Geology Reference
In-Depth Information
2. Terms applied to minerals present in the
norm
of an igneous rock and to rocks
having one or more of these minerals as major components of the norm:
d.
salic
=
s
ilicon
+
al
uminium
(for
example,
quartz,
feldspars,
feldspathoids),
e.
femic
=
fe
rric
+
m
agnesium (for example, olivine, pyroxenes).
Although most of your observations will relate to fresh surfaces it is usually
worth noting the colour of the weathered skin of an igneous rock, particularly if
it is fine-grained. Since the chemical process of weathering tends to concentrate
medium- to dark-brown insoluble residues in rocks that contain a high proportion
of mafic minerals (Table 3.1), or pale-brown to white residues if most of the
minerals are felsic, the colour of the weathered surface provides a first clue
to the composition of the rock. Fresh surfaces of most igneous rocks range in
Table 3.1
Typical chemical compositions
1
for some major silicate minerals in
igneous rocks (weight per cent). See Table 3.4 for distinguishing features in
hand specimens of these and other minerals.
Mineral
SiO
2
Al
2
O
3
FeO
+
MgO
CaO
Na
2
OK
2
OH
2
O
Fe
2
O
3
Felsic minerals
Quartz
100
-
-
-
-
-
-
-
Orthoclase
65
18
-
-
-
-
17
-
Albite
69
19
-
-
-
12
-
-
Anorthite
43
37
-
-
20
-
-
-
Muscovite
45
38
-
-
-
-
12
5
Nepheline
42
36
-
-
-
22
-
-
Mafic minerals
Olivine
40
-
15
45
-
-
-
-
Clinopyroxene (Augite)
52
3
10
16
19
-
-
-
Orthopyroxene (Fe rich)
50
1
30
16
1
-
-
-
Amphibole (Hornblende)
42
10
21
12
11
1
1
2
Biotite
40
11
16
18
-
-
11
4
1
Silicate minerals are made up of atomic frameworks in which different combinations of
cation-forming elements are always bonded to oxygen and so it is customary to quote analyses
in terms of simple oxides rather than elements.
Note also that in most mineral groups there is compositional diversity caused by the ability
of atomic frameworks to hold different cations in the same site (e.g. substitution between Fe
and Mg, or between Na and Ca occurs in many mineral groups).
