Characteristics of Rock Forming Minerals

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Rocks and Minerals

All rocks are composed of minerals. A mineral is a naturally occuring chemical compound having a definite composition and crystalline structure. The rock forming minerals are those that form most of the rocks. There are other minerals, such as the metallic minerals, which are important for their economic value. Most rock forming minerals are silicates, the main exception being calcite, which forms the sedimentary rock limestone ad the metamorphic rock marble.

Characteristics of Rock Forming Minerals:

Minerals have physical characteristics that can help geologists to recognise them.

Habit

The habit, or shape of some mineral crystals, can be very distinctive. Cubic crystals such as halite and fluorite are easy to recognise. Garnet is a complex shape based on a cube forming a dodecahedron. Quartz has a hexagonal shape. Some mineral crystals grow in pairs and are said to show twinning. This can be a feature of feldspars.

Colour

Colour can be used to identify some minerals but is not very reliable as some minerals can occur in different colours. Colour is used to distinguish between the two micas - muscovite is pale grey and biotite is black. Quartz can be purple, pink, grey, yellow, white or clear - so colour is not diagnostic. Streak is the colour of the powder of a mineral. Most silicates have a white streak. Galena is a lead grey colour and the streak is the same colour, while iron pyrite is brassy yellow, but the streak is black.

Hardness

The hardness can be measured on Mohs' scale of hardness. It measures resistance to scratching. Hard minerals scratch softer ones.

Cleavage and Fracture

Some minerals have cleavage planes, which are planes of weakness in their atomic structure. Cleavage may be termed perfect, good or poor depending on how easily it splits along a cleavage plane. In minerals like muscovite, cleavage is perfect in one direction so the mineral splits into thin sheets. There may be more than one direction of cleavage and the angles made between cleavage planes can also help in identification. Calcite has cleavage in three directions not at right angles.

Minerals that break along an irregular surface tend to fracture. They do not have cleavage because the bonds between the atoms are strong. This is fracture. Sometimes the fracture is a series of concentric curved cracks, rather like a broken glass bottle, which is called conchoidal fracture. A mineral with cleavage planes may also fracture if it breaks along a direction that is not a cleavage plane.

Lustre

The surface appearance of a mineral is described as its lustre and depends on its ability to relect light. Minerals that are shiny like metal have metallic lustre. Minerals that are dull are described as earthy. Most rock forming minerals have vitreous lustre, like glass.

Specific Gravity

Specific gravity is the ratio of the mass of a mineral compared with the mass of an equal volume of water. Because it is a ratio it has no units. Quartz has a specific gravity of 2.65, meaning that the mass of a volume of quartz is 2.65 times the mass of the same volume of water. Pale coloured minerals have S.G. of around 2.7; barite has S.G. 4.5; mafic minerals S.G. 3.3 and metallic minerals S.G. of 5.0 and over.

Reaction with Acid

Calcite reacts strongly with dilute HCl. This is a very good way to distinguish it from other pale coloured vitreous minerals.

Talc - scratched by fingernail

Gypsum - scratched by fingernail

Calcite - Scratched with coin

Fluorite - scratched by steel nail

Apatite - scratched by steel nail

Feldspar - just scratched by steel nail

Quartz - scratch glass

Topaz - scratch glass

Corundum - scratch glass

Diamond - scratch glass

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Mohs' Scale