guide to thin section microscopy - Mineralogical Society of America

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Guide to Thin Section Microscopy Optical properties: basic principles 4.1.2 Isotropy and anisotropy Isotropic materials Optically isotropic substances such as gases, liquids, glasses and the highly symmetric cubic minerals show optical behaviour that is independent of the direction of light propagation. This means, their optical properties (light velocity, refractive index and colour) are identical in all directions. The three-dimensional propagation of light in an isotropic material can be presented graphically as 3-D models for wave or ray velocity and for refractive index, both of which are spheres (Fig. 4-3). The model preferred by mineralogists is the optical indicatrix, which describes the refractive index as a direction-dependent variable. Figure 4-3. Isotropic substances; 3D surfaces of v and n Raith, Raase & Reinhardt – February 2012 A. Ray velocity surface: For each ray direction, the velocity value is represented by a specific distance from a chosen origin (v = 0). The geometric form representing all directions of ray propagation is a sphere with radius v. B. Indicatrix: For each ray direction, the refractive index is represented by a specific distance from a chosen origin and is marked off parallel to the vibration direction and perpendicular to the ray propagation direction. The geometric form representing the refractive index for all ray propagation directions is a sphere with radius n. Each ray propagation direction has an infinite number of potential vibration directions. In thin section microscopy, glasses and cubic minerals generally show a single specific refractive index and colour independent of orientation (Fig. 4-4). An indication of crystal orientation in thin section can therefore only be derived from morphological properties, for example, crystal outlines or cleavage (Fig. 3-9). Another important characteristic of optically isotropic materials is that light waves do not experience any change in vibration direction. This means that E-W vibrating plane-polarized light waves maintain their E-W orientation after passing through the isotropic materials (glass, mineral). Therefore, they are blocked by the analyzer, which is a N-S oriented polarizer (Fig. 4-4 A,E). 63
Guide to Thin Section Microscopy Optical properties: basic principles Raith, Raase & Reinhardt – February 2012 Figure 4-4. Isotropic substances Behaviour of light when passing through an isotropic substance (glass or cubic mineral): In plane-polarized light chemically homogeneous glasses or crystals of cubic minerals show the same colour independent of orientation and rotation of the stage (images B,C: MgFespinel; image D: almandine). Under crossed polarizers (image A), glass and cubic mineral grains appear black independent of orientation and rotation of the stage (image E: almandine). 64
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guide to thin section microscopy - Mineralogical Society of America

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