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Ministry of Energy Mines and Responsible for Core Review
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).

specimen of quartz

orthoclase feldspar
Orthoclase

plagioclase feldspar
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).

specimen of biotite mica

specimen of pyroxene

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.

specimen of amphibole

Olivine.bmp (48862 bytes)

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".

Talc: Talc occurs in granular or foliated masses sometimes known as soapstone. It is white to green, sometimes grey or brownish. It is very soft and will be scratched by a finger nail. It has a greasy feel specimen of talc