Minerals are the naturally occurring inorganic substances having ordered atomic structure and characteristic chemical composition, physical properties and crystal form. Various natural resources are present on the earth and they are very useful to humankind. The branch of geology studying about the minerals is known as mineralogy.
Minerals are formed by various external and internal processes like:
The mineral deposits can be formed because of the weathering of rock by different chemically active agents. The water dissolves and removes most soluble metal ions from rocks and soil near the surface. Such weathering process leaves ions of insoluble compounds in the soil to form residual deposits of metal ores such as iron and aluminium (bauxite ore).
As the sediments settle, they can form a natural accumulation of minerals by sedimentary sorting and precipitation. The flow of streams carry silt, sand, gravel and occasional small grains of gold and when the current slows down these minerals settle down on the basis of density. Some important minerals formed in this way include borax, salt and sodium carbonate.
Magmatism is the process by which there is the formation of the magma, upwelling of the magma and solidification of the magma. As the magma cools, it crystallizes into various layers of minerals containing igneous rocks. During the cooling of the magma, the early formed heavier mineral gets concentrated.
The process of plate tectonics shapes the earth’s crust as the earth’s plate collide, retreat and slide across one another at the boundaries between them and also determines where the earth’s richest mineral deposits form. This happens when the movement of the earth’s plates allow the magma to flow up onto the earth’s crust of divergent and convergent plate boundaries.
One of the most common ways of mineral resource formation is through hydrothermal processes. When two tectonic plate retreat from one another, gaps created in the earth’s crust fill with the upwelling magma and seawater. The sea water seeping into these cracks becomes super heated and dissolves metals from rock or magma. As these metal bearing solutions cool, their dissolved minerals cool and form hydrothermal mineral deposits.
1. Crystal form
The bodies that are surrounded by surfaces, usually flat, arranged on a definite plan which is an expression of the internal arrangements of the atoms are known as crystals. They are formed by the solidification of the minerals from the gases or liquid states or from the solutions.
It is used to denote the characteristic shape of the crystals arising from the variations in the number, size and shape of the face. The habit or appearance of a single crystal or aggregates of crystals helps to identify them. They may be cubic, tubular, columnar, granular, massive etc.
Some of the terms used to express habit and state of aggregation are given below:
Acicular : Needle-like crystals. Example: Selenite
Capillary and filiform : Hair like or thread like structure. Example: millerite
Dendritic : Somewhat plant-like appearance. Example: romanechite
Foliated : Separation of plants or leaves. Example: chlorite
Granular : Composed of grains. Example: halite
Reniform : kidney shaped aggregate. Example: haematite
Bladed : Elongated crystals flatted like a knife blade. Example: kyanite
It is the tendency of the minerals to break down in particular direction offering plane and smooth surface. The presence of these planes indicate the differences in strength of bonds between the atoms in the crystal. Cleavage is intimately connected with the atomic structure of minerals. Cleavage may be very well in number and may be unidirectional or many as six directions. Some important types of cleavage are:
Pinacoidal – kyanite
Pinacoidal – Gypsum
Pinacoidal – Miscas
The resistance that a smooth surface of mineral offers to scratch is known as hardness. It is the ability if the mineral against the scratching. The hardness of the mineral is related to its atomic structure. Mineral having less interionic distance are harder. A series of ten common minerals were chosen by Austrian Mineralogist F Mohns in 1824.
Color is the visual appearance of minerals. Some minerals show distinct colors which help to identify them but the color only is not true measure to identify them because weathering and other processes may change the color of the minerals. Some minerals with their distinctive colors are:
White : Calcite, magnetite etc.
Yellow : Sulphur, Chalcopyrite etc.
Red : Zinc, Cinnabar etc.
Lead grey : Galena, Granite etc.
The color of the mineral is powdered form. The streak is generally observed by rubbing the mineral on streak plate whose hardness is about 7. The streak of some minerals is not same as like that of its color. Example: black hematite gives red color.
Luster is the appearance of the minerals in reflected light.
Metallic luster: Galena, pyrite, chalcopyrite etc.
The luster of minerals may be different degrees of intensity according to an amount of reflected light from their surface.
Bend but do not get an original shape. Example: talc
Elastic : regain its original shape. Example: mica
10. Specific gravity
It depends on the chemical composition and crystal structure of the minerals. It may be defined in terms of high, medium or low value.
The resistance that a mineral offer to mechanical deformation is called tenacity.
In general mineral resources can be categorized as:
Minerals which provides the metals after processing are called metallic minerals. E.g. Iron, silver, copper, gold, chromium, nickel etc. Metallic mineral resources are chiefly formed by the processes of magmatism and process of weathering and erosion.
2. Non-metallic minerals
Minerals which yield products and other metals such as phosphorus, rocks, clay, salt, coal, diamond, granite, graphite etc. The non-metallic mineral lacks metallic shining and has high melting point.
3. Construction minerals
Rocks, stones, and soils are being used as the construction materials. Some examples are building stones like sandstone, quartz, limestone, facing stones and roofing stone like granite, marble, basalt etc.
4. Chemical minerals
Some mineral resources are used in the manufacture of different commodities by the chemical fertilizer.
Sulphur : Used in the manufacture of H2SO4 insecticides, paints, soaps etc.
Magnesite and talc : Used in the manufacture of paper, face powders, creams etc.