Optical Mineralogy

The light does contain massless particles known as photons. It does travel in the form of waves, and its speed is the highest. It can also travel in vacuum. It is known as the electromagnetic energy as it does possess both the qualities of electricity and magnetism. Light does also contain several wavelengths as gamma rays, x-rays, visible light, micro waves, and radio waves. It can absorb, reflect, refract, and transmit.

The refractive index is the ratio of the velocity of light in the air to that of in the substance. It can be measured by either immersion or becke line method. The becke line, which does appear when the microscopic stage is lowered, would move towards the mineral grain if its refractive index is higher than the oil. The becke lines are narrow banded thick lines of light.

The mineral properties can generally be studied by using the plane polarised light. It is the light which travels in a plane along a particular direction. With the upper polariser, the study of minerals can be done under crossed polars.

The color of minerals is due to absorption or reflection of wavelengths of light. For instance if a mineral does show green color, it indicates that the mineral does reflect green and does absorb all the remaining colors in the visible spectrum. Cleavage is defined as the tendency of a mineral to break along certain crystallographic orientations. They can break in one or two or even six directions. For instance, sphalerite does break along six crystallographic planes. Pleochroism is due to the unequal absorption of light in different directions. Due to this, when the microscopic stage is rotated, several colors can be seen. for instance, biotite does show variations in colors when microscopic stage is rotated. If the sample specimens do not show such variations in colors, the property can be termed as non-pleochroic.

In isotropic or cubic system specimens, the light does travel with equal velocities in all the directions, hence when the polars are crossed, nothing can be seen. The anisotropic minerals do show extinction and interference colors. If a mineral specimen does become dark four times during a complete revolution of the stage, it indicates that it does possess the property of extinction. The extinction angle can be measured. It is the angle between any cleavage direction and the point of becoming dark.

If two light waves are in the same phase, they can produce the interference colors due to increase in the intensity of light. The higher the birefringence, the higher the order of interference colors. The birefringence is the difference between the highest and the lowest refractive index. For some thin sections, there is an axis along which the light does not split into two, such sections are known as the uniaxial minerals. In biaxial minerals the light does split into two directions, for instance, mineral calcite does show double refraction. For the biaxial minerals, the angle between the two axis can also be measured which is known as 2V.

Michael Ortiz

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