NCERT Notes: Geography-Physical characteristics of minerals

 

Subject: Geography 
Category: The Fundamentals of Physical Geography  
Topic: Physical characteristics of minerals

NCERT notes on important topics for the UPSC civil services exam. These notes will also be useful for other competitive exams like banking PO, SSC, state civil services exams and so on. This article talks about the Physical characteristics of minerals.

External crystal form
  • The external crystal form determined by the internal arrangement of the molecules — cubes, octahedrons, hexagonal prisms, etc.
Cleavage
  • It denotes the splitting of a crystal along a smooth plane.
  • Not every mineral exhibits cleavage.
  • A plane of structural weakness along which a mineral is likely to split.
  • The quality of a mineral’s cleavage indicates both the ease with which the mineral cleaves and the character of the exposed surface.
Fracture
  • Fracture takes place when a mineral sample is split in a direction which does not serve as a plane of perfect or distinct cleavage.
  • A mineral fractures when it is fragmented or crushed.
  • Fracture does not result in the occurrence of clearly demarcated planar surfaces.
  • Minerals may fracture in any possible direction.
  • If the internal molecular arrangement is so complex that there are no planes of molecules; the crystal will break in an irregular manner, not along planes of cleavage.
Lustre
  • Minerals may be categorized according to whether they are opaque or transparent.
  • Each mineral has a unique lustre like silky, metallic, glossy, etc.
Colour
  • Some minerals have a characteristic colour determined by their molecular structure.
  • Eg: malachite, azurite, chalcopyrite, etc.
  • Some minerals are coloured by impurities and because of impurities quartz may be white, green, red, yellow, etc.
Streak
  • Streak is the shade of a mineral when it has been crushed to a fine powder.
  • It may be of the same colour as the mineral or may differ.
  • For eg:
    • Malachite is green and gives green streak.
    • Fluorite is purple or green but gives a white streak.
Transparency
  • Transparent: Light rays pass through so that objects can be seen plainly.
  • Translucent: Light rays pass through but will get diffused so that objects cannot be seen.
  • Opaque: Light will not pass at all through the opaque objects.
Structure
  • Structure refers to the particular arrangement of the individual crystals.
Hardness
  • Hardness is defined as the level of difficulty with which a smooth surface of a mineral specimen may be scratched.
  • It is dependent upon the strength of the bonds which compose its crystal structure.
  • Ten minerals are selected to measure the degree of hardness from 1-10. They are:
    • Talc
    • Gypsum
    • Calcite
    • Fluorite
    • Apatite
    • Feldspar
    • Quartz
    • Topaz
    • Corundum
    • Diamond
Specific gravity
  • Specific gravity is a unit-less quantity.
  • It is defined as the ratio between the weight of a given object and the weight of an equal volume of water.

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