Geology Reference
In-Depth Information
colour from dark- to light-grey, greenish-grey or brown, though many of the
lighter-coloured rock types also have purple to pink varieties. In simple terms,
this dark to light colour range results from the different combinations of dark and
light minerals that crystallise from magmas of different compositions (further
details in Section 3.5).
After noting the colour of the weathered surface, the next step in classifying
an igneous field specimen is to decide where the fresh surface lies within the
natural colour range and, if possible, to record its
colour index
, based on your
estimate of the total percentage of mafic minerals present (see Figure 3.2),
as follows:
1. Felsic/leucocratic
(0 - 33%) - rich
in
felsic
minerals
and
therefore
light-
coloured,
2. Intermediate/mesocratic (34 - 66%),
3. Mafic/melanocratic (67 - 100%) - rich in mafic minerals and therefore dark
in colour.
The accuracy with which the colour index can be estimated will depend on
the grain size of the specimen. For example, close inspection under a hand
lens of the fresh surface of a coarse-grained rock should give an estimate
within 10% of the true value (for example, Figure 3.3), but for fine-grained
rocks, the colour index becomes more difficult to determine and is obviously
inapplicable to glassy rocks (Section 3.3). In cases where individual crystals can-
not be distinguished with a hand lens, it is better to record an overall impression
of the specimen's colour, thus avoiding any misleading conclusions that could
arise later from a poorly estimated numerical value of colour index.
When working in an area of fairly similar coarse-grained rocks, such as
a granite intrusion where the colour index might vary from, say, 5 to 25%,
50 percent
75 percent
10 percent
25 percent
Figure 3.2
Comparison diagram for estimating the mafic mineral content of
two-dimensional igneous rock surfaces. It is well worthwhile using this diagram
in the field as there is a common tendency, because of the greater visual impact
of dark minerals, for colour index to be overestimated in field reports.
