Chemistry Reference
In-Depth Information
3.5.4.2 Petalite
Petalite LiAlSiO
4
O
10
is a phyllosilicate which crystallizes in the monoclinic sys-
tem as tabular crystals flattened on (001). Na or K may be substituted to Li and
iron in very small quantities to the Al. Petalite is altered into montmorillonite.
3.5.4.3 Pollucite
Pollucite (Cs, Na)AlSi
2
O
6
· H
2
O, contains no lithium, but it is associated with
lithium-bearing minerals in the same occurences. The mineral is cubic, is most
times enhedral and has no cleavage; its index (1.520) is significantly lower
than the one of quartz. Pollucite forms a series with analcime (NaAlSi
2
O
6
·
H
2
O) (§3.1.3) but the occurrences of these minerals are quite different.
3.5.5 Titanium bearing minerals
Biotite is probably, with ilmenite, the major bearer of titanium in many
rocks. Recall that biotite may contain up to 5-6 wt% TiO
2
. Titaniferous
magnetite is also an important bearer of titanium in igneous rocks.
Titanium has also its own minerals:
-
rutile
TiO
2
(tetragonal, positive unixial) and its low temperature
polymorphs;
anatase
(tetragonal, negative uniaxial) and
brookite
(orrthorhombic);
-
ilmenite
FeTiO
3;
- pseudobrookite a continuous series between Fe
2
O
3
· TiO
2
end member
and FeO · 2TiO
2
end member; very rare tardimagmatic mineral of the
volcanic alkaline rocks;
-
titanite
CaTiSiO
4
(O, OH, F);
-
perovskite
CaTiO
3
.
3.5.5.1 Rutile
TiO
2
may contain Fe
3+
, Fe
2+
, Al, Cr, V, Nb, Ta, Sn in minor amounts or trace
amounts. It is the form of high temperature of the polymorphs of TiO
2
as it
has the smallest molar volume of the three polymorphs, it is also the form of
high pressure. It thus occurs in eclogites and of high pressure amphibolites.
Rutile is less common than ilmenite in igneous rocks, but occurs in some
granitic pegmatites and anorthosites. It is rare in volcanic rocks.
Chlorite derived from titaniferous biotites commonly contains rutile
needles, sometimes grouped into 6 pointed stars, as sagenite twins.
Rutile appears as inclusions in other minerals such as quartz, as fine
needles sometimes grouped into 6 pointed stars. It is thus responsible for the
effect of asterism of star sapphires.
