Brief notes on the Physical Properties of Minerals

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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, and fracture, specific gravity including special properties like taste, order, feel, tenacity, diaphaneity as well as electrical, magnetic, radioactive properties and reaction with acids.

Form:

Mineral with a definite internal atomic structure without development of well-defined faces is said to be crystalline. U favourable physico-chemical conditions, the outer form with developed crystal faces form. In this case the mineral is said to h crystallised. Rock crystal, garnet, staurolite etc frequently s’ crystallized forms.

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A mineral is said to be cryptocrystalline when degree of crystallization is noticeable under high power microscope, term amorphous is used to describe complete lack of crystallinity. The mineral lacks the outer geometric form the term massive is Depending on the development of outward form and structure; terms are used, which have their usual meaning. These terms are explain.

Color:

Color is a physical property that attracts the first attenti colour of a mineral is determined by looking at a fresh surface in r light. It depends on the combined effect of its composition, atomic structure and nature of impurities present.

These charade the colour absorption and reflection properties of a mineral. A mi blue when it absorbs all the colours of the visible spectrum exce Black colour is due to absorption of all the colours while white col indicative of lack of absorption. Many minerals are identified by characteristic colours.

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The colour of a mineral may differ from i colour by certain extent due to the presence of impurities, play of caused by thin coating on the surface and by inconsistent reflecti refraction of light caused by stained surfaces and cleavage crac mineral is said to be colourless, when it is clear and transparent as in of rock crystal, a variety of quartz. In some instances mineral different colours.

The mineral quartz, which is an oxide of sili commonly colourless or white, but it, is also found in pink, green, and even in black colours. The corundum, which is an oxide of alum, shows pale brown, deep red and dark blue colours. Generally mi containing Al, Na, K, Ba and Mg as their main elements is colourl light coloured while those with Fe, Cr, Mn, Co, Ni, Ti, V and Cu are coloured. The elements, which control the colour of a mineral, are as chromophores.

The colour of a mineral is controlled by the presen a little amount of these elements. The mineral is called idiochromat self-colouring when the elements controlling the selective reflecti certain wavelengths are major constituents of the mineral. Sphalerite example of dichromatic mineral. Its colour changes from white – yellow- Brown – black as its composition changes from pure ZnS to a mixtir ZnS and FeS. Ruby and sapphire are examples of allochromatic varieti corundum. Ruby is deep red due to the presence of small amount o where as Fe and Ti give sapphire a deep blue colour.

Presence of in 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.

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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 show the iridescent colours due to the effect of stain on their surfaces by chemical reaction. Copper pyrites (peacock ore) is a good example showing this effect.

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