guide to thin section microscopy - Mineralogical Society of America

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Guide to Thin Section Microscopy Double refraction Raith, Raase & Reinhardt – February 2012 Figure 4-37. Diagram showing refractive index versus birefringence for rock-forming minerals. Optic class and sign (uniaxial – biaxial; positive – negative), if unknown, must be determined with conoscopic methods. Bold circles mark the optically uniaxial minerals. Glasses and cubic minerals are optically isotropic (∆n = 0). For each anisotropic mineral in a thin section of given thickness, the highest interference colour can be read from the ∆n-d interference colour chart of Raith-Sørensen. Mineral abbreviations after Whitney & Evans (American Mineralogist, 95, 185-187, 2010). 99
Guide to Thin Section Microscopy Extinction 4.2.4 Extinction positions in birefringent crystal sections Vibration direction and extinction character In optically anisotropic crystals, crystal symmetry, and in low-symmetry solid solutions also composition, controls the form and spatial orientation of the indicatrix (Figs. 4-6,7). The regular spatial relationships imply that the orientation of vibration directions with respect to morphological elements (crystal faces, crystal edges, cleavage planes, twin planes) is fixed in birefringent crystal sections. The resulting extinction behaviour allows a distinction between minerals of higher symmetry (hexagonal, trigonal, tetragonal and orthorhombic) and those of lower symmetry (monoclinic, triclinic) (Fig. 4-38). Three general types of extinction can be distinguished: Straight extinction: the vibration directions lie parallel to the morphological reference directions (ε = 0˚). Symmetrical extinction: the vibration directions bisect the angles between two equivalent morphological reference directions (ε 1 = ε 2 ). Inclined extinction: the vibration directions form any angle (ε ≠ 0˚, ≠ 90˚) with morphological reference directions. Raith, Raase & Reinhardt – February 2012 Figure 4-38. Crystal symmetry and extinction behaviour. The orientation of the crystals shown here corresponds to the extinction position. The cross in each figure indicates the two polarizer directions resp. the crosshairs. 100
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