![]() Instead, we look at the thickness of the grain boundaries around the mineral and how the mineral appears to either stand out or blend in with surrounding media or minerals. In practice, it is not necessary to measure the refractive index of the mineral or the surrounding media directly. Refractive index <0.04 different from surrounding media Refractive index 0.04 – 0.12 different from surrounding media Refractive index > 0.12 different from surrounding media For the purposes of identifying minerals in thin section, it is sufficient to describe relief as high, intermediate, or low. In a thin section, the surrounding material will be adjacent minerals. In a grain mount, the surrounding material is likely to be epoxy or mineral oil. If a mineral has a refractive index that is much higher or lower than the surrounding materials, it will stand out relative to the surrounding material and will have thick or distinct. Mineral relief is related to the refractive index of the mineral. For example, the refractive index of kyanite ranges from 1.712-1.734 ( Wikipedia). The refractive index of a mineral can vary according to crystal orientation. Refractive index is discussed in Section 2.3 Light and Optics. The refractive index of a mineral characterizes the relationship between the speed of light in a vacuum and the speed of light in that material. Fayalite (Fe-rich olivine) crystals from Eifel, Germany. ![]() Light green forsterite crystal (Mg-Fe olivine) from Naran-Kagan Valley, Kohistan District, North-West Frontier Province, Pakistan.įigure 2.6.6 C. A colorless forsterite crystal (olivine group) from Wannenköpfe, Ochtendung, Eifel region, Germany.įigure 2.6.6 B. ![]() Eclogite, California, Ward’s collection sample, 40x total magnification. Garnet (pink) and clinopyroxene (green) under plane polarized light. Sometimes the color of a mineral in plane polarized light can aid in identification (see examples in the figures below) because it is characteristic for a specific mineral.īut, be cautious about relying too heavily on color as an identification tool! Minerals within a solid solution group can have very different color characteristics in hand sample (as shown in Figure 2.6.6) and under the microscope. Some minerals that are darkly colored in hand sample will also appear to have color in thin section, even though the rock slices in thin sections are typically only 30 micrometers thick. ![]()
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