Images
Formula: Ca5(PO4)3(OH)
Anhydrous phosphate containing hydroxyl, apatite group,
apatite supergroup
Crystal System: Hexagonal
Specific gravity: 3.10 to 3.21 measured, 3.16 calculated
Hardness: 5
Streak: White
Colour: White, grey, yellow, green, violet, purple, red or brown
Solubility: Soluble in hydrochloric or nitric. Varieties containing CO3 may dissolve with slight effervescence.
Environments
Pegmatites
Metamorphic environments
Cave deposits
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
muscovite
(HOM).
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).
Localities
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).
Back to Minerals