Physical Properties of Minerals


Introduction:

A mineral can be defined as a substance which is in the form of a solid and have a definite crystalline structure. It is also defined as a naturally occurring homogenous substance with a definite chemical composition. However according to international mineralogical association in the year 1995 have given a new definition as, “a mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological process”. Every mineral has its own atomic structure. Structure and composition are the defining marks of minerals and the study of the minerals is called as mineralogy.

The minerals can be only identified by X – ray analysis and chemical tests. The X-ray analysis determines the structure of the mineral and the chemical test determines the composition of the mineral. Unfortunately, these tests require expensive equipment. Fortunately both structure and composition affect certain physical properties. It is through the proper use of these properties that mineral can reliably identified.


Physical properties:

The physical properties of minerals are used by mineralogists to help determine the identification of a specimen. Physical properties are referred to as observables. Some of the tests can be performed easily in the field to identify the mineral, while the other requires laboratory equipment. For a beginner of geology there are a number of simple tests that can be used with a good degree of accuracy. The list of tests is in a suggested order, progressing from simple experimentation and observation to more complicate either in procedure or in concept. The following physical properties of minerals can be easily used to identify a mineral.

Colour:
Minerals have distinctive colour that can be used for identification. Colour indicates the appearance of the mineral in reflected light or transmitted light. Most of the minerals have a distinctive colour that can be used for identification. In opaque minerals, the colour is more consistent. So knowing the colours associated with these minerals can be very helpful in identification. Translucent to transparent minerals have a much more varied degree of colour due to the presence of trace minerals. Therefore, colour alone is not a single identifying characteristic.

Streak:
Streak is the colour of the mineral in a powdered form. Streak shows the true colour of the mineral. In large solid form trace minerals can change the colour appearance of a mineral by reflecting the light. Trace minerals have little influence on reflection of the small powdery particles of the streak. The streak of metallic minerals appears as black because small particles of the streak absorb the light that hit them. Non-metallic particles tend to reflect most of the light so they appear as light in colour. As streak is a more accurate illustration of the mineral’s colour, it is a reliable property of minerals than colour for identification.

Cleavage:
Minerals tend to break along lines or smooth surfaces when hit sharply. Different minerals break in different ways showing different types of cleavage. Cleavage describes the way a mineral may split apart along various planes. In thin sections cleavage is visible as thin parallel lines across a mineral.

Parting:
In a crystal structure parting is also a plane of weakness. It always occurs along the planes that are weakened by some applied force. For this reason it cannot appear in all specimens, but it may appear if the same mineral has been subjected to the right stress conditions.

Fracture:
If the minerals contain no planes of weakness, it will break along random directions called fracture. Different kinds of fracture patterns are as follows: · Hackley fracture is having rough and uneven sharp edges. · Chonchoidal fracture is a smooth curved fracture with concentric ridges of the type shown by glass. · Irregular. · Fibrous.

Luster:
The general appearance of mineral surface to a reflected light is called as Luster.
The types of luster are as follows:
  • Metallic – It looks shiny like a metal. It usually gives a black coloured streak.
  • Non-metallic – These lusters can be classified as:
Ø Vitreous – It looks as glassy. Examples: quartz, tournaline.
Ø Greasy – It appears to be covered with a thin layer of oil. Example: Naphthene
Ø Silky – It looks as fibrous. Example: gypsum, serpentine.
Ø Pearly – It looks as pearl. Example: appohyllite.

Transparency:
The degree of ability to allow light to pass through the mineral is called as Transparency. The degree of transparency depends on the thickness of the mineral.

Density:
Density of any material is defined as mass per unit volume (in gm/cm3). Specific gravity is the measure of density of a mineral. It is measured using various apparatus and techniques which are covered in most mineralogy textbooks.
These are:

  1. Jolly Balance
  2. Beam Balance
  3. Pycnometer
  4. Heavy Liquids
Among the above the best methods for the measurement of density are Jolly and Beam balance. The pycnometer and heavy liquids are best suited for the density measurement of fine or very small particles.

Specific gravity:
It is the ratio of the weight of mineral to the weight of the equal amount of water. The weight of equal amount of water can be calculated by finding the difference between the weight of the mineral in air and the weight of the mineral in water.

Taste:
Soluble minerals can give taste but they are not supposed to put it into mouth or on the tongue. One cannot test this property in the classroom.

Odour:
Most of the minerals are odourless. They can give some smell when they are moistured, heated, rubbed or breathed upon.

Crystal Structure:
The particular shape is determined by the arrangement of atoms, molecules or ions that make up crystal and how they are joined is called as the crystal structure. This property can be highly diagnostic. Mineral crystals occur in various shapes and sizes. There are degrees of crystalline structure somehow difficult to see with naked eye or by using the hand lens. By using high magnification micro crystalline and crypto crystalline structures can be viewed. If there is no crystalline structure, it is called as amorphous. In nature perfect crystals are rare. The faces that develop on a crystal depend on the space available for the crystals to grow. If crystals grow into one another or in a restricted environment, it is possible that no well-formed crystal faces will be developed. However, crystals sometimes develop certain forms more commonly than others, although the symmetry may not be readily apparent from these common forms. The term used to describe general shape of a crystal is habit.

Some common crystal habits are as follows:
Individual Crystals.

  • Cubic - cube shapes.
  • Octahedral - shaped like octahedrons.
  • Tabular - rectangular shapes.
  • Equant - a term used to describe minerals that have all of their boundaries of approximately equal length.
  • Acicular - long, slender crystals.
  • Prismatic - abundance of prism faces.

Hardness:

Hardness is one of the best properties of minerals to use for identifying a mineral. Hardness is a measure of the mineral’s resistance to scratching. Hardness is the most important diagnostic feature of the mineral. It reflects the strength of the chemical bonds between atoms and depends on the type of the chemical bonds. For minerals with an ionic type of chemical bond, a low degree of hardness is a characteristic. While the minerals with a purely covalent bond are very hard. For this Halite and Diamond are the best examples. Minerals with a mixed type chemical bonds usually have lowered hardness like hydrogen and Vanderwalls bonds. There are several methods of determining hardness. In mineralogical practise Mhos scale is employed. Mhos scale is a set of ten minerals whose hardness is known. The following minerals are listed according to the degree of hardness based on the standards of Mhos scale of hardness:

  • Talc Mg3Si4O10(OH)2
  • Gypsum CaSO4•2H2O
  • Calcite CaCO3
  • Fluorite CaF2
  • Apatite Ca5(PO4)3(OH,Cl,F)
  • Orthoclase KAlSi3O8
  • Quartz SiO2
  • Topaz Al2SiO4(OH,F)2
  • Corundum Al2O3
  • Diamond C (pure carbon)

Tenacity:
The resistance of the mineral to breaking, bending or crushing is called is called as Tenacity. Tenacity can be described by the following terms.

  • Malleable – It can be hammered into thin sheets.
  • Ductile – It bends easily and return to its original shape.
  • Flexible – It bends somewhat and doesn’t return to its original shape.
  • Elastic – It bends but doesn’t return to its original shape.
  • Brittle – It breaks or powders easily.
Electrical properties:
The electrical properties applicable to mineral are

1.Conduction
2.Pyroelectricity
3.Piezoelectricity
4.Polarisation

1.Conduction:
It is the ability of a mineral to conduct electricity. Only very small number of minerals are good conductors. These are metallic elements and the minerals graphite, chalcopyrite, pyrhotite. These conductors can be placed between a wire carrying electricity, and the electricity pass through it. Conduction is an important property that can distinguish true metals from metallic sulphites and oxides

2.Pyroelectricity:
Pyroelectricity is where a mineral develops electrical charges when exposed to temperature changes. Some minerals develop an electrical charge when heated, others when cooled.

3.Piezoelectricity:
Piezoelectricity is where a mineral develops electrical charges when put under stress. Piezoelectric minerals will develop charges when rubbed or struck repeatedly. These properties have been discovered with crystals of quartz, tourmaline, calamine, boracite and others.

4.Polarisation:
It is connected with a shift if positive and negative charges in different directions under the action of electrical field. The polarisation occurs in minerals-dielectrics and is not accompanied by electron conductivity. Polarisation can be electronic, ionic, orientational. Polarisation is measured in terms of dielectrical permeability.

Magnetic properties:

There are several minerals in this nature can react when placed within a magnetic field. Some minerals can be strongly attracted to the magnet, and others are weakly attracted, and some minerals are repelled. There are also several minerals that attracted to magnetic field only when heated. A magnetic field is an area encompassing a magnet or electrical current that has the ability to attract or repel certain objects placed in the field. The closer the object is to the magnet or electrical current, the more powerful the magnetic effect. The presence of iron in a mineral is responsible for the magnetic properties of a minerals is virtually in all cases. Magnetic properties of minerals are difined as follows

  • Ferromagnetic: It is a property that only a few minerals exhibits strong attraction to magnetic field. Examples: Magnetite, Pyrrhotite, Hematite-Ilmenite solid solutions (Fe2O3-FeTiO3).
  • Paramagnetic: It is a property that the minerals exhibits weak attraction to magnetic field, which is undetectable. Most paramagnetic minerals become strongly magnetic when heated. A small number of paramagnetic minerals such as platinum, are not essentially paramagnetic, but contain iron impurities which are responsible for the paramagnetism. However there are some specimens which are lacking of iron also exists, and these are not paramagnetic. Example: Haematite.
  • Diamagnetic: It is a property which is repelled by a magnetic field. Example : Bismuth
Other properties:

Iridescence:
It results from diffracted and reflected light from closely spaced fractures, cleavage planes.

Asterism:
Its a star like figure appearing on the surface of the mineral caused by reflection or scattering of light off needle inclusions of minerals which parallel crystallographic axes.

Luminescence:
Its a special glowing of the mineral caused by the impurities in a mineral.

Fluorescence:
It is emission of light wavelength caused by electron excitation in transition metals during bombardment of a mineral with ultraviolet or X-rays.

Radioactivity:
Radioactivity is the transformation of unstable isotopes of one chemical element into the other elements with the emission of radioactive particles. Radioactivity is studied by means of a special instrument such as radiometer.

Thermo luminescence:
If the mineral is subjected to heat below the red hot temperature it gives emissions of light and wavelength. This property is known as thermo luminescence.

QUESTIONS AND ANSWERS:

1.What is meant by a mineral?
Ans: A mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological process.

2.What are the different physical properties that can be useful for identifying a mineral?
Ans: Colour, crystal system, streak, cleavage, hardness, luster etc.

3.What are the types of electrical properties?
Ans: Conductivity, pyroelecticity, piezoelectricity, polarisation.

4.Which minerals exists ferromagnetic property?
Ans: Magnetite, ilmenite, pyrhotite.

5.What are the different magnetic properties of a mineral?
Ans: Ferromagnetic, paramagnetic, ferrimagnetic, anti ferromagnetic and diamagnetic

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