Apatite is the name for a group of calcium phosphate minerals that share a similar structure and composition, with the three main members being fluorapatite, chlorapatite, and hydroxylapatite. These minerals are chemically defined by a formula allowing for the substitution of fluorine, chlorine, or a hydroxyl group. Apatite is found across a wide range of geological environments, occurring as an accessory mineral in igneous, metamorphic, and sedimentary rocks.
This mineral group is the most abundant and widespread phosphate-bearing mineral in the Earth’s crust. Consequently, apatite serves as the world’s primary source of phosphorus, an element necessary for organic life and a component of bones and teeth in vertebrates.
Although it is found almost everywhere, it usually appears as tiny, disseminated grains or cryptocrystalline fragments within the host rock.
The Visual Spectrum of Apatite
Apatite is known for its expansive range of colors. While it is most commonly found in shades of green, the mineral can also be vibrant blue, yellow, violet, pink, colorless, and even reddish-brown. The specific color is influenced by trace elements and radiation exposure within its crystal structure.
The mineral’s appearance can range from completely transparent (often used for faceted collector’s gemstones) to translucent or opaque in its more common forms. Specimens with high clarity and vivid color are prized by collectors. The surface typically exhibits a vitreous luster, although some specimens may present a slightly greasy or sub-resinous shine.
Certain varieties of apatite also display chatoyancy, or the “cat’s-eye” effect, caused by a dense, parallel alignment of microscopic inclusions. Blue apatite is particularly noted for its striking, electric hue.
Defining Crystal Structure and Habit
Apatite belongs to the hexagonal crystal system, meaning its structure is based on six-fold symmetry around a central axis. When the mineral grows in an unrestricted space, it often forms distinct, well-defined prismatic crystals with six sides. These crystals are typically terminated by either a flat pinacoid face, a hexagonal pyramid, or a combination of both.
Apatite exhibits various forms beyond perfect crystals. It frequently occurs as massive, granular, or compact aggregates that show no distinct external crystal faces. A cryptocrystalline variety, known as collophane, forms the bulk of phosphorite rock and is a fine-grained, earthy mass.
In some geological settings, such as pegmatites or metamorphic rocks, apatite can form large, well-developed crystals. Other habits include globular or reniform (kidney-shaped) lumps, as well as fibrous or stalactitic crusts. The size and perfection of the crystal shape depend heavily on the temperature and pressure conditions during the mineral’s formation.
Key Diagnostic Physical Properties
The primary physical characteristic of apatite is its hardness, which serves as the index mineral for a value of 5 on the Mohs scale. This relatively low hardness means the mineral can be easily scratched by a steel knife blade or quartz. This softness is why natural specimens often appear worn or scratched and is a primary reason it is not widely used in jewelry.
Apatite is a brittle material. It possesses a poor or indistinct cleavage in one direction, meaning it does not break cleanly along smooth, flat planes. Instead, when it breaks, it typically exhibits a conchoidal fracture—a smooth, curved, shell-like surface characteristic of materials like glass.
The specific gravity, or density, is another important diagnostic property. Apatite has a specific gravity that generally ranges from 3.1 to 3.3, which is slightly denser than many common rock-forming minerals. This density, combined with its characteristic crystal habit and low hardness, helps distinguish apatite from other minerals it may resemble, such as beryl or tourmaline.