Hydroxyapatite is made of calcium, phosphorus, oxygen, and hydrogen, arranged in a specific crystal structure with the chemical formula Ca₁₀(PO₄)₆(OH)₂. In plainer terms, it is a calcium phosphate mineral with hydroxide groups woven into its lattice. This is the same mineral that makes up roughly 70 percent of your bone by weight and over 95 percent of tooth enamel, though the version inside your body is never chemically “pure” in the textbook sense. That gap between the idealized formula and the messy biological reality turns out to be one of the most interesting things about hydroxyapatite.
What the Formula Actually Tells You
The formula Ca₁₀(PO₄)₆(OH)₂ says that for every unit of hydroxyapatite, there are ten calcium atoms, six phosphate groups (each a phosphorus atom bonded to four oxygens), and two hydroxide groups (each an oxygen bonded to a hydrogen). Those components arrange themselves into a hexagonal crystal, and the hydroxide ions line up along the central axis of the structure like beads on a string.1SpringerMaterials. hydroxyapatite, HAP (Ca5[PO4]3[OH] rt) Crystal Structure Computer modeling of this arrangement confirms that the lattice is remarkably stable, held together by strong ionic and electrostatic forces between the calcium ions and the surrounding phosphate and hydroxide groups.2Journal of Physics and Chemistry of Solids. Computer simulation of stoichiometric hydroxyapatite: Structure and substitutions
Calcium and phosphate do the heavy lifting. Calcium provides the positive charge that binds the structure together, while phosphate groups form the rigid scaffolding. The hydroxide ions are a smaller part by volume, but they are what distinguish hydroxyapatite from other calcium phosphates. Without those hydroxide groups, or with different ions substituted in their place, you get a different mineral entirely.
How Biological Hydroxyapatite Differs from the Textbook Version
If you could zoom into a bone or tooth and compare what you see with the neat formula above, you would find discrepancies everywhere. Biological hydroxyapatite is always “impure,” loaded with extra ions and compositional quirks that the idealized formula does not capture. Calcium and phosphate are indeed the major components of the hydroxyapatite crystals in teeth and bone,3PubMed Central. Functional Role of Inorganic Trace Elements on Enamel and Dentin Formation: A Review but the resemblance to a pure lab crystal ends there.
The biggest difference is carbonate. In your bones and teeth, carbonate ions routinely substitute into the crystal lattice, replacing either phosphate groups or hydroxide groups. This makes biological hydroxyapatite a “carbonated” version of the mineral.4PubMed Central. The Mineral-Collagen Interface in Bone Carbonate content in bone mineral typically runs around 5 to 8 percent by weight, which is enough to subtly distort the crystal structure and make the mineral more soluble than a pure sample would be. That extra solubility is actually useful: it lets the body dissolve and remodel bone tissue as needed. A perfectly stable, perfectly pure hydroxyapatite crystal would be much harder for your body to work with.
Bone mineral also contains acidic phosphate ions concentrated at the surfaces of its nanocrystals, sitting within a disordered outer layer rather than inside the well-ordered crystalline core.5Scientific Reports. Bone mineral: new insights into its chemical composition This amorphous surface layer is thought to play a role in how bone interacts with the surrounding collagen and water, and it means that the surface chemistry of a bone crystal is quite different from its interior. A biomimetic hydroxyapatite nanocrystal studied in the lab showed a calcium-to-phosphorus ratio of about 1.9, notably higher than the stoichiometric ratio of 1.67 that the textbook formula predicts.6PubMed Central. Compositional Analysis of the Dental Biomimetic Hybrid Nanomaterials Based on Bioinspired Nonstoichiometric Hydroxyapatite with Small Deviations in the Carbonate Incorporation These deviations are the norm in biology, not the exception.
The Trace Elements That Swap In
Beyond carbonate, biological hydroxyapatite routinely incorporates a cast of trace elements. Strontium, zinc, magnesium, and fluoride all find their way into the lattice.7Ceramics International. Synthesis and characterisation of nanophase hydroxyapatite co-substituted with strontium and zinc Each substitution works a little differently. Magnesium, zinc, and strontium carry a positive charge and replace calcium atoms, while silicate can replace phosphate groups. When silicate swaps in for phosphate, the number of hydroxide ions adjusts to keep the overall charge balanced.8PubMed Central. Advanced Mg, Zn, Sr, Si Multi-Substituted Hydroxyapatites for Bone Regeneration
These substitutions are not random contamination. Many of them influence how well the mineral performs in your body. Magnesium, for example, tends to inhibit crystal growth, keeping biological hydroxyapatite crystals small and fine-grained. Strontium has attracted interest because it may help reduce bone resorption. Fluoride is the most famous substitute: when it replaces hydroxide, you get fluorapatite, a closely related mineral that is harder and less acid-soluble than hydroxyapatite. Fluorapatite and hydroxyapatite are considered the most chemically stable calcium phosphates and have the lowest solubility, which is why both play central roles in dental health.9PubMed Central. Hydroxyapatite and Fluorapatite in Conservative Dentistry and Oral Implantology-A Review
Carbonate substitution deserves a closer look because it happens in two distinct ways. Carbonate can replace hydroxide ions along the central channel of the crystal (called A-type substitution) or it can replace phosphate groups in the surrounding lattice (B-type substitution). The two types distort the crystal differently. A-type substitution expands the crystal in one direction, while B-type expands it in another. In biological systems, B-type dominates because it forms more readily at body temperature. A-type requires high temperatures and is mostly seen in lab-synthesized samples.10PubMed Central. Carbonate substitution in the mineral component of bone: Discriminating the structural changes, simultaneously imposed by carbonate in A and B sites of apatite
Why Tooth Enamel Dissolves in Acid
Understanding what hydroxyapatite is made of explains one of the most familiar phenomena in dentistry: cavities. The phosphate and hydroxide groups in the crystal are sensitive to acidity. When the pH in your mouth drops below a certain threshold, those groups begin to react with hydrogen ions, destabilizing the lattice and allowing calcium and phosphate to dissolve out. This is the chemical basis of tooth demineralization.
The solubility of enamel hydroxyapatite is not fixed. It changes with pH, becoming dramatically more soluble as conditions turn acidic. Measurements show that the solubility product of human dental enamel shifts by several orders of magnitude between a mildly alkaline pH of 7.6 and a moderately acidic pH of 4.6.11Archives of Oral Biology. The hydroxyapatite solubility product of human dental enamel as a function of pH in the range 4.6–7.6 at 20 °C That steep change explains why sugary or acidic foods and drinks are so damaging: even a small pH drop at the tooth surface can tip the balance from mineral stability to mineral loss.
Fluorapatite resists this process better because the fluoride ion sits more snugly in the crystal lattice than the hydroxide it replaces, making the whole structure less reactive to acid. This is why fluoride in toothpaste and drinking water helps prevent cavities: it gradually converts surface hydroxyapatite into the tougher fluorapatite variant.
How Hydroxyapatite Grabs Onto Proteins
Hydroxyapatite’s composition also makes it unusually good at interacting with proteins, which matters both for biology and for engineering. The crystal surface has alternating charged regions: positively charged calcium sites and negatively charged phosphate sites. Proteins latch on via their own charged groups, with negatively charged protein segments binding to calcium sites and positively charged segments binding to phosphate sites.12Biomaterials. Molecular simulation of protein adsorption and desorption on hydroxyapatite surfaces The process is fast and driven mostly by electrostatic attraction.13Chemical Engineering Journal. Impacts of the surface charge property on protein adsorption on hydroxyapatite
This dual-charge surface is why hydroxyapatite has been used for decades as a column packing material in protein chromatography, a laboratory technique for separating mixtures of proteins. It is also why, inside the body, bone mineral integrates so effectively with collagen: the mineral’s charged surface readily bonds to the protein matrix that gives bone its flexibility. Researchers can even tune the surface charge by attaching different amino acids to hydroxyapatite particles, shifting which proteins bind preferentially.14PubMed Central. Modulating protein adsorption onto hydroxyapatite particles using different amino acid treatments
Making Synthetic Hydroxyapatite
Because hydroxyapatite is so useful in medicine and dentistry, researchers have developed many ways to synthesize it. All of them start with a calcium source and a phosphate source, but the conditions under which those two are combined determine the final product’s purity, crystal size, and shape. Precipitation from a water-based solution is the most common approach: you dissolve a calcium compound and a phosphate compound, adjust the pH to strongly alkaline conditions, and the hydroxyapatite crystals precipitate out.15PubMed Central. Environmentally sustainable processes for the synthesis of hydroxyapatite
Other methods include sol-gel processing, hydrothermal synthesis at high temperature and pressure, mechanochemical grinding of dry precursors,16Journal of the European Ceramic Society. Synthesis of hydroxyapatite-based powders by mechano-chemical method and their sintering and microwave-assisted reactions. Each route produces hydroxyapatite with different crystal sizes, surface areas, and defect levels, which matters because those properties control how the material behaves in the body.
Grain size, in particular, makes a significant difference. Nano-grain hydroxyapatite consistently outperforms its larger-grained counterpart on mechanical strength, surface energy, and biological compatibility. Compressive strength, hardness, and fracture toughness all increase as grain size shrinks, and cell studies show that osteoblasts adhere, proliferate, and differentiate better on nano-grain surfaces.17Materials Science and Engineering: C. Effect of grain size on mechanical, surface and biological properties of microwave sintered hydroxyapatite This tracks with the fact that biological hydroxyapatite crystals are naturally nanoscale, typically only tens of nanometers long. The body seems to prefer its building material in very small pieces.
Dental and Orthopedic Applications
Knowing what hydroxyapatite is made of has translated directly into practical products. In dentistry, nano-hydroxyapatite particles are now used in toothpastes designed to remineralize early enamel damage. The particles bind to damaged enamel surfaces and fill in tiny pores, restoring surface integrity. Unlike fluoride, which works mainly at the outermost surface, hydroxyapatite particles can penetrate into deeper layers of an early cavity lesion.18PubMed Central. The use of hydroxyapatite toothpaste to prevent dental caries Clinical trials have also found that hydroxyapatite toothpaste reduces tooth sensitivity, apparently by plugging the exposed tubules in dentin that transmit pain signals.18PubMed Central. The use of hydroxyapatite toothpaste to prevent dental caries Reviews of the evidence describe the remineralizing effect as comparable to or better than conventional fluoride for initial enamel lesions.19PubMed Central. Nano-hydroxyapatite and its applications in preventive, restorative and regenerative dentistry: a review of literature
In orthopedics, hydroxyapatite coatings on metal implants have been used since the late 1980s. The idea is straightforward: a titanium hip or knee implant is strong but biologically inert, so coating it with hydroxyapatite gives bone something familiar to grow onto. Plasma-sprayed coatings have been shown to promote faster and stronger bone fixation compared to uncoated metal.20PubMed. Material fundamentals and clinical performance of plasma-sprayed hydroxyapatite coatings: a review The clinical record, however, is mixed on long-term durability. Some coatings degrade or delaminate over years of use, and the quality of the initial coating process turns out to matter a great deal. Electrochemically deposited coatings offer an alternative method that also increases bone ongrowth, though studies have not found a clear winner between the two techniques.21PubMed Central. The effect on bone growth enhancement of implant coatings with hydroxyapatite and collagen deposited electrochemically and by plasma spray
Is Nano-Hydroxyapatite Safe to Put in Your Mouth?
Given that nano-hydroxyapatite is showing up in more toothpastes and oral care products, safety is a reasonable question. The short answer is that current evidence points toward it being safe for oral use. One study specifically designed to mimic worst-case toothbrushing exposure tested a 3.1 percent nano-hydroxyapatite suspension against human gingival cells and found no signs of toxicity, no damage to cell structure, no increase in harmful reactive oxygen species, and no irritation potential.22Scientific Reports. Nano-hydroxyapatite in oral care cosmetics: characterization and cytotoxicity assessment Any particles that are swallowed dissolve immediately in stomach acid, releasing nothing more than calcium and phosphate ions that the body handles routinely.
The one nuance worth knowing is that lab studies using cells grown in flat dishes (rather than intact tissue) have shown that nano-hydroxyapatite particles can be taken up by individual cells and cause damage in that artificial setting. But the oral lining has a natural barrier layer, and given that oral epithelial cells turn over every five to seven days, absorbed particles at the surface are shed before they could accumulate or penetrate deeper.23PubMed Central. Nanohydroxyapatite in dentistry: A comprehensive review The gap between what happens to isolated cells on a dish and what happens in an intact mouth is large enough that regulatory agencies in Japan and the EU have approved nano-hydroxyapatite as an oral care ingredient.
What Happens When You Heat It
Hydroxyapatite’s composition also governs how it behaves at high temperatures, which matters for anyone manufacturing ceramics, implant coatings, or bone substitutes. The crystal is stable up to a point, but heating it above roughly 770–1000°C causes it to start losing its hydroxide ions. This is essentially the mineral drying out at the atomic level.24PubMed. Thermal decomposition and reconstitution of hydroxyapatite in air atmosphere Push the temperature higher, above about 1100–1400°C, and the crystal structure breaks down entirely, decomposing into other calcium phosphate phases that have different solubility, different mechanical properties, and different biological behavior.25Ceramics International. Phase transformation on hydroxyapatite decomposition Raman spectroscopy work has tracked these transitions in real time, showing that substantial hydroxide loss begins near 770°C and is complete by around 850°C.26Journal of Materials Science. High-temperature phase transformations of hydroxylapatite and the formation of silicocarnotite in the hydroxylapatite–quartz–lime system studied in situ and in operando by Raman spectroscopy
This thermal sensitivity is the central challenge in making plasma-sprayed implant coatings. The plasma flame can reach thousands of degrees, so controlling how much of the hydroxyapatite coating converts to unwanted phases during spraying is critical. Too much decomposition means a coating that dissolves too fast inside the body or bonds poorly to the underlying metal. Manufacturers walk a narrow processing window, trying to get good adhesion and density without cooking the hydroxyapatite into something else.
Hydroxyapatite Beyond the Human Body
Humans are not the only organisms that build with calcium phosphate. Vertebrates in general use carbonated hydroxyapatite for bones and teeth, and the basic composition is remarkably conserved across fish, reptiles, birds, and mammals. Some invertebrates, however, have evolved their own variations on the theme. Certain rock-boring bivalves in the genus Lithophaga mineralize their outer shell layer with fluorapatite rather than hydroxyapatite, swapping fluoride for hydroxide throughout.27Biological Journal of the Linnean Society. Nanocrystalline fluorapatite mineralization in the calciphile rock-boring bivalve Lithophaga: functional and phylogenetic significance This gives them a harder, more acid-resistant coating that may help them bore into limestone.
Meanwhile, the barnacle Ibla cumingi produces a phosphatic mineral that is structurally and compositionally distinct from both vertebrate bone and the fluorapatite of brachiopod shells. Its mineral is poorly ordered and rich in hydrogen phosphate, resembling an early, disordered precursor to crystalline apatite rather than the well-organized hydroxyapatite of bone.28PubMed Central. Characterization of the phosphatic mineral of the barnacle Ibla cumingi at atomic level by solid-state nuclear magnetic resonance: comparison with other phosphatic biominerals These examples show that nature uses the same basic calcium-phosphate toolkit in very different ways, tuning composition and crystallinity to meet different functional demands. Hydroxyapatite, as humans know it, is just one point on a broad spectrum of biologically produced apatites.