Apatite is a group of phosphate minerals that are found in many different environments throughout the world. Some of the most common places apatite occurs naturally include:
Igneous Rocks
Apatite is a common accessory mineral found in many different igneous rocks. As magma cools and igneous rocks form, apatite crystallizes out early due to its high solubility. Apatite is found in mafic rocks like gabbro and basalt, but larger well-formed crystals are more likely to occur in felsic rocks like granite and rhyolite. Apatite is so common in granitic rocks that its presence can be used as a marker to identify rocks that crystallized from silica-rich magma.
Metamorphic Rocks
During metamorphism, apatite has a high resistance to alteration. While other minerals around it may change as temperature and pressure increase, apatite remains stable. Consequently, it is a common accessory mineral found in metamorphic rocks like gneiss, schist, and marble that originally formed under high-grade conditions. The apatite found in metamorphic rocks likely crystallized out of the protolith rock before metamorphism occurred.
Sedimentary Rocks
Small amounts of apatite can be found in some sedimentary rocks, especially phosphorite rock deposits that form from marine seabeds. Apatite erodes out of igneous and metamorphic source rocks and accumulates in coastal or offshore sediments where it becomes concentrated. Over long periods of geologic time, these apatite-rich sediments lithify into sedimentary phosphate rock. Major deposits are found along continental margins and island arcs where upwelling ocean waters bring ample nutrients that allow apatite to accumulate.
Soils and Bone Deposits
Apatite frequently occurs in soils that form in subtropical or tropical regions with seasonally dry climates. In these soil environments, apatite is most concentrated in the sections of soil horizons that contain calcium carbonate nodules. Apatite also occurs as a natural mineral in fossilized bones and teeth. As organic phosphatic material from bones and teeth fossilize and replace with minerals, apatite is a major component of those replacement minerals. Consequently, apatite comprises a significant portion of vertebrate fossils.
Hydrothermal Deposits
Apatite forms in the vesicles, fractures, and pipes of volcanic and plutonic hydrothermal systems. As hot aqueous fluids move through the host rocks, apatite precipitates if the temperatures and pressures allow. Significant accumulations of apatite can build up over time in hydrothermal vein deposits. Apatite is also found as a gangue mineral in some hydrothermal ore deposits of tin, tungsten, iron, and rare earth elements.
