Anhydrous phosphate containing hydroxyl, apatite group
Specific gravity: 3.10 to 3.21
Colour: White, grey, yellow, green, violet, purple, red or brown
Solubility: Soluble in hydrochloric or nitric. Varieties containing CO3 may dissolve with slight effervescence.
Hydroxylapatite is much less common than fluorapatite. It is formed by the
reaction of limestone with phosphatic solutions derived from guano. In caves it
is associated with brushite and calcite. In
talc schists it is associated with
talc and serpentine. It has been found as a
fracture-localised alteration of montebrasite in a complex
granite pegmatite, associated with
montebrasite, crandallite and
Other common associates include kaolinite, quartz, siderite, titanite and smectite group minerals (which includes montmorillonite, saponite and nontronite) (Mindat).
Hydroxylapatite is stable over the pH range 4 to 12 (fairly acid to highly alkaline) (Dana).
At the Willy Wally gully near Merriwa, New South Wales, Australia, hydroxylapatite was seen in some vesicles scattered over or coating saponite vesicle linings, and occasionally coating arborescent saponite. Its presence in other vesicles may be masked by overlying chabazite-Ca or lévyne-Ca/offretite (AJM 16.2.81).
At the Fairview deposit, South Australia, hydroxylapatite is the most abundant mineral. Green hydroxylapatite has been noted overgrowing fluorapatite and turquoise (AJM 17.1.18).
At the Mount Deverell variscite deposit, Milgun Station, Western Australia, hydroxylapatite encrusts surfaces and cavities in variscite veins; it also forms microscopic crystals on segelerite and mitridatite. The variscite deposits are hosted by marine sedimentary rocks (AJM 20.2.26). Hydroxylapatite generally is stable at a pH greater than 6 under weathering zone conditions. When the pH rises to this level variscite becomes unstable and may alter to hydroxylapatite (AJM 20.2.35-36).
In alpine regions of Switzerland hydroxylapatite occurs in talc schists (Dana).
At the serpentine quarry near Holly Springs, Cherokee county, Georgia, USA, hydroxylapatite has only been found in talc and chlorite schists, which indicates that it is formed by metamorphism in the presence of much water and with the simultaneous formation of other minerals rich in the hydroxyl group. These talc schists are formed by metamorphism of igneous rocks with a silica content below 45 weight-%. An upper temperature limit is set by the fact that at one atmosphere pressure talc dissociates into enstatite between 8,000 and 8,400oC, and hydroxylapatite dissociates at 1,200 to 1,500oC. Minerals of the apatite group generally contain fluorine; hydroxyl is substituted for fluorine only under very unusual conditions in some pegmatites (AM 28.356-371).
At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, hydroxylapatite is a late-stage mineral in partially altered phosphate pods. In some cases the cores of hydroxylapatite botryoids are composed of hydroxylherderite. In one occurrence the core of the botryoids was fairfieldite overgrown by hydroxylherderite and then by hydroxylapatite (R&M 94.6.509).
At the Clay Canyon variscite mine, Fairfield, Utah, USA, hydroxylapatite is the main constituent of crusts up to one centimetrre thick separating crandallite layers from variscite cores (Minrec 41.4.340-342).
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