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Hydroxyapatite
Chemical Formula	Ca5(PO4)3(OH)
Species	Phosphates
Crystal System	Hexagonal
Mohs Scale	5
Specific Gravity	3.14-3.21
Color	Colorless, white, gray, yellow, yellowish green
Streak	White
Luster	Vitreous to subresinous, earthy
Refractive Index	nω = 1.651 nε = 1.644
Diaphaneity	Transparent to translucent
Cleavage	Poor on and
Fracture	Conchoidal
Crystal Habit：As tabular crystals and as stalagmites, nodules, in crystalline to massive crusts

Hydroxylapatite, also called hydroxyapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca₅(PO₄)₃(OH), but is usually written Ca₁₀(PO₄)₆(OH)₂ to denote that the crystal unit cell comprises two entities. Hydroxylapatite is the hydroxyl endmember of the complex apatite group. The OH^- ion can be replaced by fluoride, chloride or carbonate, producing fluorapatite or chlorapatite. It crystallizes in the hexagonal crystal system. Pure hydroxylapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of dental fluorosis.

Up to 50% of bone by weight is a modified form of hydroxylapatite (known as bone mineral). Carbonated calcium-deficient hydroxylapatite is the main mineral of which dental enamel and dentin are comprised. Hydroxylapatite crystals are also found in the small calcifications (within the pineal gland and other structures) known as corpora arenacea or 'brain sand'.

Medical uses

Hydroxylapatite can be found in teeth and bones within the human body. Thus, it is commonly used as a filler to replace amputated bone or as a coating to promote bone ingrowth into prosthetic implants. Although many other phases exist with similar or even identical chemical makeup, the body responds much differently to them. Coral skeletons can be transformed into hydroxylapatite by high temperatures; their porous structure allows relatively rapid ingrowth at the expense of initial mechanical strength. The high temperature also burns away any organic molecules such as proteins, preventing an immune response and rejection.

Many modern implants, e.g. hip replacements and dental implants, are coated with hydroxylapatite. It has been suggested that this may promote osseointegration.^{[citation needed]}

Porous hydroxylapatite implants are used for local drug delivery in bone.

Pathology

Hydroxyapatite deposits in tendons around joints in the medical condition calcific tendinitis.

Supplement

Microcrystalline hydroxylapatite (MH) is marketed as a "bone-building" supplement with superior absorption in comparison to calcium. It is a second-generation calcium supplement derived from bovine bone. In the 1980s, bone meal calcium supplements were found to be contaminated with heavy metals, and although the manufacturers claim their MH is free from contaminants, people are advised to avoid it because it has not been well-tested. However, the limited tests seem to show positive results. A 1995 randomized placebo-controlled study of 40 people in Europe found that it was more effective than calcium carbonate in slowing bone loss. A 2007 randomized double-blind controlled study of an MH supplement called the Bone Builder found significant positive effects in bone mineral density (BMD) compared to placebo. Hydroxylapatite has been used by Noel Fitzpatrick to facilitate bionic development in animals, by coating steel rods in hydroxylapatite to encourage natural growth of skin around it. As a component of nanocomposites, hydroxylapatite is finding uses as a potential new bone replacement materials.

Chromatography

The mechanism of hydroxylapatite (HA) chromatography is complicated and has been described as "mixed-mode" ion exchange. It involves nonspecific interactions between positively charged calcium ions and negatively charged phosphate ions on the stationary phase HA resin with protein negatively charged carboxyl groups and positively charged amino groups. It may be difficult to predict the effectiveness of HA chromatography based on physical and chemical properties of the desired protein to be purified. For elution, a buffer with increasing phosphate concentration is typically used.

Use in archaeology

In archaeology, hydroxylapatite from human and animal remains is analysed in order to reconstruct ancient diets. The mineral fractions of bone and teeth act as a reservoir of trace elements, including strontium. It has been established that the ratio of strontium to calcium in bone hydroxylapatite broadly reflects an animal's diet during the period before its death when the bone was being formed (5–10 years in the case of human remains), or in the case of dental mineral in childhood. In either case analysis of Sr/Ca ratios allows an individual's diet to be classified as carnivorous, herbivorous or omnivorous and either predominantly marine or terrestrially based. However the difficulty of compensating for post-mortem contamination of archaeological samples through interaction with groundwater continues to cast doubt on the reliability of the method. Stable isotope analysis is considered a more viable alternative, although strontium and other trace mineral analyses of dental samples are commonly used in situations where this is impossible because the collagen content of the bone has completely decayed (i.e. for palaeolithic samples).

Animal structures

The clubbing appendages of the Peacock mantis shrimp are made of an extremely dense form of the mineral, which is being investigated for potential synthesis and engineering use.