Chemistry Reference
In-Depth Information
3.5.3 Phosphates
3.5.3.1 Apatite
Apatite Ca
5
(PO
4
)
3
(OH, F, Cl) is an accessory mineral common in all types of
rocks. Apatite varies mostly by substitution:
F
OH
Cl
The end members are called fluor-apatite/hydroxyl-apatite/chlor-apa-
tite. Fluor-apatite and fluor-hydroxylapatite are the most common.
There is also a substitution:
PO
4
3-
CO
3
2-
+
F
-
Francolite is an apatite rich in CO
3
2-
and fluorine.
Apatite may contain silica in substitution to phosphorus; the charge
deficit is compensated either by Rare Earth (or thorium) as a substitution
to calcium:
Ca
2+
+
PO
4
3-
REE
3+
+
SiO
4
4-
or by the coupled substitution of phosphorus by sulfur which forms a con-
tinuous solid solution toward ellestadite (Ca
5
(SiO
4
, SO
4
)
3
(OH, Cl, F)):
2 PO
4
3-
SO
4
2−
+
SiO
4
4-
Apatite is an ubiquitous mineral.
Fluor-apatite (hydroxy-apatite and fluor-) is found in both basic and
acidic igneous rocks, including pegmatites. It appears, however, only beyond
some grade of differentiation. Thus, in the Bushveld (tholeiitic series) it
begins to crystallize in the upper part of the Upper Zone. In the alkaline vol-
canic series of Velay, it is missing in basalts/basanites and poorly differenti-
ated hawaiites, and it appears in differentiated hawaiites and is subsequently
present in mugearites, benmoreites, trachytes and phonolites.
Apatite is very abundant in differentiated alkaline rocks, particularly
in plutonic rocks. It is a major constituent of some rocks of carbonatite
complexes: at Palaborwa (/Phalaborwa, Transvaal): it is mined in apatite-
magnetite-phlogopite-calcite-apatite rocks and olivine-phlogopite-clinopy-
roxene-olivine rocks. Alkaline rocks and carbonatite complexes provide
23% of world production of phosphates.
Fluor-apatite is the major constituent of bones and teeth of vertebrates
and fish scales.
The major deposits of sedimentary phosphates (76% of world produc-
tion) are sediments of areas of upwelling in the coastal shelves. Fragments
