Hydroxyapatite crystals on matrix
Needle-like hydroxyapatite crystals on stainless steel. Scanning electron microscope picture from University of Tartu.
Nanoscale coating of Ca-HAp, image taken with scanning probe microscope
A 3D visualization of half of a hydroxyapatite unit cell, from x-ray crystallography

Naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities.

- Hydroxyapatite

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Tooth enamel

One of the four major tissues that make up the tooth in humans and many other animals, including some species of fish.

Labeled molar
Parts of a tooth, including the enamel (cross section).
Histologic slide showing a developing tooth. The mouth would be in the area of space at the top of the picture.
Histologic slide showing enamel formation
The effects of bruxism on an anterior tooth, revealing the dentin and pulp which are normally hidden by enamel
Common dentistry trays filled with fluoride foam
An X-ray showing enamel and dentin replaced by an amalgam restoration
Irreversible enamel defects caused by an untreated celiac disease. They may be the only clue to its diagnosis, even in absence of gastrointestinal symptoms, but are often confused with fluorosis, tetracycline discoloration, or other causes. The National Institutes of Health include a dental exam in the diagnostic protocol of celiac disease.
Teeth of a rottweiler

The primary mineral is hydroxyapatite, which is a crystalline calcium phosphate.


Calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth.

Parts of a tooth, including dentin
Dentinal sclerosis

By volume, 45% of dentin consists of the mineral hydroxyapatite, 33% is organic material, and 22% is water.


Faceted blue apatite, Brazil
Apatite in photomicrographs of a thin section from the Siilinjärvi apatite mine. In cross-polarized light on left, plane-polarized light on right.
An apatite mine in Siilinjärvi, Finland.

Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal.


Phosphate mineral with the formula Ca53F (calcium fluorophosphate).

Fluorapatite grains in carbonate groundmass. Photomicrographs of thin section from Siilinjärvi apatite ore.
Fluorapatite. São Geraldo do Baixio, Doce valley, Minas Gerais, Brazil.

Along with hydroxylapatite, it can be a component of tooth enamel, but for industrial use both minerals are mined in the form of phosphate rock, whose usual mineral composition is primarily fluorapatite but often with significant amounts of the other.


For other uses, see Bone (disambiguation) or Bones (disambiguation).

A bone dating from the Pleistocene Ice Age of an extinct species of elephant
Cross-section details of a long bone
Micrograph of cancellous bone
Bone cells
Light micrograph of decalcified cancellous bone tissue displaying osteoblasts actively synthesizing osteoid, containing two osteocytes.
Transmission electron micrograph of decalcified woven bone matrix displaying characteristic irregular orientation of collagen fibers
Structure of a long bone
One way to classify bones is by their shape or appearance.
Endochondral ossification
Light micrograph of a section through a juvenile knee joint (rat) showing the cartilagineous growth plates
Radiography used to identify possible bone fractures after a knee injury
Reduced bone mineral density in Osteoporosis (R), increasing the likelihood of fractures
Human femurs and humerus from Roman period, with evidence of healed fractures
Skeletal fluorosis in a cow's leg, due to industrial contamination
Leg and pelvic girdle bones of bird
Bones of slaughtered cattle on a farm in Namibia
Cells in bone marrow
Scanning electron microscope of bone at 100× magnification
Structure detail of an animal bone

The matrix is hardened by the binding of inorganic mineral salt, calcium phosphate, in a chemical arrangement known as bone mineral, a form of calcium hydroxylapatite.


In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.

Crystals of serandite, natrolite, analcime, and aegirine from Mont Saint-Hilaire, Quebec, Canada
Schist is a metamorphic rock characterized by an abundance of platy minerals. In this example, the rock has prominent sillimanite porphyroblasts as large as 3 cm.
Hübnerite, the manganese-rich end-member of the wolframite series, with minor quartz in the background
When minerals react, the products will sometimes assume the shape of the reagent; the product mineral is termed a pseudomorph of (or after) the reagent. Illustrated here is a pseudomorph of kaolinite after orthoclase. Here, the pseudomorph preserved the Carlsbad twinning common in orthoclase.
Topaz has a characteristic orthorhombic elongated crystal shape.
Contact twins, as seen in spinel
Diamond is the hardest natural material, and has a Mohs hardness of 10.
Pyrite has a metallic lustre.
Perfect basal cleavage as seen in biotite (black), and good cleavage seen in the matrix (pink orthoclase).
Galena, PbS, is a mineral with a high specific gravity.
Carnotite (yellow) is a radioactive uranium-bearing mineral.
Aegirine, an iron-sodium clinopyroxene, is part of the inosilicate subclass.
Natrolite is a mineral series in the zeolite group; this sample has a very prominent acicular crystal habit.
Muscovite, a mineral species in the mica group, within the phyllosilicate subclass
Asbestiform tremolite, part of the amphibole group in the inosilicate subclass
An example of elbaite, a species of tourmaline, with distinctive colour banding.
Epidote often has a distinctive pistachio-green colour.
Black andradite, an end-member of the orthosilicate garnet group.
Native gold. Rare specimen of stout crystals growing off of a central stalk, size 3.7 x 1.1 x 0.4 cm, from Venezuela.
Red cinnabar (HgS), a mercury ore, on dolomite.
Sphalerite crystal partially encased in calcite from the Devonian Milwaukee Formation of Wisconsin
Pink cubic halite (NaCl; halide class) crystals on a nahcolite matrix (NaHCO3; a carbonate, and mineral form of sodium bicarbonate, used as baking soda).
Gypsum desert rose

Moreover, living beings often synthesize inorganic minerals (such as hydroxylapatite) that also occur in rocks.

Human skeleton

Internal framework of the human body.

A human skeleton on exhibit at the Museum of Osteology, Oklahoma City, Oklahoma
During construction of the York to Scarborough Railway Bridge in 1901, workmen discovered a large stone coffin, close to the River Ouse. Inside was a skeleton, accompanied by an array of unusual and expensive objects. This chance find represents one of the most significant discoveries ever made from Roman York. Study of the skeleton has revealed that it belonged to a woman.

However, bones are not entirely made of calcium, but a mixture of chondroitin sulfate and hydroxyapatite, the latter making up 70% of a bone.


Anion, salt, functional group or ester derived from a phosphoric acid.

Phosphoric acid speciation
Phosphate mine near Flaming Gorge, Utah, US, 2008
Train loaded with phosphate rock, Métlaoui, Tunisia, 2012
Sea surface phosphate from the World Ocean Atlas
Relationship of phosphate to nitrate uptake for photosynthesis in various regions of the ocean. Note that nitrate is more often limiting than phosphate. See the Redfield ratio.

These structures are made of crystalline calcium phosphate in the form of hydroxyapatite.

Dental fluorosis

Common disorder, characterized by hypomineralization of tooth enamel caused by ingestion of excessive fluoride during enamel formation.

Mild fluorosis: in its usual mildest form, fluorosis appears as opaque white patches on the enamel
Amelogenesis imperfecta: this condition can be mistaken for fluorosis
Enamel hypoplasia caused by untreated celiac disease: this condition is often confused with fluorosis
Severe fluorosis: brown discolored and mottled enamel of an individual from a region with high levels of naturally occurring fluoride
Severe fluorosis: the enamel is pitted and discolored

Hydroxyapatite is converted to fluoroapatite in a three step process.

Tooth decay

Breakdown of teeth due to acids produced by bacteria.

Destruction of a tooth by dental caries and disease.
(A) A small spot of decay visible on the surface of a tooth. (B) The radiograph reveals an extensive region of demineralization within the dentin (arrows). (C) A hole is discovered on the side of the tooth at the beginning of decay removal. (D) All decay removed; ready for a filling.
Diagrammatic representation of acidogenic theory of causation of dental caries. Four factors, namely, a suitable carbohydrate substrate (1), micro-organisms in dental plaque (2), a susceptible tooth surface (3) and time (4); must be present together for dental caries to occur (5). Saliva (6) and fluoride (7) are modifying factors.
A Gram stain image of Streptococcus mutans.
"Stephan curve", showing sudden decrease in plaque pH following glucose rinse, which returns to normal after 30–60 min. Net demineralization of dental hard tissues occurs below the critical pH (5.5), shown in yellow.
Tooth decay
Microbe communities attach to tooth surface and create a biofilm. As the biofilm grows an anaerobic environment forms from the oxygen being used up. Microbes use sucrose and other dietary sugars as a food source. The dietary sugars go through anaerobic fermentation pathways producing lactate. The lactate is excreted from the cell onto the tooth enamel then ionizes. The lactate ions demineralize the hydroxyapatite crystals causing the tooth to be degraded.
The progression of pit and fissure caries resembles two triangles with their bases meeting along the junction of enamel and dentin.
The faster spread of caries through dentin creates this triangular appearance in smooth surface caries.
The tip of a dental explorer, which is used for caries diagnosis
A dental infection resulting in an abscess and inflammation of the maxillary sinus
Tooth samples imaged with a non-coherent continuous light source (row 1), LSI (row 2) and pseudo-color visualization of LSI (row 3).
G. V. Black Classification of Restorations
Rampant caries caused by methamphetamine abuse.
Toothbrushes are commonly used to clean teeth.
Annual caries incidence increases exponentially with annual per capita sugar consumption. Data based on 10,553 Japanese children whose individual lower first molar teeth were monitored yearly from the age of 6 to 11 years of age. Caries plotted on a logarithmic scale, so line is straight.
Common dentistry trays used to deliver fluoride.
Fluoride is sold in tablets for cavity prevention.
An amalgam used as a restorative material in a tooth.
A tooth with extensive caries eventually requiring extraction.
An image from Omne Bonum (14th century) depicting a dentist extracting a tooth with forceps.

These minerals, especially hydroxyapatite, will become soluble when exposed to acidic environments.