Geology Reference
In-Depth Information
3.4 Mineral Identification
The most informative, yet also the most difficult, part of a field description of
igneous rocks is the identification of the constituent minerals. Igneous rocks
consist of abundant or
major
minerals, together with minor minerals that are
much less abundant. Major and minor minerals may be termed
essential
and
accessory
minerals. Essential minerals are those that are necessary to the naming
of the rock. They are usually major minerals, for example quartz in a granite,
but are sometimes present in minor amounts, for example olivine, in an olivine
basalt. Accessory minerals are those which are present in such small amounts
that they are disregarded in the definition of the rock; for example Fe-Ti oxide
in a gabbro, or biotite in a granite.
Even though most rocks contain only two or three essential minerals, together
with a few accessory minerals,
it is rarely easy to identify all the essential min-
erals and often impossible to name the accessory minerals
. The problem is most
acute for fine-grained mafic rocks in which many of the mafic minerals have a
similar appearance. Even in coarse-grained leucocratic rocks, it can be difficult
to distinguish among the different minerals (for example, between plagioclase
and alkali feldspar). However, as you gain in experience of the properties of
minerals and of the types that commonly occur together, the more confident
your naming of rocks will become. It is worth remembering that most igneous
rocks represent an assemblage of minerals that crystallised almost in chemical
equilibrium with one another from a magma. So, for example the mafic min-
erals olivine and pyroxene
or
biotite and amphibole frequently occur together
but other combinations, such as olivine and biotite, are much less common.
The kinds of combinations that occur most frequently in igneous rocks are
summarised in the mineral-rock classification diagrams in Section 3.5.
In the course of observing the colour and texture of a rock you will have
formed an impression of the approximate number and some of properties of
the individual minerals. The next stage is to identify some of those minerals
with the aid of Table 3.4, which summarises mineral properties under five major
headings: colour, cleavage, lustre, habit and hardness. Generally, it is best to start
with the light-coloured felsic minerals, as these are often most easily identified.
A few points about the use of Table 3.4:
1.
Colour
is an obvious property of a mineral which, like that of a rock
(Section 3.2), reflects its overall chemical composition. Generally,
alkali/alkaline earth silicates (Na, K, Ca silicates) are light-coloured
whereas silicate minerals rich in transition elements, particularly iron, are
dark-coloured.
2.
Cleavage
is the tendency of minerals to split along well-defined planes
that are related to weaknesses in their atomic structures. Cleavage surfaces
are flat planes that reflect light evenly and are often responsible for lustre
