Hydroxylapatite
Hydroxylapatite, also frequently called hydroxyapatite, is a mineral. It is a naturally occurring 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 molecules.  Hydroxylapatite is the hydroxyl endmember of the complex apatite group. The OH- ion can be replaced by fluoride, chloride or carbonate. It crystallizes in the hexagonal crystal system. It has a specific gravity of 3.08 and is 5 on the Mohs hardness scale. Pure hydroxylapatite powder is white. Naturally occurring apatites can however also have brown, yellow or green colorations. Compare to the discolorations of dental fluorosis.
Seventy percent of bone is made up of the inorganic mineral hydroxylapatite. Carbonated-calcium deficient hydroxylapatite is the main mineral of which dental enamel and dentin are comprised.

# Medical uses
Hydroxylapatite can be found in teeth and bones, within the human body. Therefore, it can be 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 graft vs. host disease (GVHD).
Some modern dental implants are coated with hydroxylapatite. It has been suggested that this may promote osseointegration, but there is not yet conclusive clinical proof of this.
Bioactive Glasses are the only man made materials known to bond to both bone and soft tissue and have been clinically used as a bone grafting material for over 20 years in dental, maxillofacial and orthopedic procedures.  The material has been used in both solid form, as a middle ear prosthetic for conducted hearing loss, as well as in particulate form for filling boney defects throughout the body.  Unlike hydroxyapatite, which is said to be osteoconductive by conducting new bone growth along the materials surface, Bioactive Glasses are osteostimulative in that the material stimulates the recruitment and differentiation of osteoblasts which produce new bone.  As a result, Bioactive Glass rapidly enhances the production of new bone and is completely resorbed by the body and replaced by new bone.  These materials are usually produced in a high temperature (1350 C) melt process but can also be produced by the Sol-gel process which results in a controlled porosity and resorbability.  
Bioactive Glasses have also been used in oral care applications as a tooth remineralizer (Calcium Sodium Phosphosilicate) in both professional dental and consumer oral care products.

# Hydroxyapatite uses in chromatography
The mechanism of Hydroxyapatite (HAP) 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 HAP resin with protein negatively charged carboxyl groups and positively charged amino groups.  It may be difficult to predict the effectiveness of HAP 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.