Geology Reference
In-Depth Information
Table 8.4
The mineral formula and site occupancies of an amphibole
1
2
2a
3
4
5
Oxide
RMM of oxide
Analysis as mass
% oxides
Analysis as moles
of oxides*
Moles of
metals
†
Moles of
oxygen
(as O
2
)
‡
Cations per
24 oxygens
§
Site occupany
totals
SiO
2
60.09
57.73
0.9607
0.9607
1.9214
7.786
Z site
8.000
iv
#
0.214
vi 1.700
al
2
O
3
101.94
12.04
0.1181
0.2362
0.3543
Fe
2
O
3
159.70
1.16
0.0073
0.0146
0.0219
0.118
FeO
71.85
5.41
0.0753
0.0753
0.0753
0.610
C site
5.056
MnO
70.94
0.10
0.0014
0.0014
0.0014
0.011
MgO
40.32
13.02
0.3229
0.3229
0.3229
2.617
CaO
56.08
1.04
0.0185
0.0185
0.0185
0.150
B site
1.975
Na
2
O
61.98
6.98
0.1126
0.2252
0.1126
1.825
a site
¶
K
2
O
94.20
0.68
0.0072
0.0144
0.0072
0.117
0.117
h
2
O
18.02
2.27
0.1260
0.2520
0.1260
2.042
Oh site 2.042
total
100.43
2.9615
24
2.9615
=
×
* Column 1 divided by relative molecular mass.
†
Column 2 × number of cations in oxide molecule. (This column does not appear in Table 8.3 because there all oxide
molecules had only one cation.)
‡
Column 2 × number of oxygens per oxide molecule.
§
Column 2a × 24/2.9615.
#
iv represents Al allocated to 4-fold co-ordinated (tetrahedral) Z sites to make up shortfall in Si. vi represents remaining
Al allocated to 6-fold (octahedral) C sites.
¶
The A-site is only partly occupied (i.e. on the atomic scale, some A-sites are filled while others are empty.)
(b) The ions are distributed between a greater variety
of structural sites (discussed in Box 8.5) than in an
olivine. As a result the agreement between site
occupancies and the ideal formula is less close than
for olivine.
(c) There is insufficient Si to fill the 8 tetrahedral
Z-sites per formula unit. We assume that the
remainder are occupied by Al ions (symbolized
Al
iv
), but most of the Al is left over and gets all-
ocated to the octahedral C sites (where it is denoted
as Al
vi
)
4
in company with Fe
3+
, Fe
2+
, Mg
2+
and Mn
2+
.
(d) Ca
2+
cannot enter the octahedral C sites and must
be allocated to the larger B site, which also accom-
modates Na
+
.
(e) K
+
is too large to enter any but the A site. In the
example given in Table 8.4, it falls a long way short
of filling all of the A-sites available, and in many
amphiboles this site is vacant (a situation indicated
in an amphibole formula by '□').
Formula calculations like these have several important
applications in mineralogy.
• Firstly, they help to confirm the accuracy of an anal-
ysis, as the site totals for a good analysis of a simple
mineral like olivine should approximate to whole
numbers (Table 8.3).
• Knowing what kind of crystallographic site an element
occupies helps in understanding why, for example,
pyroxenes contain no potassium, or why the structure
of augite is different from hypersthene (Box 8.5).
• Formula calculation also plays a part in classifying
mineral groups that involve complicated solid sol-
utions. The nomenclature of the amphiboles, for
example, rests heavily on chemical parameters such
as the
number of silicon atoms per 8 tetrahedral sites
(item (c) above).
The 'vi' here signifies the 6-fold co-ordination of octahedral
sites (see Table 7.1).
4
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