Chemistry Reference
In-Depth Information
3.5.8 Oxides of niobium, tantale and
zirconium of alkaline rocks
- perovskite
CaTiO
3
(cubic)
- baddeleyite ZrO
2
(monoclinic)
-
pyrochlore*
(Nb) - microlite (Ta) series
(Ca, Na)
2
(Nb, Ta)
2
O
6
(OH, F)
(cubic)
-
zirconolite*
(Ca, REE, U, Th)Zr
(Ti, Nb, Ta)
2
O
7
(several polymorphs)
Perovskite
is a mineral of rocks undersaturated in silica; it is discussed
in §3.5.5.4.
Baddeleyite
ZrO
2
(monoclinic) is a rare mineral of silica undersaturated
rocks: syenites, gabbro, anorthosites, carbonatites, kimberlites. Baddeleyite
is also a heavy mineral that are concentrated in some clastic sediments like
black sands.
Pyrochlore
and
microlite
(Ca, Na)
2
(Nb,Ta)
2
O
6
(OH, F) (respectively
Nb and Ta end members) are cubic minerals. They are found in alkaline
silica-saturated or under-saturated pegmatites and in carbonatites. They are
niobium - tantalum ores (Brazil, Quebec, Nigeria).
There are several polymorphs of formula CaZrTi
2
O
7
: the most common
form is
zirconolite ss
(metamict); 3T zirconolite is trigonal, 3O zirconolite
is orthorhombic, 2M zirconolite is monoclinic and zirkelite is cubic (Bayliss
et al., 1989). Zirconolite
s.l
. may by confused under the microscope with
rutile or titanite and its determination requires either the SEM or the electron
microprobe. The precise determination of the different polymorphs requires
to know the structure. This mineral is considered very rare. But it may be
relatively common in alkaline rocks. Zirconolite is used to fix the actinides.
3.6 MINERALS OF SEDIMENTARY ROCKS AND
ALTERITES
Clastic may contain any mineral reworked as clasts. Some groups of minerals
are however more specific of sedimentary rocks: these are carbonates (discussed
above), clays, iron and aluminum hydroxides (diaspore,
α
-AlO(OH), dis-
cussed above, boehmite
-FeO(OH),
limonite FeO(OH)-
n
H
2
O; non treated here) and evaporite minerals.
β
-AlO(OH), gibbsite Al(OH)
3
), goethite
α
3.6.1 Clay minerals
3.6.1.1 Structure and chemical composition
The clays are hydrated aluminum silicates. They are sheet silicates (phyllo-
silicates) made of silicates sheets connected by interlayers.
