Thematic Articles

Apatite has numerous applications that benefit society. The atomic arrangement of the apatite crystal structure and its rich and variable chemistry impart unique properties, which permit a wide range of technological and scientific applications in an array of disciplines outside of the traditional Earth sciences, including ecology, agronomy, biology, medicine, archeology, environmental remediation, and materials science. In our daily lives, apatite is essential for sustaining and enhancing human life through agricultural amendments, through bone replacements, through fluorescent lights, and through environmental remediation of contaminated soils. Apatite is truly a technological gem.

Apatite can provide geologists with an exceptionally wide range of ages and temperatures to investigate processes that operate from Earth’s surface right down to the lower crust. Apatite is a widespread accessory mineral in igneous, metamorphic, and clastic sedimentary rocks and can be dated using four radioactive decay schemes, each with a different temperature window for isotopic closure: Lu–Hf (675–750 °C); U–Pb (350–550 °C); apatite fission track (60–110 °C); (U–Th)/He (40–80 °C). The fission-track and (U–Th)/He methods are popular for studying upper-crustal and near-surface processes, whereas the U–Pb and Lu–Hf systems are used to investigate the thermal, tectonic, and magmatic histories of the deeper crust.

The distribution and abundances of H2O and other volatiles in our Solar System are of fundamental interest because of the important roles volatiles play in geological and biological processes. Apatite, Ca5(PO4)3(F,Cl,OH), is a ubiquitous accessory mineral and provides a consistent window into volatile abundances and processes across the Solar System and throughout its history. Consequently, the chemical composition of apatite can be used as a tool for exploring such diverse topics as the compositions and roles of the Solar System’s earliest fluids on asteroids, the volatile abundances of planetary bodies, and the habitability of past environments (e.g. on Mars) for life as we know it.

Apatite may be a minor constituent in magmatic rocks but it is a powerful research tool because it is ubiquitous and it incorporates magmatic water, halogens, S, C, and trace elements including Sr, U, Th, and the rare earth elements. Recent advances in experimental and analytical methodologies allow geologists to analyze apatite textures and compositions in great detail. This information improves understanding of the behavior of volatiles and trace elements both in terrestrial igneous melts and their related fluids and in extraterrestrial bodies, such as the Moon and Mars. With more research, the petrological power of apatite can only increase with respect to understanding eruptive, pluton-building, and mineralizing magmatic systems.

Apatite is a superb mineral by which to investigate the nature of fluids that have passed through and altered a rock (metasomatic processes). Its ubiquity allows it to act as a reservoir for P, F, Cl, OH, CO2, and the rare earth elements. It is also a powerful thermochronometer and can be chemically altered by aqueous brines (NaCl–KCl–CaCl2–H2O), pure H2O, and aqueous fluids containing CO2, HCl, H2SO4, and/or F. Thus, apatite is the perfect tracker of metasomatic fluids, providing information on the timing and duration of metasomatism, the temperature of the fluids, and the composition of the fluids, all of which can feed back into the history of the host rock itself.

Apatite is ubiquitous in igneous, metamorphic, and sedimentary rocks and is significant to more field of study than perhaps any other mineral. To help understand why, one needs to know apatite’s structure, composition, and crystal chemistry. Apatite has a robust hexagonal atomic framework based on two distinct metal-cation sites (M1, M2), a tetrahedral-cation site (T), and an anion column along four edges of the unit cell. These cation and anion sites can, among them, incorporate more than half of the long-lived elements in the periodic table, giving rise to the “apatite supergroup,” which contains over 40 mineral species. The structure and composition impart properties that can be technologically, medically, and geologically very useful.