Chemistry Reference
In-Depth Information
Twins may by simple or repeated. Among the repeated twins there are:
￿
Polysynthetic twins : the joined individual crystals form parallel
lamellae (examples: twins in cordiérite; albite and pericline twins in
the plagioclases, grunerite).
￿
Cyclic twins : individual crystals form a more or less circular asso-
ciation ( example : twins of leucite, cordierite).
The unit cell of the twinned system has a higher symmetry than the unit
cell of the individual crystals.
1.1.2 A given chemical composition
A given chemical composition is not sufficient to define a mineral. Indeed
polymorphs are minerals with the same chemical composition but a differ-
ent structure: diamond (cubic) and graphite (hexagonal), calcite (trigonal)
and aragonite (orthorhombic) are well known examples. Other examples
are given by the various polymorphs of silica or feldspar.
The chemical composition varies within certain limits depending on:
￿
the presence of trace elements included in the lattice;
￿
substitutions between elements:
￿
such substitutions can be made of atom to atom, for example, the
substitution Mg
Fe 2+ ;
￿
or by laws of substitution involving several elements. An example is
the widespread substitution in silicates
Si IV Mg VI
Al IV Al VI
(where Al IV is aluminum in 4-folds coordination, in tetrahedral site, sur-
rounded by 4 atoms of oxygen and Al VI aluminum in 6-fold coordination,
and octahedral site, surrounded by 6 atoms of oxygen).
Micas and amphibole groups are minerals where the laws of substitu-
tion are particularly diverse.
The substitutions in minerals are governed by strict laws (rules of
Goldschmidt):
￿
electrical equilibrium must be respected: for instance in the above exam-
ple the valency of silicon is 4, the one of the magnesium is 2: 4
+
2
=
6,
the valency of aluminum is 3: 3
+
3
=
6;
￿
only elements that have ionic radii difference of less than 15% can
replace each other; the difference may be bigger but the substitution
is then only partial. Iron replaces magnesium in common silicates and
carbonates, but the replacement of magnesium by calcium is impossible
(or at least very limited) in carbonates due to the difference in ionic
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