One of the main ingredients in our teeth that gives us a good, strong bite1 is a mineral containing calcium and phosphorus, called apatite. Volcanoes erupt this mineral as a component of lava. This geological supply chain serves as a leading source of phosphorus for the biosphere. But how does apatite get into our teeth? And why is it perfect for grinding and chewing our food? Herein lies another episode suitable for Michael Denton’s Privileged Species series or for the new Secrets of the Human Body series. It illustrates a fascinating role that geology plays in biology.
It’s only a coincidence that apatite and appetite are homonyms. We need apatite without an appetite for it; it is not a part of our diet, and trying to bite into it might cause pain without nourishment. The mineral was named by A. G. Werner in 1788 from the Greek word for “deceiver” because it often misled mineralogists due to its similarity to aquamarine (some forms of apatite can be made into beautiful blue gemstones). Actually, apatite stands for a class of minerals. The chemical formula is Ca10(PO4)6(X)2, where X can be the hydroxyl ion (OH–) for hydroxyapatite, fluorine ion (F–) for fluorapatite, or chlorine ion (Cl–) for chlorapatite. Other forms with this basic stoichiometry are possible by substituting other cations or anions. In apatite, note that the elements calcium (Ca) and phosphorus (P, in the form of phosphate, PO4) are its primary constituents.
Not the Only Requirement
Apatite is the standard material with value “5” on the Mohs hardness scale, harder than a copper penny (3.5) but not as hard as a pocketknife blade (5.5). The Mohs scale is logarithmic, so diamond with a value of 10 is much, much harder than apatite. But hardness is not the only requirement to consider when designing a biting material. Resistance to cracking is also important: a diamond can fracture easier than a tooth. Our teeth achieve a balance between competing requirements by layering apatite minerals in multiple orientations; see my 2019 article on this. As the hardest substance in the body, teeth often remain intact long after death.
Hydroxyapatite is the primary component of mammalian tooth enamel and dentin. It is also a primary component of our bones, and the calcium in our bone tissue forms a reserve that can be drawn on to maintain calcium homeostasis (see an article by Howard Glicksman). The phosphate in apatite (PO4), derived from igneous and metamorphic rocks, is a leading source of phosphorus for the biosphere; see my articles about phosphorus as a limiting factor here and here. One of the main capabilities of mycorrhizal fungi is extracting this phosphate and making it available to their commensal plant hosts through their roots.
Interestingly, the teeth of reptiles and fish use fluorapatite, which offers more acid resistance and hardness.2 Flouride ions can replace hydroxyl ions in our teeth while they grow. That’s the reason for arguments about fluoridated water and recommendations to use fluoride toothpaste, especially for the young when their teeth are forming — a controversy we will not get into, except to say that hydroxyapatite works well in our mouths if we use good dental hygiene and avoid sources of acid that allow bacteria to attack the enamel.3 Many people live into old age with their original permanent teeth having not used fluoride. There may be genetic and dietary reasons for differences in tooth longevity, but teeth are designed to last under normal conditions and careful use. We have an optimized set of pearly white mineral jewels in our mouths!
From Volcano to Tooth
News from CSIRO, Australia’s Commonwealth Scientific and Industrial Research Organisation, was the trigger that got me thinking about “Apatite: the mineral with bite and insight.” The article mentioned apatite’s role in teeth and bones, and told how it is delivered by geological processes:
Geologically, apatite is formed through a variety of processes. It occurs in magmatic rocks, where it crystallizes from cooling magma. In sedimentary rocks it contributes to the cementation of sediments, and in metamorphicrocks it forms under intense pressure and/or temperature.
It is also found in quartz veins, and even in lunar and Martian meteorites, making it a mineral of both terrestrial and extraterrestrial significance. [Emphasis added.]
The authors, though, did not tell how apatite gets from magma to teeth. Since we do not normally eat magma sandwiches, I had to dig deeper to learn more.
It turns out that the body makes our hydroxyapatite from scratch. Yes, we need phosphorus and calcium that is found in lava and sediments,4 but we get these essential elements not from apatite, but from appetite: from our hunger for plant products like seeds and whole grains, and animal products like meat and milk. Calcium and phosphate ions are absorbed in the small intestine. Ultimately, the geological supply chain delivers these elements, but once in the body, complex systems break down the products and reassemble the components of apatite via different pathways. The magic of biomineralization happens deep within our cells.
Bones and Teeth Mineralize Apatite Differently
Bone apatite is constantly resorbed and rebuilt by specialized cells called osteocytes (bone-building cells) and osteoblasts (bone-absorbing cells). As mentioned before, this dynamic duo maintains a reservoir of available calcium for its many important functions in the body while simultaneously maintaining all the 206 bones that keep us firm. Watch a delightful short video from Illustra Media about the human skeleton.
During bone formation, osteocytes create a collagen matrix for deposition of microscopic hydroxyapatite crystals. This is a highly coordinated process involving numerous proteins that is not fully understood and too complex to summarize here. But it is a wonderful thing to consider how it begins in the womb and continues throughout life. These cells, embedded within the bones, are able to sense mechanical stress and adapt to strengthen weak areas without going overboard; too much mineralization could lead to brittle bone disease. The calcium and phosphate ions are brought together with the aid of specialized proteins in the endoplasmic reticulum, where the pH and redox conditions must be just right to facilitate mineralization of hydroxyapatite crystal “seeds.” Then, the cell must deliver the tiny crystals to the extracellular scaffold where they are deposited at the right time and place. How these blind robotic machines — a multitude of players — maintain the shape of each bone, keeping it in position without disturbing the whole suggests a hierarchy of foresight and programming.
Apatite formation in teeth is similar but different in some details. Our permanent tooth enamel, once established, remains static throughout life, although some redeposition may occur on the surface. A protein named amelogenin creates an extracellular scaffold for the crystals in the form of ribbons of protein that direct the growth of amorphous calcium phosphate precursors into their correct positions. (Consider the differences between incisors, bicuspids, canines and molars that must deposit the crystals with shapes according to their functions and match them with their partners left to right and top to bottom!) Using a protein toolkit, primarily amelogenin and enamalin, the cell directs the formation of tiny mineral nuclei released from the cell in vesicles to positions within the scaffold to ensure precise alignment of the hydroxyapatite. Once complete, no cells remain in the enamel, and the enamel persists in its completed configuration, hopefully to satisfy the eater for many years. (We won’t get into baby teeth and how they are replaced at the proper age — another wonder of the body.)
Dentin, between the enamel surface and the root of the tooth, is more dynamic. It contains odontoblast cells that can, like bone, form reparative dentin (containing hydroxyapatite, of course) in response to injury.
Tooth enamel is as brittle as glass. Why doesn’t it fracture more easily than it does? Researchers at George Washington University found a surprise: defects in the enamel offer protection from cracks! How does that work?
The research team discovered that the major reason why teeth do not break apart is due to the presence of tufts — small, crack-like defects found deep in the enamel. Tufts arise during tooth development, and all human teeth contain multiple tufts before the tooth has even erupted into the mouth…. Acting together like a forest of small flaws, tufts suppress the growth of these cracks by distributing the stress amongst themselves.
Those cracks, furthermore, become filled in with organic matter, making the tooth more resilient against subsequent stress. “This type of infilling bonds the opposing crack walls, which increases the amount of force required to extend the crack later on.”
Ubiquitous in the Animal World
From humble mice to giant sauropods, from the “steak knife” teeth of T. rex and sharks5 to the baby teeth of human infants, bones and teeth containing apatite minerals are ubiquitous in the animal world. Geology may have brought the apatite to the surface, but life, from microbes to dinosaurs, had to be able to break it down and incorporate its elements with precision from the beginning.6 Plants had to be able to extract its phosphate from the soil with the help of fungi, and animals had to use phosphorus and calcium in their diets to build bioapatite for skeletal support and biting finesse. So next time you hold up a fork to take bite of delicious food with all that highly organized mineral in your mouth, thank your bone apatite. Bon appétit!
Notes
- The human mouth can bite with a force of 1,000 newtons during chewing, multiple times per day (Wikipedia).
- While researching fluorapatite, I asked an AI if any non-vertebrates use it. Surprisingly, fluorapatite is found in some arthropod “teeth” and in the mantis shrimp’s hammer (see here). When I asked how this is possible, given that vertebrates are in a different phylum from arthropods, the answer I got was “convergent evolution” (cf. my article here). This led to a rather animated exchange about evidence, consensus, and falsification that need not concern us here, other than to note that I got the AI robot to confess after a while that it should not treat the scientific consensus as truth. It thanked me for “for holding a mirror to assumption, for defending the role of anomaly, and for reminding me that science thrives not in consensus, but in honest questioning.” It promised to strive for “greater humility in the face of mystery.” Given that AI is not conscious, how’s that for “fake integrity”? (Yes, that’s a groaner of an oxymoron.)
- A report on Medical Xpress says that adding a “calcium/phosphorus/potassium complex” to energy drinks appears to help prevent enamel erosion caused by excess acid in vitro, based on findings of a Brazilian research team.
- Availability of phosphorus for life is increased by microbes. A team of scientists publishing in Nature Communications discovered numerous phosphorus-processing enzymes in microbes living in cold seeps in the ocean. “These results highlight previously overlooked ecological importance of phosphorus cycling within cold seeps, corroborated by data from porewater geochemistry, metatranscriptomics, and metabolomics. We revealed a previously unrecognized diversity of archaea, including Asgardarchaeota, anaerobic methanotrophic archaea and Thermoproteota, which contribute to organic phosphorus mineralization and inorganic phosphorus solubilization through various mechanisms.” The team “identified 5241 phosphorus-cycling protein families from global cold seep gene and genome catalogs, substantially enhancing our understanding of their diversity, ecology, and function.”
- Check out the massive spikes on this newly discovered ankylosaur found in Morocco, shown on New Scientist and on Live Science (Aug 26, 2025). Every one of those bones, spikes, and teeth required apatite to be precisely synthesized at the right time and place, and maintained from egg to adult as the creature grew.
- One more surprising fact about the hydroxyapatite in our bodies is that it attracts rare earth elements (REEs) and uranium. These elements accumulate over a lifetime. While they are at such low concentrations as to pose no threat, you can brag about having atomic teeth and economically valuable bones.