Chemistry Reference
In-Depth Information
Systematic mineralogy
3.1 MAJOR TECTOSILICATES:
QUARTZ - FELDSPARS - FELDSPATHOIDS
In tectosilicates (
framework silicates), tetrahedra (SiO
4
)
4−
form a three-
dimensional lattice in which each oxygen placed at an apex of a tetrahe-
dron is shared between two neighboring tetrahedra. The silicon Si
4+
ions in
the center of each tetrahedron can be replaced by aluminum Al
3+
ions. All
the aluminum in the tectosilicates is 4-fold coordinated. Two neighboring
tetraedra sharing an apex cannot contain both Al
3+
ions: the replacement of
Si by Al can not exceed 50%. This substitution of Si by Al can be completely
disordered or partially ordered. Disordered forms whose entropy is higher,
are high temperature polymorphs.
The deficit of the charge between Si
4+
and Al
3+
ions is compensated by
large cations K
+
, Na
+
, Ba
+
and Ca
++
which are 6-fold or more coordinated.
Small and/or heavily charged cations induce a local imbalance and do not fit
into these structures: tectosilicates do not contain ions such as Fe, Mg, Mn,
Ti. Due to the larger dimensions of their cations, lead or Rare Earths can
enter these structures.
The structure of tectosilicates is relatively open and molecules of large
dimension, Cl
2
, CO
3
, SO
4
, S
2
can enter the lattice (scapolite, feldspathoids
of the sodalite group). Zeolites contain open cavities where water molecules
can be introduced; this water is very weakly bound to the lattice.
As Si is replaced of by Na Al or K Al, or 2 Si by Ca Al
2
, all tectosilicates
are strictly saturated in alumina.
Tectosilicates include:
=
1 silica group minerals;
2 feldspars;
3 feldspathoids that are under-saturated in silica in comparison to
feldspars;
4
analcime that is not a feldspathoid but is close to them by its chemical
composition and its occurrences;
