Uniaxial Minerals

Uniaxial Minerals - 1 Optics of Uniaxial Minerals...

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1 Optics of Uniaxial Minerals Introduction Uniaxial minerals are defined as minerals that have one and only one direction along which light passes with the vibrations (remember, vibrations are always perpendicular to the direction of propagation) moving at equal speed (and hence with a unique resistance or refractive index). That unique direction along which the ray propagates is known as the optic axis of the crystal, and coincides with the c-axis of the mineral. Uniaxial minerals are ones that crystallize in the tetragonal, hexagonal and trigonal systems. Interaction with light Light passing through a uniaxial crystal at an orientation other than the optic axis will therefore break into 2 rays: an ordinary ray “o”, and an extraordinary ray “e” (Fig. 1). The ordinary ray always vibrates in that plane perpendicular to the optic axis, and hence has a fixed velocity inversely proportional to the refractive index in that direction designated by “ ω ” (or in some texts “n ω ”). The planes of vibration of the extraordinary ray will vary with the orientation of the crystal relative to the direction of incident light, and their vibration velocity will therefore vary with this variation in orientation. The maximum or minimum value for this velocity will be constrained by the maximum or minimum value of the refractive index of this crystal ( ε ), depending on whether the crystal is positive or negative. A mineral in which the extraordinary ray is slower than that of the ordinary one (i.e. ε > ω ) is considered to be optically positive, and vice versa. Remember though, that in uniaxial minerals, ε always coincides with the c axis , regardless of whether the mineral is positive or negative. Pleochroism: Pleochroism is the change in color of the mineral as it is rotated in plane-polarized “white” light. It is a phenomenon observed only in some colored anisotropic minerals (uniaxial or biaxial). It arises from the preferential absorption of a specific wavelength in one (or two) specific directions coinciding with specific directions as ω and ε in the mineral (Fig. 2). For example, in tourmaline, the absorption of a particular wavelength in the direction of ω is > than in the direction of ε . A crystal of tourmaline therefore appears darker in color when oriented with its long axis perpendicular to the direction of vibration of the polarizer. Twinkling:
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Uniaxial Minerals - 1 Optics of Uniaxial Minerals...

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