We know that most rocks are hard; they have different colours-grey, brown, red, black or mixtures of colours. Some can be broken with the hand whereas some break only when they are hammered. Rocks vary in their nature and properties because the mineral they are made up of vary in their properties. There are thousands of minerals present in the earth; they can be segregated on the basis of their physical and chemical properties. Some properties of minerals are discussed here.
Properties of minerals
Minerals have specific colouration in pure state. A mineral called calcite is white in colour, feldspar is whitish pink whereas gypsum is yellowish in colour. Minerals give colour to the rocks. Although their colours change due to various impurities present, nevertheless the colour of a rock can give a clue about the minerals it contains.
If you rub a mineral on the back of a porcelain tile, it will leave a streak. This streak is specific for a particular mineral and always remains the same. Fluorite is pale purple in colour but it leaves a white streak. Gypsum gives a white streak although it is yellowish in colour. The colour of the streak is a reliable clue in identifying a mineral.
Luster is a property which gives a mineral its characteristic appearance. It depends on whether light is absorbed, refracted of reflected by the mineral. Metallic, glassy, luminous and dull are different kinds of luster. Table 5.5 shows the luster possessed by different minerals.
Crystals have regular arrangement of atoms and molecules. Most minerals are crystalline but as these crystals rub against each other their shapes change. When melted lava pours out from the mouth of a volcano it may flow down the mountainside and harden into a rock that does not contain a crystal.
Talc is the softest mineral and diamond is the hardest. German scientist Friedrich Mohs, devised a scale of hardness which is varied from 1 to 10. A mineral can scratch another mineral softer than it. Table 5.3 shows minerals placed according to their increasing order of hardness.
The specific gravity of an object is the ratio of the mass of the object to the mass of an equal volume of water. Metals have a higher specific gravity than non-metals. Therefore specific gravities of metallic and non-metallic minerals can be easily demarked.
As minerals are inorganic substances containing metals or non-metals and groups like sulphides, carbonates, etc, they can be identified by chemical tests. For example, a mineral called calcite will give effervescence when it reacts with an acid. This because calcite is basically calcium carbonate and the carbon dioxide liberated gives off effervescence.
Another example is galena, a sulphide ore. When it reacts with acid, hydrogen sulphide gas is produced which smells like rotten eggs.