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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

A standard thin section is highly irregular in detail. The boundaries between adjacent grains are not generally vertical and may be stepped or curved. The top and bottom surfaces of the section are not polished but pitted and grooved to some degree. If there is a difference in the refractive index between mineral and cement, these irregularities concentrate or scatter light by reflection and refraction. The irregularities concentrate or scatter light by reflection and refraction. The effect is to give an impression of three-dimensional relief. If the difference in the refractive index is small, the irregularities will be barely visible and the mineral has low relief. A large difference results in high relief which can be found when viewed under polarized microscope for geologist. Relief should be observed in plane-polarized light with low- or medium-power objective lens, and with the diaphragm below the stage nearly closed. Relief may be improved by lowering the condenser system of the petrographic polarizing microscope used in optical mineralogy. The description of relief is somewhat subjective and the apparent relief depends on both the thickness of the section and the degree of irregularity of its surfaces. Nevertheless, we can usefully make estimates of refractive index using relief. If the mineral has very high relief found under polarized microscope, it is safe to state that its refractive index is higher than that of the Canada balsam. However, if the relief is low or moderate, the refractive index may be higher or lower than that of the Canada balsam. Hence, it is necessary to make a Becke line test.



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Tuesday, December 29th, 2009 at 2:45 pm
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Optical Mineralogy
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Click Here For Best Selection Of High Quality Polarizing Microscope