Examination of Minerals under Polarized-Light-Microscope

First of all, we need to prepare a thin section of mineral or rock with the help of a grinder, and then get it polished from its one side. This slide is then cemented to a slip of glass with Canada balsam (n=1.54). Canada balsam is a y-+ellowish resin obtained from the balsam fir and used for mounting preparations on microscope slides. The mineral chip is then ground down on the other side with successively finer grades of carborundum powder. When a suitable thickness is left (about 0.03 mm) a coverslip is cemented on it. After having the slide the polarizer is set with its plane of polarization parallel to one of the two cross-hairs (usually E-W cross-hair) of the eyepiece. The vibration direction of the analyzer will be normal to that of the polarizer. To examine minerals in the polarized light, the analyzer is removed from the microscope tube. The properties that are commonly examined in polarized light are colour, relief, refractive index, form, cleavage and pleochroism. Some of the important properties are explained in the following paragraphs.

Examination of Mineral under Polarizing Microscope

(1). Relief:

When a mineral grain is brought into focus in the polarized light, it stands out in relief. To study the relief of mineral grain, the sharpness of its outline and the roughness of its surface is observed. The relief of a crystal is a function of its own refractive index and that of the cement.

  1. If the difference between the refractive indices of the mineral and cement layer is less, the crystal will appear flat and featureless with a faint outline. It is then said to have a ”low relief”.
  2. If the difference in the refractive indices of the mineral and the cement is high, the grain outline will appear bold and cracks on its surface will become conspicuous. In this case, the mineral is said to have a ”high relief”.

(2). Refractive Index:

The relief indicates the difference in refractive index between the mineral slide and the cement layer. The refractive index of the grain may be above or below that of the cement. To determine whether the grain is of higher or lower refractive index than the cement, ”Backe Line” test is carried out.

Backe Line Test:¬†In the Becke Line test, a higher power objective is used and the iris diaphragm of the polarizing microscope below the stage is partly closed. A tapering edge of a mineral grain is selected and brought into focus. If now the microscope tube is raised, a narrow line of light, called ”Backe line”, will appear at the grain boundary. The Backe line moves towards the medium of the higher refractive index when the tube is raised. This test is very useful for isotropic minerals, which have only one value of the refractive index.

In order to determine the accurate value of refractive index of a mineral, a set of suitable liquids of known refractive index is required. These liquids are called immersion media”. A mineral grain under examination is immersed in a drop of liquid of known refractive index. Then by the use of Becke line, the refractive index of the minerals and liquid are compared. Thus if the Backe line moves into the mineral grain, a new mount is made by using a liquid of a higher refractive index. This procedure is repeated (with different liquids) till an exact match between the liquid and mineral grain is obtained. In such a case, the refractive index will be equal to the refractive index of that liquid.

(3). Pleochroism:

A crystal is said to be ”Pleochroic” when it, on rotating in polarized light, shows a change in quality or quantity of colour. This change in colour is due to the change in the absorption of light vibrating in different directions of the crystal.

  1. Uniaxial crystals have two vibration directions. Such a crystal shows two different colours for these vibration directions. This property is called dichroism. For example, in tourmaline, the rays vibrating parallel to the length of the crystal are much less absorbed than those vibrating at right angles to the length.
  2. In biaxial crystals, there are three principal vibration directions, X, Y, and Z. Along these vibrations, three distinct colours are observed. This phenomenon is called ”pleochroism”.

Pleochroism or dichroism is described by noting the colour of light transmitted when each principal vibration direction fo a mineral coincides with the plane of the polarizer. For example, in hornblend, the pleochroism is described as follows: X-pale yello, Y-yellow green, and Z-green in the three directions of light vibration.

Related Posts:

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  2. Minerals of Mica Group
  3. Minerals of Olivine Group
  4. Minerals of Garnet Group
  5. Minerals of Chlorite Group
  6. Minerals of Clay Group
  7. Minerals of Serpentine Group
  8. Polymorphs/ Minerals of Al2SiO5
  9. Calcium Minerlas
  10. Aluminium Minerals
  11. Iron Minerals
  12. Manganese Minerals
  13. Copper Minerals
  14. Miscellaneous Ores
  15. Optical Properties of Minerals
  16. Refractive Index of Minerals
  17. Isotropic and An-isotropic minerals
  18. Double Refraction of light in minerals 
  19. Uni-axial Minerals
  20. Bi-axial Minerals
  21. Optical Indicatrix of Minerals
  22. Polarized Light Microscope

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