Mine

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Essentials of Mineralogy

CHAPTER

4
PHYSICAL PROPERTIES

Both the physical properties of minerals and gemstones and their beauty are dependent on their chemical composition and atomic structure. Physical properties can be useful in distinguishing between both mineral species and individual minerals within a group or series.

Minerals are identified by their distinguishing physicalproperties which are discussed in details in this chapter. The physical properties are colour (which depends on the crystal structure, symmetry, different bondings in the crystals, mode of crystallization, defects/impurities in the mineral structure, absorption capacity of different wavelengths of the light spectrum), lustre (depending on type of atomic bonds, symmetry, refraction, absorptioncapacity of different wavelengths of the light spectrum), crystal habit (symmetry), cleavage (symmetry), parting (symmetry), tenacity (types of bonds and symmetry), hardness (types of bonds, symmetry and packing of atoms), specific gravity (types of bonds, symmetry and packing of atoms), thermal properties (heat flow, atomic bonds), electrical property (types of bonds, surface conditions), magneticproperties (chemical composition, atomic bonding), and radioactivity (chemical composition).

4.1 PROPERTIES CONTROLLED BY SYMMETRY
Colour
Colour is one of the physical properties most commonly used to describe a mineral. Colour is caused due to absorption, or lack of absorption of different wavelengths of light by the mineral/material.

S. Mukherjee, Applied Mineralogy: Applications inIndustry and Environment, DOI 10.1007/978-94-007-1162-4_4, © Capital Publishing Company 2011

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Physical Properties 81

Atoms present in the minerals when exposed to white light, absorb certain wavelengths and emit another wavelength to get rid of the extra energy, provided by the absorbed wavelength. The different types of bonding decides the energy state of the participating electrons andhence produce different colours. Some minerals are nearly always of the same colour like azurite (blue) and sulfur (yellow) and they are known as idiochromatic minerals. Many minerals come in a variety of colours—the changes are caused by slight chemical impurities or through exposure to heat. Example of such minerals are quartz, diamond, beryl, corundum etc. and they are known as allochromaticminerals. Ions of certain elements are highly absorptive of selected wavelengths of light. Such elements are called chromophores; they possess strong pigmenting capabilities. The elements vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu) are chromophores. A mineral whose chemical formula stipulates the presence of one or more of these elements maypossess a vivid and distinctive colour. Colour-change occurs when the surface is exposed to moisture and air—it tarnishes or oxidizes. Colour also varies with the change of oxidation state, for example, beryl (aquamarine) = Fe++, is blue; beryl (heliodor) = Fe+++, is yellow (the oxidation state of impurities present in beryl). Some minerals have common names (varietal names) that describe a specimenwith a certain colour e.g. quartz—rock crystal (colourless), smoky quartz (brown), citrine (yellow), amethyst (violet), rose quartz (pink). Colour can be described as metallic or non-metallic and is often described along with lustre. Colours may also be caused due to the following reasons: Charge transfer can only occur in compounds that have at least two elements in different and variable oxidationstates. Charge transfer can produce very intense colours in gems and minerals. The term charge transfer refers to the process where electrons are swapped between elements. Examples of elements that can participate in charge transfer are: Fe2+ and Fe3+; Ti3+ and Ti4+; Mn2+, Mn3+ and Mn4+ etc. Colour centres are imperfections in crystals that cause colour [defects that cause colour by absorption...