Hydroxylapatite-<i>M</i>

Chemical formula: Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>(OH)

Hydroxyapatite-M is a monoclinic polymorph of hydroxyapatite, a mineral from the apatite group, which is the main inorganic component of vertebrate bones and teeth.

## Characteristics Hydroxyapatite-M is a monoclinic polymorphic variety of hydroxyapatite, belonging to the apatite group. It occurs much less frequently than its hexagonal counterpart. It forms prismatic, tabular, or acicular crystals, often gathered into radial or fibrous aggregates. It can also be found in the form of granular masses, crusts, or earthy masses. It is a brittle mineral. ## Physical Properties It is characterized by a hardness of approximately 5 on the Mohs scale. Its luster is most often vitreous to resinous on crystal faces, and on fracture surfaces, it can be dull or earthy. It is a transparent to translucent mineral. Its density is approximately 3.1-3.2 g/cm³. ## Colors and Varieties Hydroxyapatite-M occurs in various colors; most often it is white, colorless, yellowish, greenish, or brown. The coloration depends on chemical impurities. There are no named commercial or gemological varieties; classification is based on crystal structure (polymorphism). ## History and Name The name "hydroxyapatite" comes from the Greek words "hydro" (water), referring to the presence of the hydroxyl group (OH), and "apatein" (to deceive, to mislead), because apatites were often mistaken for other minerals, such as beryl or tourmaline. The suffix "-M" (formerly "-2M") denotes its monoclinic crystal structure. The mineral was formally described as a distinct phase by Chris J. Stanley and co-workers in 2010, although its existence was known earlier. ## Applications Due to its rarity, Hydroxyapatite-M is primarily of scientific and collector's interest. Synthetic hydroxyapatite (regardless of the polymorph) is widely used in medicine as a biomaterial for the production of bone implants, coatings for endoprostheses, and in dentistry, due to its biocompatibility and presence in natural bone tissue.

Properties

Mohs hardness
5
Luster
Vitreous to Resinous
Streak
White
Density
3.1-3.2
Cleavage
Indistinct on {1010}
Fracture
Conchoidal to Uneven
Transparency
Transparent to Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of Hydroxyapatite-M in field conditions is practically impossible without advanced analytical techniques. It can be preliminarily recognized by the typical prismatic or tabular crystal habit characteristic of apatites, vitreous luster, and a hardness of approximately 5. It reacts vigorously with acids. Definitive differentiation from hexagonal hydroxyapatite requires X-ray diffraction (XRD) analysis. ## Distinguishing from Similar Minerals It can be confused with other minerals from the apatite group (fluorapatite, chlorapatite) and with hexagonal hydroxyapatite. Differentiation from these requires specialized chemical and crystallographic studies. It may also resemble some feldspars or quartz, from which it differs by lower hardness. ## Crystal Forms It forms elongated, prismatic crystals, often terminated by pyramids. It also occurs as tabular crystals. It often forms fibrous, radial, rosette-like aggregates, as well as massive, granular, and crust-like occurrences.

Geological environment

## Genesis Hydroxyapatite-M is a secondary mineral, forming in the oxidation zones of ore deposits, especially in phosphorus-rich rocks. It forms under conditions of low temperatures and pressures. It can also crystallize in gas vesicles in volcanic rocks, such as basalts. ## Mineral Associations It co-occurs with minerals such as strengite, cyrilovite, rockbridgeite, dufrenite, wavellite, quartz, goethite, and other secondary iron and manganese phosphates. ## Localities The most important localities from which well-formed specimens originate include the Fowey Consols mine in Cornwall (Great Britain) - which is its type locality. It is also known from Wheal Alfred in Cornwall and from the Fuchs an der Stümpelfurche quarry in Hesse (Germany).

Rarity

Rare

For collectors

## Quality Criteria Most valued by collectors are specimens with well-formed, sharp, and undamaged crystals with distinct luster. Intense color (e.g., greenish or yellow) and aesthetic arrangement of crystals on the rock matrix enhance its attractiveness. Association with other rare phosphates is also important. ## Popular Localities Collector-grade specimens mainly come from historical mines in Cornwall, Great Britain, which are the type locality for this mineral. Material from these localities is rare and sought after.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, avoiding prolonged soaking. Ultrasonic cleaners should be avoided, as they can cause cracks. ## What to Avoid The mineral is sensitive to acids, which cause its rapid dissolution. Contact with household and laboratory chemicals should be avoided. It should not be heated or exposed to sudden temperature changes. ## Storage Hydroxyapatite-M specimens should be stored in separate, padded boxes or display cases to prevent scratching by harder minerals. They should be protected from dust and moisture. Brittle aggregates require particular care during handling.

Sources

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