found to be perpendicular to the other three axes. When viewed under petrographic polarizing light microscope, mineral Apatite is commonly found exhibiting a uniaxial negative figure. The indices of refraction found range from 1.629 to 1.667. Apatite is commonly found exhibiting a moderately high positive relief when viewed with the aid of polarizing microscope. The name Apatite was derived from the Greek word apatos, which means deception. This is an allusion to the crystal shapes and wide variety of colors of the mineral, which is often confused with other valuable minerals such as peridot, beryl and olivine.
Apatite is actually a group of three related minerals, which can be distinguished from each other through the predominance of either fluorine, chlorine, or the hydroxyl group. These three ions can be found together existing in one specimen but other specimens may contain nearly 100 percent of one or the other ion. They can also be found substituting freely in the crystal lattice of the specimen. These three related minerals forming the Apatite group are the Flourapatite, the most common one, the Clorapatite, which is rarely seen in existence, and the hydroxylapatite. There is also one rare occurrence of carbonate-apatite under some carbon-phosphate substitution process. A cryptocrystalline apatite that makes up the fossil bones and most phosphate rocks is called Collophane. Impure and massive Apatite variety is called Phosphorite. It has been known that Apatite mineral makes up the teeth and bones of almost all vertebrate animals. The phosphates found in bones and teeth are known members of the Apatite group of minerals. The three minerals forming the group Apatite are usually considered contained altogether in one specimen due to the difficulty in distinguishing their individual presence in hand sample by just using simple and ordinary methods. The sediments in sedimentary basins and thermal history of mountain or orogenic belts can be determined by the fission tracks in mineral Apatite.
The hardness measure of mineral Apatite when it is evaluated with the use of the Mohs scale method is usually 5. The specific gravity measure of the mineral is commonly found ranging from 2.9 grams per cubic centimeters to 3.5 grams per cubic centimeters. The density of the most common Apatite is commonly found ranging from 3.1 grams per cubic centimeters to 3.2 grams per cubic centimeters. Apatite is commonly found in shades of green or gray with bluish or yellowish tints. But it is also important to note that almost any color can be possibly found in hand samples of mineral Apatite. When Apatite specimen is rubbed on a white porcelain streak plate, it is commonly found leaving a white streak. When viewed in reflected light of polarized microscope, Apatite is commonly found exhibiting a vitreous luster. .
Apatite is commonly found colorless in thin section and in grain mount. Strongly colored samples of Apatite may display pale colors corresponding to the hand sample color. This is usually found displaying weak to moderate pleochroism and absorption when viewed under polarizing microscope. Apatite specimens usually appear forming prismatic crystals with first order pyramid termination and often with basal planes, which are commonly found clearly exhibited when the mineral is evaluated under geological polarizing light microscopes. When the mineral is viewed between crossed nicols of polarizing microscopes, inclusions of typical hexagonal prisms with terminated hexagonal piramid and pinacoid faces can be found. They commonly appear in variation of massive granular to compact masses. They can be also found formed in reniform or acicular habit. Apatite minerals are commonly found in small euhedral and subhedral elongate prismatic crystals. It shows hexagonal cross sections found in transmitted light of polarized light microscope. They may be found in anhedral grains or columnar aggregates. Apatite mineral crystal can be also found transparent to opaque in appearance. They usually appear transparent in thin section. Mineral Apatite is usually non-pleochroic even between crossed nicols of polarizing light microscopes. They may also exhibit colloform habit, which is showing crystals forming from a gel or colloidal mass and is usually wonderfully exhibited when viewed with the aid of polarizing light microscopes. A clay-like texture without any crystalline affinities found visible might be included.
Apatite is commonly found having basal cleavage that are usually relatively poor and another prismatic cleavage that is also poor but are generally not observed. Cleavage does not have a strong influence on fragment orientation in grain mount. Twinned sections of Apatite are quite rare but have been reported on contact planes. The elongate sections of Apatite through crystals show parallel extinction and they are usually found length fast. The indices of mineral Apatite usually vary in a general way with composition. The basal section in thin section of Apatite usually yield uniaxial negative figures with diffuse isogyres on a first-order gray field. The interference figures on small grains in thin section may be difficult to obtain.
Apatite is commonly found as an accessory mineral in many types of sedimentary, igneous, and metamorphic rocks. However, Apatite is usually found as small-disseminated grains or cryptocrystalline fragments in these rock types. Alkalic rocks are found having the largest deposits of Apatite. Well-formed large Apatite crystals are commonly found formed in certain metamorphic rocks. They can also be found in magmatic rocks, as hydrothermal veins in iron rich rocks. Apatite can also be found in plutonic, hornfels, and granite pegmatites. They are considered not so rare mineral in Sweden but good quality crystals are usually scarce. Apatite minerals can be found in type of localities like Germany, Russia, and Bancroft, Ontario. There is also reported existence in Durango, Mexico.
