Chemistry Reference
In-Depth Information
radii of these elements. When two elements have the same charge, the
one that has the smallest ionic radius is preferentially incorporated: so,
in an isomorphous series, the magnesium-bearing members are stable at
higher temperatures than the iron-bearing members;
in the lattice, a site may remain all or partially empty; for instance,
the substitution 2 Al
VI
3 (Fe, Mg)
VI
, (where the sign represents
a vacant site), is (limited) substitution occurring between dioctahedral
micas and trioctalhedral micas.
The composition of minerals is even more constrained as the tempera-
ture is higher. Knowledge of the precise compositions of minerals and their
variations is very important to reconstruct the conditions of the formation
of rocks.
Figure 1.7
Valency, ionique radius (data by Shannon and Prewitt, 1969 and
Shannon, 1976).
