DEFINITION OF MINERAL
Mineral is a naturally occurring homogeneous substance with a definite (sometimes variable within certain limits) chemical composition and is commonly characterised by the presence of a typical internal atomic structure with or without the development of external crystalline form. These are usually formed by the inorganic natural processes.
For example, quartz, the common mineral of the earth crust, is generally formed by consolidation of magma and has a fixed chemical composition (SiO₂). It is characterized by its own internal atomic structure, which is expressed by development of hexagonal/trigonal crystalline form. Thus, it is a mineral. For similar reasons, corundum, haematite, magnetite, chromite, bauxite, graphite, calcite, magnesite, orthoclase, microcline, plagioclases, biotite, muscovite, olivine, topaz, talc, granet, beryl, hornblende, augite, sillimanite, kyanite pyrite, chalcopyrite, gypsum, apatite, fluorite etc. are all minerals.
PHYSICAL PROPERTIES OF MINERALS
Each mineral has its own set of physical properties by which it can be identified and distinguished from other minerals. These are form, colour, lustre, streak, hardness, cleavage, fracture, specific gravity including special properties like taste, order, feel, tenacity, diaphaneity as well as electrical, magnetic, radioactive properties and reaction with acids.
Mineral with a definite internal atomic structure without the development of well-defined faces is said to be crystalline. Under favourable physico-chemical conditions, the outer form with well developed crystal faces develops. In this case the mineral is said to be crystallised. Rock crystal, garnet, staurolite etc. frequently show crystallized forms. A mineral is said to be cryptocrystalline when the degree of crystallization is noticeable under high power microscope.
The term amorphous is used to describe complete lack of crystallinity. When the mineral lacks the outer geometric form, the term massive is used. Depending on the development of outward form and structure, certain terms are used, which have their usual meaning. These terms are explained in Table 4.1.
Colour is a physical property that attracts the first attention. The colour of a mineral is determined by looking at a fresh surface in reflected light. It depends on the combined effect of its composition, internal atomic structure and nature of impurities present. These characters affect the colour absorption and reflection properties of a mineral. A mineral is blue when it absorbs all the colours of the visible spectrum except blue. Black colour is due to absorption of all the colours while white colour is indicative of lack of absorption. Many minerals are identified by their characteristic colours.
The colour of a mineral may differ from its true colour by certain extent due to the presence of impurities, play of colours caused by thin coating on the surface and by inconsistent reflection and refraction of light caused by stained surfaces and cleavage cracks. A mineral is said to be colourless, when it is clear and transparent as in case of rock crystal, a variety of quartz. In some instances a mineral shows different colours. The mineral quartz, which is an oxide of silica, is commonly colourless or white, but it is also found in pink, green, brown and even in black colours. The corundum, which is an oxide of aluminium shows pale brown, deep red and dark blue colours.
Generally minerals containing Al, Na, K, Ba and Mg as their main elements are colourless or light coloured while those with Fe, Cr, Mn, Co, Ni, Ti, V and Cu are dark coloured. The elements, which control the colour of a mineral, are known as chromophores. The colour of a mineral is controlled by the presence of a little amount of these elements.
The mineral is called idiochromatic or self-colouring when the elements controlling the selective reflection of certain wavelengths are major constituents of the mineral. Sphalerite is an example of idiochromatic mineral. Its colour changes from white – yellow – brown – black as its composition changes from pure ZnS to a mixture of ZnS and FeS. Ruby and sapphire are examples of allochromatic
varieties of corundum. Ruby is deep red due to the presence of small amount of Cr where as Fe and Ti give sapphire deep blue colour. Presence of minor quantity of Cr imparts green colour to emerald (a variety of beryl). Structural defect is also responsible for variable colour of certain minerals. Purple, smoky and black colours of quartz are due to damage of the crystal structure by radiation to different extents. Oxidation or reduction of certain element like iron and the presence of minute inclusions of other minerals also control the colour of minerals to certain extent. The diagnostic colours of some of the minerals are given in
When viewed from different angles, some minerals show a series of colours, which is known as play of colours. It is shown by diamond and is due to dispersion of white light to its constituent colours. Some varieties of feldspars, when rotated, show a series of colours over broader surfaces. This phenomenon is known as change of colours. Opalescence is milky appearance exhibited by opal and moonstone (a variety of K-feldspar). Iridescence is a display of colours produced due to interference of light rays from minute cracks and fractures. Minerals like quartz, calcite and mica at times show this effect. Some minerals