Estimation of refractive index
The difference between the refractive indices of the medium and the mineral can be estimated from the relief. As the refractive index of the medium is known, we can calculate the difference in the refractive index of the mineral.
Correct lighting is necessary to observe the degree of relief. If you work with maximum lighting (diaphragms at maximum openings) the relief is undervalued. To assess the relief, it is necessary to close the aperture diaphragm (the one located next to the condenser) until a clear drop in light intensity is noted.
Determining the refractive index values is a very important step for mineral identification. This can be simple in the case that the mineral occurs in grains surrounded by an inclusion medium of known refractive index value, but it is much more difficult in the case of minerals included in a thin layer of rock.
In the images that follow, the medium is Canada Balsam with N = 1.54, and it is assumed that all the mineral grains have higher indices than that of the cement.
By pressing the images you can observe mineral grains with refractive indices ranging between the margins indicated.
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