Chemistry Reference
In-Depth Information
So one can understand that the ages obtained on a population of
zircons or even on a single crystal of zircon are discordant or of uncer-
tain interpretation. The current method used involves single point sepa-
rate measurements in each zone recognized in a single crystal age. For
example, this method provided on reworked zircons in metapelites of
the garnet zone of Montagne Noire following ages (Ma) (Gebauer et al.,
1989): 3148, 2900-2765 (Archean), 2140-1750 (Paleoproterozoic), 1000
(Mesoproterozoic), 600-556 (Neoproterozoic, Cadomian orogeny), 435
(Silurian, Caledonian orogeny). Debate on the significance of these ages
remains open.
3.5.7 Titano- and zircono-silicates and
silicates of the alkaline rocks
In (plutonic and/or volcanic) peralkaline rocks, the excess of alkali over
aluminum causes the appearance of alkali ferro-magnesian minerals (sodic
amphiboles and pyroxenes), and among the accessory minerals, of sodic
titano- silicates and zircono-silicates.
Here are some more or less common minerals of this group (the ones
shown on the CD are indicated with an asterix*):
- aenigmatite* Na
2
Fe
2+
5
Ti Si
6
O
20
(triclinic)
- astrophyllite* (K, Na)
3
(Fe
2+
, Mn)
7
Ti
2
(Si
4
O
12
)
2
(O, OH, F)
7
(triclinic)
- lamprophyllite Na
2
(Sr, Ba)
2
Ti
3
(SiO
4
)
4
(OH, F)
2
(monoclinic)
- rinkite Na(Na,Ca)
2
(Ca, Ce)
4
Ti(F
2
/(O, F)
2
/(Si
2
O
7
)
2
) (monoclinic)
- mosandrite (Na,Ca,Ce)
3
Ti (SiO
4
)
2
F
(monoclinic)
-
eudyalite*
(Na, Ca, Fe
2+
)
6
Zr (Si
3
O
9
)
2
(OH, F, Cl)
(trigonal)
- catapleiite
(Na,Ca)
2
Zr (Si
2
O
7
) SiO
4
· 2H
2
O
(monoclinic)
-
låvenite*
(Na, Ca, Mn, Fe)
3
(Zr, Nb, Ti)
Si
2
O
7
(OH, F)
(monoclinic)
- wöhlerite*
Ca
2
Na (Zr, Nb) Si
2
O
7
(O, OH, F)
2
(monoclinic)
-
rosenbuchite
(Na, Ca, Mn)
3
(Zr, Ti, Fe
3+
)
(SiO
4
)
2
(OH, F)
(triclinic)
There are others …
These are minerals of nepheline syenites and associated pegmatites.
Most of them occur in rocks under-saturated in silica. Aenigmatite, astro-
phyllite (and very rarely, eudyalite, låvenite and catapleiite) are also found
in peralkaline granites. Aenigmatite also occurs in volcanic rocks: alka-
line rhyolite, trachyte, phonolite. Wöhlerite also occurs in fenites and
carbonatites.
