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| Sunday, 13 March 2005 |
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Junior Observer |  |
| News Business Features Editorial
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Minerals rare and common
We use a wide variety of minerals in our day-to-day lives without even knowing it. A mineral is any substance found in the earth's crust that does not come from a living thing. Minerals are usually divided into two groups, metallic and non-metallic. Examples of metallic minerals are hematite (iron), bornite (copper) and bauxite (aluminium). The non-metallic minerals include quartz, asbestos and calcite. Minerals are usually found in forms called crystals.It is easy to confuse minerals with rocks, but there is a difference. A mineral has a definite composition. The chemical make-up of a mineral is almost always the same, no matter where it is. So the same mineral found in any part of the world will have the same lustre, hardness and other properties. Rocks, however, are made up of minerals like quartz, feldspar and mica. But a piece of granite found in one place might have different amounts of these minerals from a piece of granite in another place.
Minerals are found in many places. They may be found in rocks or in sand and gravel. And they are formed in many different ways. Many minerals crystallised or hardened years ago from a very hot, melted mass of rock called magma. Diamond, mica and feldspar are examples of minerals that formed from molten magma. Certain minerals are very valuable., mainly because of their beauty and scarcity. These are gems. Diamonds, garnet, topaz and zircon are some valuable gem minerals. Some minerals, on the other hand, are found almost everywhere. The most common mineral is quartz. There are about 200 different kinds of quartz, and they are found in almost every part of the earth. How to identify minerals Most common minerals can be identified by inspecting or testing their physical properties. These properties are colour, streak, transparency, lustre, hardness, cleavage (splits), fracture, specific gravity, and crystal form. Colour Usually, we first notice the colour of a mineral. Some minerals are easily identified by colour because they are never any other colour. For example, malachite is always green. Keep in mind, however, that colour by itself isn't enough to identify a mineral. Chemical impurities can change the colour of a mineral without changing its basic make-up. For example, quartz in its purest form is colourless and clear as glass. Quartz with traces of iron becomes violet (amethyst). With traces of manganese, it turns pink (rose quartz). If quartz is exposed to radiation, it turns brown (smoky quartz). Streak
When a mineral is rubbed firmly across an unglazed tile of white porcelain (a streak plate), it leaves a line of powder. This is called the streak. The colour of the streak is always the same, whether the mineral has impurities or not. For example, quartz leaves a white streak, whether it's violet (amethyst), pink (rose quartz) or brown (smoky quartz). Transparency Transparency describes how well light passes through a mineral sample. There are three degrees of transparency: transparent, translucent and opaque. You can see objects through a transparent mineral. You can see light, but not objects through a translucent mineral. You can't see anything through an opaque mineral. Lustre Lustre is the way the surface of a mineral reflects light. Lustre should be observed on a cut or freshly broken, untarnished surface. There are two general types of lustre - metallic and non-metallic. Hardness The hardness scale was established by the German mineralogist, Friedrich Mohs. The Mohs' hardness scale places ten common or well-known minerals on a scale from one to ten. One is the softest mineral and ten is the hardest. Cleavage When a mineral sample is broken with a hammer, it breaks along planes of weakness that are part of its crystalline structure. These breaks are cleavages. Some minerals break only in one direction. Others break in two or more directions. Fracture
Not all minerals cleave easily. Some fracture instead. Unlike cleavages, which are usually clean and flat breaks, fractures can be smoothly curved, irregular, jagged or splintery. The most common types of fracture are conchoidal (quartz) , fibrous or splintery, hackly (copper), uneven or irregular. Specific gravity Specific gravity is the density of a mineral. Special equipment is usually needed to find out a mineral's exact specific gravity. With a little practice, you can guess a mineral's specific gravity by hand. Some mineral samples will feel heavier than others, even if all your samples are the same size. The heavier ones have a greater specific gravity. Fact File * Geologists have classified nearly 3,000 minerals. * Of the 92 elements (basic chemical substances) that occur naturally in the Earth's crust, 22, including gold and silver are sometimes found in a pure state. The others occur in combinations with one or more elements. * Talc or soapstone feels soapy to the touch. * Gypsum is used to make Plaster of Paris. * Halite or rock salt tastes salty. * Radioactive minerals are located with the help of a Geiger counter. |
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