Geology Reference
In-Depth Information
3.
Mineralogy
- usually between two and four main minerals occur in an
igneous rock. For coarse-grained rocks, the number and nature of the main
minerals present, their relative abundances and inter-relationships provide a
clear guide to rock identification. Many fine-grained rocks also contain some
large crystals that may be useful for diagnostic purposes. In many cases of
fine-grained rock identification, it is difficult to make full use of features
1 - 3 and to record in the field a description, that is more detailed than say,
'fine-grained grey lava'. In addition, but not part of the rock identification,
there are inclusions of 'foreign' material. These may be found in samples
and should be described separately, as they are important when assessing
the mode of occurrence and origin of the igneous rock body.
4.
Weathering
- although the diagnostic features of igneous rocks are often
less easily recognised from weathered than from freshly broken surfaces, the
opposite is sometimes true for coarse-grained mafic rocks. It is important
to note the amount of weathering and to explore both weathered and fresh
outcrop as they often reveal different things about the rocks texture. Bear
in mind, however, that the effects of weathering may penetrate below the
surface to a depth varying from a few millimetres in fine-grained rocks to
as much as several metres in the case of coarse-grained or heavily-fissured
material and in the case of poorly consolidated pyroclastic rocks. (These
depths may be increased by at least a factor of 10 in areas which have, or
are presently experiencing, wet, tropical climates.)
3.2 Colour and Composition
The basic composition of an igneous rock is reflected in the mineralogy which
crystallises from it. It is important, therefore, to remember some key things
with regard to understanding the types of minerals that we expect to see in
igneous rocks of different compositions, as these reflect known crystallisation
sequences. Additionally, it can be helpful in determining likely mineral species
where minerals which are easy to identify are found with minerals that are not
clear or difficult to identify, or when you find minerals that should not be in
sequence with each other which may represent magma mixing/crystals in dise-
quilibrium with melt compositions, and so on. The most important conceptual
diagram which will help understand crystallisation sequences is Bowens Reac-
tion Series (Figure 3.1), based on experimental melting experiments of different
compositions of igneous material. Importantly it includes the major rock types
from mafic through intermediate to felsic (acidic) rocks, it also indicates the
likely temperatures that different compositions have been at.
Once you have an understanding of what minerals crystallise from what
magmas types, we can use this information to get a good understanding of the
