Carbonate-hydroxylapatite

Chemical formula: Ca₅(PO₄,CO₃)₃(OH,F,O)

Carbonate-hydroxylapatite is a variety of apatite enriched in carbonates, being a key mineral component of vertebrate bones and teeth.

## Characteristics Carbonate-hydroxylapatite, often referred to as dahllite, is a variety of hydroxylapatite in which some phosphate groups (PO₄) have been replaced by carbonate groups (CO₃). It rarely forms well-developed, distinct crystals. It usually occurs in the form of compact, granular, radial, or reniform aggregates, as well as concretions and oolites. It is the main inorganic component of bones and teeth, where its presence is of fundamental biological importance. ## Physical properties This mineral is relatively soft, with a Mohs hardness of approximately 5. It is brittle and characterized by an uneven or conchoidal fracture. The luster is typically vitreous to resinous or dull in the case of earthy aggregates. It is transparent to translucent. ## Colors and varieties Most often it is colorless, white, gray, yellowish, or brownish. The coloration often depends on impurities and formation conditions. The name "dahllite" has historically been used for its fibrous, resinous varieties, although it is not currently a formally recognized mineral species name, but merely a synonym for carbonate-hydroxylapatite. ## History and name The name "carbonate-hydroxylapatite" precisely describes its chemical composition – it is hydroxylapatite containing carbonate groups. The synonymous name "dahllite" was given in 1888 by Waldemar Christofer Brøegger and H. Bäckström in honor of the Norwegian geologists, brothers Tellef Dahl and Johan Dahl.

Properties

Mohs hardness: 5
Luster: Vitreous to Resinous
Streak: White
Density: 3.0-3.1
Cleavage: None
Fracture: Uneven to Conchoidal
Transparency: Transparent to Translucent
Crystal system: Hexagonal

Diagnostic features

## Identification A key diagnostic feature is its reaction with acids – even dilute hydrochloric acid causes effervescence and the release of CO₂. A Mohs hardness of 5 (can be scratched by steel) and often encountered reniform or radial forms are also helpful in identification. ## Distinguishing from similar minerals It can be confused with other varieties of apatite (e.g., fluorapatite) or with calcite and aragonite. It is distinguished from other apatites by its reaction with acid. It is distinguished from calcite and aragonite by its higher hardness (calcite has a hardness of 3) and lack of perfect cleavage. ## Crystal forms Well-formed crystals are rare. It usually forms fibrous, radial, reniform, spherical (spherulitic) aggregates, as well as massive, earthy, or compact masses. It also occurs as oolites and concretions in sedimentary rocks.

Geological environment

## Genesis This is a mineral formed mainly in sedimentary processes in marine environments, often with the participation of living organisms. It forms as a result of diagenesis and weathering of phosphorites and other phosphorus-rich rocks. It can also form in the oxidation zones of ore deposits and as a result of low-temperature hydrothermal processes. ## Mineral associations It co-occurs with other phosphate minerals, such as fluorapatite and hydroxylapatite, as well as with calcite, dolomite, quartz, chalcedony, and clay minerals. ## Localities As a rock-forming mineral, it is widely distributed worldwide. Significant phosphorite deposits containing carbonate-hydroxylapatite are found in Morocco, Tunisia, Algeria, the United States (Florida, North Carolina), Russia (Kola Peninsula), and China. Occurrences in the form of dahllite are known from Norway (Odegården, Bamble) and Germany.

Rarity

Common

For collectors

## Quality criteria For collectors, the most desirable specimens are those with well-formed, unusual shapes, such as precise spherulites, smooth reniform concretions, or aesthetically pleasing radial aggregates. An intense, resinous luster and interesting color, e.g., honey-yellow or brownish, are also valued. Specimens on a contrasting rock matrix are more attractive. ## Popular localities Classic "dahllite" specimens come from localities in Norway, especially from the Bamble region. High-quality spherical and reniform aggregates are found in phosphorite deposits in Morocco and Florida.

Care and storage

## Cleaning Specimens should be cleaned gently, using a soft brush and distilled water. Avoid hard tools that can scratch the surface (Mohs hardness 5). ## What to avoid The mineral is sensitive to acids, which cause its rapid dissolution with the release of carbon dioxide. Avoid contact with hydrochloric acid, acetic acid, and other household chemicals. Prolonged exposure to strong sunlight is not recommended, nor are sudden temperature changes. ## Storage Specimens should be stored under stable conditions, away from chemicals. It is best to keep them in separate boxes or on a soft surface to prevent scratching by harder minerals.

Sources

IMA Mineral List (CNMNC)