Background Information:
Properties of Minerals Minerals can be made up of many chemicals found naturally in the Earth’s crust. Some minerals contain only one or two chemical elements, while others contain many additional compounds. The differences in the chemical composition and of the crystalline structure of minerals means that different minerals all display different properties.
Colour, lustre and streak These properties are all very easily observed with the naked eye. Colour is the most obvious thing to note about individual minerals. The colour is directly linked to the chemicals that make the mineral. If a mineral contains many impurities, these can affect the colour. Quartz is naturally colourless, but varieties are known to be pink, blue, purple and black – according to the impurities. Lustre is another property that can be observed easily with the naked eye. Lustre means the way the mineral reflects light. Quartz is said to have glassy (or vitreous) lustre. Streak is also known as ‘powder colour’. When you draw the mineral across an unglazed white tile, it will leave a coloured streak of the powder. Often this will be a different colour to the visible colour of the mineral. Pyrite has yellow-gold crystals but displays a dark greenish-black streak.
Crystal Properties ‘Crystal habit’ describes the way that well-formed crystals grow naturally. For example, quartz is said to have a hexagonal habit, and pyrite has a cubic habit. The ‘cleavage’ of a mineral refers to how it breaks. It is related to the crystal structure of the mineral. If a mineral breaks in a regular way to give smooth surfaces, then it is said to have cleavage, and those surfaces are called cleavage planes. Some minerals do not display any cleavage, but instead show fracture surfaces. Different minerals can have different types of fracture. Twinning is related to the crystal structure of the mineral. Twinning occurs when a mineral repeatedly changes the direction in which it is growing. In large crystals, twinning results in crystals growing out of each other.
Hardness and Density In geology, hardness is the ability of one mineral to scratch another. The Moh’s hardness scale rates minerals from 1 to 10, with 1 being the softest, and 10 the hardest. The density of minerals can be measured with special equipment in the laboratory. A pure sample of the mineral must first be weighed, and then submerged in a displacement tank to determine the volume, and density can be calculated using the following mathematic equation which will give a mass density with units of Kg/m3:
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Special Light Effects. In special geological laboratories, it is possible to examine thin sections of rocks under a microscope. When looking at rocks so close up, it is possible to closely examine the minerals in the rock. These microscopes often have a function to view such slides using polarised light. Many minerals demonstrate a variety of properties under these conditions, and this is a vital tool for mineral identification and study for geologists.