Chemistry Reference
In-Depth Information
3.5.5.3 Titanite
Titanite CaTiSiO
4
(O, OH, F) may contain aluminium and ferric iron in sub-
stitution to titanium:
Ti
4+
+
O
2-
(Al, Fe)
3+
+
(F, OH)
-
Rare Earths, ferrous iron, magnesium may also replace calcium.
There may be traces of niobium, tantalum, vanadium, uranium and tho-
rium. The presence of these radioactive elements allows the use of titanite
for radiometric dating. Their radiation can also destroy the lattice of titanite
(metamict state).
Titanite is a common mineral in the acidic to intermediate igneous
rocks, provided they contain sufficient calcium. It is relatively abundant
in nepheline syenite. Titanite is also common in metamorphic rocks rich
enough calcium, especially some metapelites and particularly amphibolites
and calc-silicate-gneisses.
3.5.5.4 Perovskite
Perovskite is orthorhombic pseudo-cubic mineral, which has a very com-
pact structure: it is the type of stable minerals in mantle conditions. Its sim-
plified formula is CaTiO
3
. Calcium is commonly replaced by Rare Earths
(dominant cerium and less abundant lanthanum) and yttrium (minor), and
to a lesser extent by Fe
2+
, Sr and/or Na. Titanium is replaced by niobium,
often associated with minor amounts of zirconium, and to a lesser extent,
tantalum:
(
Ca
, Ce, La, Y, Fe
2+
, Sr, Na) (
Ti
, Nb, Ta, Zr)
O
3
titanite).
It is a mineral of alkaline igneous rocks under-saturated in silica, where
it is associated with nepheline, leucite, melilite. It is often found in mafic and
ultramafic: rocks like ijolites, jacupirangites, nephelinite, kimberlites and in
the carbonatites. It is also found in nepheline syenites and their pegmatites.
It is known in meteorites.
It is also found in skarns.
Perovskite is under-saturated in silica (quartz
+
perovskite
=
3.5.6 Zircon
Zircon
ZrSiO
4
is a common accessory mineral of the igneous rocks. It is
more common in intermediate to acidic rocks. It is relatively abundant in
differentiated alkaline rocks such as nepheline syenite.
