Geoscience Reference
In-Depth Information
Table 9.1 Classification of igneous rocks
Colour (% dark minerals)
Type
Example
Less than 40
Felsic
Granite
40-70
Intermediate
Diorite
70-90
Mafic
Gabbro
Over 90
Ultramafic
Peridotite
Silica (% SiO 2 )
Type
Example
Over 66
Acid
Rhyolite
52-66
Intermediate
Andesite
45-52
Basic
Basalt
Less than 45
Ultrabasic
Komatiite
Average chemical composition (%)
Granite
Grano-diorite
Diorite
Gabbro
Peridotite
Andesite
Basalt
Komatiite
SiO 2
70.4
66.9
51.9
48.4
43.5
54.9
50.8
44.9
TiO 2
0.4
0.6
1.5
1.3
0.8
1.0
2.0
0.3
Al 2 O 3
14.4
15.7
16.4
16.8
4.0
17.7
14.1
3.1
Fe 2 O 3
1.0
1.3
2.7
2.6
2.5
2.4
2.9
2.3
FeO
1.9
2.6
7.0
7.9
9.8
5.6
9.1
11.5
MnO
0.1
0.1
0.2
0.2
0.2
0.2
0.2
0.2
MgO
0.8
1.6
6.1
8.1
34.0
4.9
6.3
33.0
CaO
2.0
3.6
8.4
11.1
3.5
7.9
10.4
3.8
Na 2 O
3.2
3.8
3.4
2.3
0.6
3.7
2.2
0.01
K 2 O
5.0
3.1
1.3
0.6
0.3
1.1
0.8
0.01
P 2 O 5
0.2
0.2
0.4
0.2
0.1
0.3
0.2
0.03
Note : These are families of rocks and exhibit much variation around the typical; thus, an average is somewhat
meaningless. The table presents only the rough chemical content to be expected.
Source : Chemical compositions are from Nockolds et al .(1978) and Smith and Erlank (1982).
Another good way to classify rocks is by dividing them according to their
chemical content. Rocks rich in SiO 2 such as rhyolite are called acid; intermediate
rocks have rather less SiO 2 ; basic rocks are about 50% SiO 2 or less and include
basalt; ultrabasic rocks are very poor in SiO 2 .
Chemistry can be of further help. Not content with analysing fine-grained
and glassy rocks, petrologists considered what a lava would have turned into if
it had cooled slowly, crystallizing completely. Various schemes were devised to
estimate the minerals that would have formed if a lava of given composition had
crystallized. Of course, these schemes are artificial because so many variables
are involved, but eventually standard ways of recalculating the chemical analysis
were accepted. The standard, or normative , composition can then be used to
classify the rock.
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