Chemistry Reference
In-Depth Information
of bones and fish scales serve as nuclei for precipitation of apatite (rich in
CO
3
2−
). Cryptocrystalline phosphates are called collophane. They are mainly
made of apatite, but their exact composition is uncertain. Apatite also forms
nodules and crusts in the anoxic zone on the continental slopes.
Phosphorites (and guano) are the accumulation of debris and excre-
ment of vertebrates trapped in karsts; their mining interest is no more than
historical (1% of world production).
Apatite is a common accessory in metamorphic rocks as well as of
ortho- and para-origin. It also occurs in hydrothermal alpine clefts.
3.5.3.2 Monazite
Monazite (Ce, La, Th) (PO
4
) is the bearer of REE in calcium-poor granites,
syenites and pegmatites. Some monazites are rich in europium. Monazites
also contain uranium, aluminum and ferric iron in very small quantities.
Monazite is used for radiometric U/Pb dating. The obtained ages are gener-
ally concordant.
3.5.3.3 Xenotime
Xenotime YPO
4
(tetragonal) is a rare phosphate of Rare Earths (mostly
heavy lanthanides: dysprosium, erbium, terbium and ytterbium) that also
contain uranium and thorium. It may also contain traces of calcium, sili-
cium and arsenic. It is mostly a mineral of granite pegmatites and evolved
igneous rocks. It may be a clastic and may be concentrated in black sands
with other heavy minerals. Diagenetic xenotime is also reported, particu-
larly as coating on detrital zircons. It is an ore of heavy Rare Earths.
3.5.4 Lithium bearing minerals
These minerals appear mainly in lithian pegmatites; they are also found in
associated evolved granites, greisens and tin veins. Some are presented in
other parts of this topic. These are: lithium micas (§3.2.1.5), spodumene
(§3.2.4.1), amblygonite, petalite, elbaite (lithian tourmaline §3.5.2.1). Pol-
lucite, cassiterite (§3.7), the potassium feldspars, orthoclase and microcline,
albite and quartz may be associated to these lithium minerals.
3.5.4.1 Amblygonite
Amblygonite (Li, Na) AlPO
4
(F, OH) is a fluophosphate which forms a series
with the montebrasite, term low in fluoride. These triclinic minerals can be
confused with feldspars but are distinguished under the microscope by their
cleavages and by their alterations. These are lithium ore and some crystals
may be used as gemstones.
