Sign of elongation

In a thin section, the an-isotropic minerals contain two vibration directions, one ”fast” and the other “slow”. To identify these directions, the mineral section is turned from a position of extinction through 45°. Then through the slot in the polarizing microscope tube, the gypsum plate is inserted.

1. If the X-direction of the gypsum plate and the mineral coincide, the interference colour will rise in its order.
2. If the optical direction of the two are opposed to each other, the interference colour will fall.

Hexagonal and tetragonal crystals are frequently elongated on the c-axis. If such an orientation is known, the optical sign of the elongated grain can be determined. The sign of elongation is said to be ”positive”, when the slow vibration direction is parallel to the direction of elongation of a mineral grain. It is said to be “negative” when the fast direction coincide with the length of grain.

Relevant posts:

1. Optical Properties of Minerals
2. Refractive Index of Minerals
3. Isotropic and An-isotropic minerals
4. Double Refraction of light in minerals 
5. Uni-axial Minerals
6. Bi-axial Minerals
7. Optical Indicatrix of Minerals
8. Polarized Light Microscope
9. Examination of Minerals under Polarized Light Microscope
10. Examination of minerals in cross polar
11. Extinction in minerals and Types of extinction
12. Accessory plates used with a polarizing microscope