Chemistry Reference
In-Depth Information
Appendix - Calculation
of the structural formula
of a mineral
The chemical analysis of mineral gives the
percentage by weight
of the dif-
ferent oxides that compose this mineral. The analysis is presented by order
of decreasing valency these oxides: SiO
2
, TiO
2
, Al
2
O
3
, Fe
2
O
3
, Cr
2
O
3
, FeO,
MnO, MgO, CaO, Na
2
O, K
2
O, H
2
O, F, Cl. The structural formula shows
the distribution of the
atoms
in different sites of the unit cell (or in one of its
multiple or sub multiples).
The principle of this calculation is very simple:
1
The number of atoms of each element in 100 g of the sample is obtained
by dividing the percentage by weight of oxides by the molecular weight
of each oxide.
2
A simple rule of three, distributes the atoms in the unit cell by fixing one
or more parameters of this cell: it may be number of total number of
oxygens, the number of cations in the entire or part of the unit cell, etc.
The main difficulty is the choice of this or these parameters.
When the distribution of iron between Fe
2
O
3
and FeO (and manganese
between Mn
2
O
3
and MnO) and H
2
O is known
, it is usual to calculate the
structural formula
on the basis of a fixed number of anions
(O, OH, F, Cl).
Chemical analysis of minerals are most often made using an electron
microprobe which do not measure elements lighter than chlorine, nor H
and O. Consequently, only the total iron is determined, usually in the form
of ferrous iron, which we will note FeO*. It is the same for manganese.
The
previous calculation based on a content of oxygen and hydroxyl can no
longer be done
and can only lead to
inaccurate
results.
The method of calculation must be adapted to each particular case.
I
- if we can assume that
there is no water, nor ferric iron
in the struc-
tural formula, it is possible to calculate this formula on a base on a number
of oxygen or on a number of cations.
Example 1:
A plagioclase (Na, K)
1−x
Ca
x
Al
1+x
Si
3−x
O
8
[albite (x
=
0),
anorthite (x
1)]. Calculation of the structural formula can be based either
on 8 oxygens, or on 5 cations.
=
