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The apatite group comprises three minerals:
Fluorapatite: Ca5(PO4)3F
Hydroxylapatite: Ca5(PO4)3(OH)
Chlorapatite: Ca5(PO4)3Cl
All of these are anhydrous phosphates containing hydroxyl or halogen
Fluorapatite is the commonest mineral in the apatite group.
Varieties
Collophane is a carbonate-rich variety of apatite
Properties
Crystal System: Hexagonal
Specific gravity:
Fluorapatite: 3.1 to 3.25 measured, 3.18 calculated
Hydroxylapatite: 3.10 to 3.21 measured, 3.16 calculated
Chlorapatite: 3.17 to 3.18 measured, 3.17 calculated
Hardness: 5
Streak: White
Colour: Colourless to white, grey, also green, blue, pink, yellow, brown, violet, purple, red
Common impurities: OH,Cl,transition elements,La,Ce,Pr,Nd,Sm,Eu,Gd,Dy,Y,Er,Mn
Environments
Plutonic igneous environments
Pegmatites
Carbonatites
Sedimentary environments
Metamorphic environments
Hydrothermal environments
Apatite is a primary and secondary mineral, widely
distributed with important concentrations in carbonatites.
It is the most common rock-forming phosphate and a major mineral in lithified phosphate-rich sediments.
It occurs the oxidation zone of hypothermal (high temperature) veins and in Alpine cleft-type veins.
It is a common secondary mineral in high-temperature hydrothermal
alteration zones
(AofA).
Apatite is a common constituent of
marble,
skarn and
magnetite deposits.
It may also be found in
quartzolite,
granite,
syenite,
diorite,
rhyolite,
trachyte,
andesite and
basalt.
Apatite crystallises in the hexagonal system, point group 6/m. Crystals are common, tabular parallel to {0001}
or short to long prismatic with {1010},
{1011} and minor {0001}, sometimes together with other prisms and
bipyramids
(OJ).
Localities
In the pegmatite at the Sapo mine, Minas Gerais, Brazil,
apatite crystals are zoned, with a core of
fluorapatite and an outer overgrowth of
hydroxylapatite. They are associated with
microcline, albite variety
cleavelandite, quartz and
muscovite
(Min Rec 40.4.279-288).
The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned
iron mine, with
both underground and open cast workings. The iron ores contain
magnetite as the ore mineral and occur predominantly as masses of all sizes
enclosed in a large skarn body formed by contact metasomatism of
dolomitic limestone at the
margins of a granite intrusion. In parts of the underground workings
magnetite is also found in
marble in contact with the
granite. The skarn rocks
consist mainly of tremolite,
actinolite, diopside and
garnet.
Apatite is found in the skarn rocks
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
At Nacozari, Mexico, turquoise
pseudomorphs after apatite have been found
(KL p211).
Morocco
Apatite in Morocco is the most ubiquitous phosphate mineral and is found in most sedimentary, igneous and
metamorphic settings. Well crystallized apatite, usually
fluorapatite, occurs most frequently in igneous/hydrothermal rocks
such as evolved gabbros and
granitoids. It usually forms hexagonal, rectangular, acicular or
rounded tiny grains with an average grain size of 0.1–0.2 mm, but it is also occasionally a main rock-forming
mineral in some alkaline igneous systems (primarily those of Imilchil and Dakhla regions).
The major producing localities for apatites are connected with alkaline igneous intrusions. The apatite
deposits form millimeter to centimetre thick veins hosted mainly by
syenite rocks. The main components of these rocks are always alkali
feldspars (mostly albite), pyroxene,
amphibole, quartz,
prehnite, calcite,
magnetite, epidote and
titanite.
The apatite crystals occur as hexagonal prisms that can reach 15 cm in length and 3 cm in width. Both
fluorapatite and
chlorapatite have been identified. The more abundant is
fluorapatite formed in a white matrix, which consists of
albite, K-feldspar,
calcite, clinopyroxene,
magnetite, prehnite and
quartz. The most beautiful Moroccan gem-quality apatite is mined
from this vein type, where the apatite is characterised by its perfect euhedral habit, green-yellow colour,
transparency and glassy lustre.
Less common are chlorapatites, which are associated with veins of
dark matrix consisting mainly of amphibole ±
prehnite; these are euhedral in habit, typically cloudy, pale green to
grey, with variable lustre, and they frequently exhibit enclosed needles of
amphibole.
Other significant Moroccan fluorapatite deposits are reported in
Dakhla Gleibat Lafhouda-Twihinate carbonatite
occurrences. These occur as euhedral to subhedral crystals that range in size from 0.2 to 2 mm. The most important
concentrations are associated with iron oxide resting on top of the
carbonatites. Coarse white to pale yellow apatite,
as euhedral and subhedral crystals to 8 cm, comprising up to 40 volume percent of the rock, are hosted by these
iron oxide rocks
(R&M 98.2.172-173).
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