Hydroxylapatite

Chemical formula: Ca₅(PO₄)₃OH

Hydroxyapatite is a mineral from the apatite group, being the main inorganic component of vertebrate bones and teeth.

## Characteristics Hydroxyapatite is a calcium phosphate and the hydroxyl end-member of the apatite series. It rarely forms well-developed, hexagonal, prismatic, or tabular crystals. Much more often, it occurs as compact, granular, reniform, radial, or earthy aggregates. As the main mineral component of bones and teeth, it is ubiquitous in the biological world. In its mineral form, it is less common than its fluorine counterpart, fluorapatite. ## Physical Properties This mineral is characterized by a hardness of 5 on the Mohs scale, which is the reference value for this hardness class. It has a vitreous to resinous luster. It is brittle, and its density is approximately 3.1-3.2 g/cm³. Crystals can be transparent to translucent, while massive forms are usually opaque. ## Colors and Varieties Hydroxyapatite can exhibit various colors: it can be colorless, white, gray, yellow, greenish, or brown. The coloration is often a result of impurities from other elements. No specific commercial varieties are distinguished, and the name primarily refers to its chemical composition. ## History and Name The mineral's name, given in 1856 by Carl Rammelsberg, refers to its chemical composition – indicating the presence of a hydroxyl group (OH) and its belonging to the apatite group. The name "apatite" comes from the Greek word *apatein* (to deceive, to mislead), because its diverse forms and colors often led to confusion with other minerals, such as beryl or tourmaline. ## Applications Synthetic hydroxyapatite is widely used in medicine as a biomaterial due to its biocompatibility with bone tissue. It is used in the production of bone implants, coatings for metal implants (e.g., hip joint prostheses), and in dentistry as a material for fillings and bone regeneration. It also has significance in scientific research and as a potential adsorbent for pollutants.

Properties

Mohs hardness: 5
Color: White, grey, yellow, green, violet, purple, red or brown
Luster: Vitreous
Streak: White
Density: 3.10
Cleavage: Poor on {0001) and on {10<mi>1</mi>0}
Fracture: Conchoidal,Sub-Conchoidal,Fibrous
Transparency: Transparent,Translucent,Opaque
Crystal system: Hexagonal

Diagnostic features

## Identification A key diagnostic feature is the hexagonal habit of crystals, if they are well-formed. A reference hardness of 5 on the Mohs scale is also an important clue – the mineral can be scratched by a knife but does not scratch glass. It reacts vigorously with acids, which is a destructive test and should be performed with great caution on a low-value fragment. ## Distinguishing from Similar Minerals Hydroxyapatite is difficult to distinguish from other minerals of the apatite group (fluorapatite, chlorapatite) without advanced chemical analyses (EDS, XRD). It can be confused with beryl (much harder, 7.5-8), some forms of calcite (much softer, 3, effervesces with cold, dilute acids), or scheelite (much denser and often exhibits fluorescence under UV light). ## Crystal Forms Crystals most often occur as hexagonal prisms, frequently terminated by pyramids or flat basal faces (pinacoids). It also occurs in radial, reniform (botryoidal), spherical (spherulitic) aggregates, and as massive, granular, or earthy aggregates.

Geological environment

## Genesis Hydroxyapatite forms in various geological environments. It can crystallize in pegmatites and hydrothermal veins. It is also a product of metamorphic processes in carbonate rocks, such as marbles. It also forms as a result of sedimentary and diagenetic processes in sedimentary rocks, often as the main component of phosphorites of organic origin (accumulations of bones, guano). ## Mineral Associations This mineral often co-occurs with other phosphates, such as fluorapatite. In pegmatites, it is associated with feldspars, micas, and quartz. In metamorphic environments, it occurs with calcite, dolomite, and diopside. In Alpine-type veins, it can occur with chlorite and albite. ## Localities Well-formed hydroxyapatite crystals are known from many places around the world. Valuable specimens come from Panasqueira in Portugal, the Khibiny Massif on the Kola Peninsula in Russia, and Brazil (Minas Gerais). In the USA, beautiful crystals have been found in mines in San Diego County, California, and in Maine. In Europe, it is also known from Alpine-type veins in Austria and Switzerland.

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are well-formed, sharp, vitreous, and transparent crystals with intense coloration, especially yellow or greenish. Specimens where hydroxyapatite crystals are aesthetically embedded on a contrasting rock matrix, for example, on albite or in association with other minerals, are also highly prized. Crystal size and lack of damage significantly increase the value of a specimen. ## Popular Localities Specimens of the highest collector's value come from several classic localities. Prismatic, yellowish-green crystals from the Sapo mine in Minas Gerais (Brazil) are highly sought after. Specimens from California (e.g., from the Elizabeth R. mine in Pala) also have historical significance. In Europe, localities in the Alps (e.g., in the Habach Valley in Austria) are considered classic.

Care and storage

## Cleaning Hydroxyapatite specimens should be cleaned very carefully. It is best to use a soft brush to remove dust. If necessary, the specimen can be rinsed with distilled water and then thoroughly dried. Ultrasonic cleaners should be avoided, as they can cause cracks. ## What to Avoid Hydroxyapatite is sensitive to acids, which cause its rapid dissolution. Contact with household acids (vinegar, lemon juice) and chemical acids should be strictly avoided. It should also not be heated or exposed to sudden temperature changes. ## Storage Specimens should be stored in separate, padded boxes or on stands to prevent scratching by harder minerals (e.g., quartz). It does not require special conditions regarding light or humidity, but stable room conditions are always recommended.