Geology Reference
In-Depth Information
Box 8.4 Features of a silicate analysis
table  8.4.1 shows two ways of presenting the chemical
analysis of a silicate material (in this case an olivine):
Only five oxides have concentrations significant enough to
be listed in this olivine analysis. In other minerals or rocks,
a wider range of elements would require analysis. a typical
silicate analysis would include the following major elem-
ents (those making up more than 0.1% of the total and
so included in the analysis total): SiO 2 , al 2 O 3 , Fe 2 O 3 , FeO,
MnO, MgO, CaO, Na 2 O, K 2 O, tiO 2 , p 2 O 5 , h 2 O (see table 8.5
in exercise 8.2 at the end of the chapter) and CO 2 . One
might also look for certain trace elements (Ni being the
prominent one in the case of olivine) whose concentr-
ations would mostly fall below 0.1% and have no significant
effect on the total (see Figure 9.1).
Table 8.4.1 typical silicate analysis (olivine)
Percentage of
element*
Percentage of
oxide
Si
18.42
SiO 2
39.41
Fe (ferrous)
12.79
FeO
16.46
Mn
0.16
MnO
0.21
Mg
26.10
MgO
43.27
Ca
0.16
CaO
0.23
O
41.95
total
99.58
*the composition is expressed here in percentages of each element
(i.e. the number of grams of the element per 100 g of the sample).
Such units are commonly referred to as 'weight percent', although
'mass percent' is a more accurate description. Oxygen appears as
a separate item, but in fact it is unnecessary to analyse for it: the
valency of each element requires that it combines with oxygen in
stoichiometric proportions, even in complex silicates. It is thus pos-
sible to calculate the amount of oxygen present from the percent-
ages of the other elements in the rock and their valencies. reporting
mass % to two decimal places is consistent with the precision of
analytical methods currently in use for major elements.
a more convenient format for including oxygen is simply to report
the analysis in terms of the percentage of each oxide (g oxide per
100 g sample), as shown in the right-hand column. there is no sepa-
rate entry for oxygen, as it has all been allocated to individual oxides.
the entries in columns 1 and 2 are related to each other as
follows:
RMM
RAM
of oxide
of element
oxide% metal%
=
×
×
n
where n is the number of metal atoms per molecule of oxide. (It is
necessary to specify FeO and Fe 2 O 3 separately, or to assume that
all of the iron is in one form or the other.)
the oxide analysis is followed by a total, which for an accurate
analysis should fall between 99.5 and 100.75% (all analytical
measurements attract a small statistical error). If the total exceeds
100.75, one would suspect an unacceptable error in analytical
procedure or calculation. a low total would suggest an error in the
other direction, or neglect of an important constituent.
Calculating site occupancies
larger site. It occurs in amphiboles and micas in a large
('A') site associated with the silicate rings (Box 8.5), but
this site does not exist in the pyroxene structure, which
therefore excludes K.
In framework silicates there are no compact Y sites
to accommodate Mg 2+ and Fe 2+ . Ca, Na and K occupy
sites with somewhat irregular geometry, ranging in
co-ordination number from 6 to 9. In the more open
structures of the zeolites these sites are larger still, and
indeed may be substantially larger than the Na + and K +
ions occupying them. This, in combination with the
weak charge on these ions, means that they are readily
removed and replaced by other cations, a process
known as ion exchange that zeolites share with clay
minerals (Box 8.2).
The analysis of a silicate mineral (Box 8.4) is easier to
understand if it is recalculated into a form directly
comparable with the mineral's chemical formula.
Olivine analysis
We have seen that, in the formula of pure forsterite
(Mg 2 SiO 4 ), 2 magnesium ions and 1 silicon atom are
associated with 4 atoms of oxygen. How many atoms of
Mg (and Fe, Mn and Ni) and of Si would on average be
associated with 4 oxygen atoms in a general olivine
analysis like that given in Box  8.4? To answer this we
need to recalculate the analysis in terms of the relative
numbers of atoms, instead of mass percentages of oxides.
 
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