Common Rock-forming Minerals
While rocks consist of aggregates of minerals, minerals
themselves are made up of one or a number of chemical elements with a definite
chemical composition. Minerals cannot be broken down into smaller units with different
chemical compositions in the way that rocks can. More than two thousand three hundred
different types of minerals have been identified. Luckily many are rare and the common
rocks are made up of a relatively small number of minerals.
Identifying the common minerals
Since minerals are the building blocks of rocks, it is
important that you learn to identify the most common varieties. Minerals can be
distinguished using various physical and/or chemical characteristics, but, since chemistry
cannot be determined readily in the field, geologists us the physical properties of
minerals to identify them. These include features such as crystal form, hardness (relative to
a steel blade or you finger nail), colour, lustre, and streak (the colour when
a mineral is ground to a powder). More detailed explanations of these terms and other
aspects of mineral identification may be found in field handbooks or textbooks. Generally
the characteristics listed above can only be determined if the mineral grains are visible
in a rock. Thus the identification
key distinguishes between rocks in which the grains are visible and those in which the
individual mineral components are too small to identify.
The six commonest minerals
The six minerals olivine, quartz, feldspar, mica, pyroxene
and amphibole are the commonest rock-forming minerals and are used as important tools in
classifying rocks, particularly igneous rocks. Except for quartz, all the minerals listed
are actually mineral groups. However, instead of trying to separate all the minerals which
make up a group, which is often not possible in the field, they are dealt with here as a
single mineral with common characteristics.
| Quartz: Quartz is a glassy
looking, transparent or translucent mineral which varies in colour from white and grey to
smoky. When there are individual crystals they are generally clear, while in larger masses
quartz looks more milky white. Quartz is hard - it can easily scratch a steel knife blade.
In many rocks, quartz grains are irregular in shape because crystal faces are rare and
quartz does not have a cleavage
(ie, it does not break on regular flat faces). |
|
Orthoclase
Plagioclase |
Feldspar: Feldspar is the
other common, light-coloured rock-forming mineral. Instead of being glassy like quartz, it
is generally dull to opaque with a porcelain-like appearance. Colour varies from red,
pink, and white (orthoclase) to green, grey and white (plagioclase).
Feldspar is also hard but can be scratched by quartz. Feldspar in igneous rocks forms well
developed crystals which are roughly rectangular in shape, and they cleave or break along
flat faces. The grains, in contrast to quartz, often have straight edges and flat
rectangular faces, some of which meet at right angles. |
| Mica: Mica is easily
distinguished by its characteristic of peeling into many thin flat smooth sheets or
flakes. This is similar to the cleavage in feldspar except that in the case of mica the
cleavage planes are in only one direction and no right angle face joins occur. Mica may be
white and pearly (muscovite) or dark and shiny (biotite). |
|
|
Pyroxene: The most common
pyroxene mineral is augite. Augite is generally dark green to black in colour and
forms short, stubby crystals which, if you look at an end-on section, have square or
rectangular cross-sections. |
| Amphibole: The most common
amphibole is hornblende. Hornblende is quite similar to augite in that both are
dark minerals, however hornblende crystals are generally longer, thinner and shinier than
augite and the mineral cross-sections are diamond-shaped. |
|
|
Olivine: Olivine, or peridot
in the jewellery trade, is yellow-green, translucent and glassy looking. Crystals are not
common; it usually occurs as rounded grains in igneous rocks or as granular masses.
Olivine is almost as hard as quartz; it does not have a well-developed cleavage. |
Quartz and feldspar are light-coloured
minerals; mica, pyroxene, amphibole and olivene are dark-coloured. The colour of a rock
will be determined by the proportions of light and dark-coloured minerals present. If most
of the grains are quartz and feldspar then the overall appearance of the rock will be
light, while the opposite will be true if the minerals are mainly mica, pyroxene,
amphibole or olivine. The colour of a rock with between 25 and 50% dark minerals is
intermediate.
Other common rock-forming minerals
Calcite: Calcite is a very
common mineral in sedimentary rocks. It is commonly white to grey in colour. Individual
crystals are generally clear and transparent. Calcite is softer than quartz and can be
scratched easily by a steel knife blade. In a rock, calcite grains are often irregular to
rhomb-like in shape. Calcite's major distinguishing characteristic though is its vigorous
reaction with dilute hydrochloric acid. Dolomite is very similar to calcite but
does not react well with acid unless powdered first.
Clays: Clay minerals are very
fine grained and difficult to tell apart in the field. They can vary in colour from white
to grey, brown, red, dark green and black. Clays are plastic and often sticky when wet;
they feel smooth when smeared between the fingers.
Magnetite: Magnetite is
common in igneous and metamorphic rocks, and some sediments, though usually in only small
amounts (1 - 2 %). It is black in colour with a metallic lustre, occurring in small
octahedra (like two pyramids stuck together). Easily recognized by its strongly magnetic
character.
Pyrite: The commonest of the
sulphide minerals, i.e. those minerals containing sulphur as a principle component. It
occurs in all rock types, though usually only in small amounts. It is a pale brassy yellow
in colour with a metallic lustre and often forms cube-shaped crystals. Also known as
"fool's gold".
