Prolate Ellipsoid - uniaxial positive indicatrix3D Model
The optical indicatrix is a 3-dimensional depiction of the refractive index in differing directions through crystals - the result is an ellipsoid because the refractive index varies (anisotropic). For the case of uniaxial minerals, there is just one orientation where a section through this ellipsoid forms a perfect circle (represented by the yellow disc) - all other sections result in an ellipse. The circular section is perpendicular to the Z crystallographic axis, which as a result is concident with the ‘optic axis’ (O.A.). The significance of this is that when anisotropic crystals are viewed in thin section under crossed polarised light, they mostly display interference phenomena - however crystals viewed down the optic axis behave as if they were isotropic (black under crossed polars) because light is vibrating in all directions with the same refractive index.
In this case the indicatrix is positive because it is prolate (like a rugby ball). Compare with negative indicatrix.
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