Geology Reference
In-Depth Information
Figure 3.3
Three samples of equigranular (equal grain sizes) coarse-grained
igneous rocks with colour indices of 10, 40 and 70. The left specimen is felsic/
leucocratic, that in the middle is intermediate/mesocratic, and that on the right
is mafic/melanocratic.
then it is permitted field practice to use the prefixes
leuco
-,
meso
-and
mela
-in
conjunction with the word 'granite' to describe, respectively, samples with mafic
mineral contents that are below average, average and above average for the
whole intrusion.
Because the colour of an igneous rock reflects the minerals it contains and
hence its chemical composition, it follows that the colour index conveys some
information about the chemical composition of the rock. It will be obvious from
Table 3.1 that the most important chemical constituent of most igneous rocks,
expressed in the conventional oxide form, is silica - SiO
2
. Almost all igneous
rocks contain between 40 and 75% SiO
2
by weight; the higher the silica content,
the more felsic minerals the rock must contain and the lower will be its colour
index - and vice versa for its content of mafic minerals.
There are obvious similarities between this colour index classification and the
much used geochemical classification of igneous rocks into
acid, intermediate
and
basic
categories, using their silica contents. This classification dates from
the period when silicate minerals were classified, erroneously, as salts of hypo-
thetical silicic acids. Although the terms acid, intermediate, basic and ultrabasic
are defined strictly in terms of the abundance of SiO
2
in igneous rock chemical
analyses (Table 3.1), they are also applied loosely in the field; for example most
felsic/leucocratic rocks may be described as acid. Table 3.2 provides a ready
comparison of the different terms used in describing the colour of an igneous
rock; in general, it is best to use those on the right for field descriptions.
Although the implications of colour for an assessment of rock composition
have been described in some detail, clearly there are many other clues to be
considered before assigning field names to igneous rock samples. Therefore, do
not jump to hasty conclusions but, after noting the colour of the weathered and
