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1. Applied to substances whose optical or other physical properties vary according to the direction from which they are observed. In optical mineralogy the term is applied to minerals other than cubic that split the light passing through them into two vibration directions, each with a different velocity (provided the light is not travelling along an optic axis), as a result of the light being doubly refracted (see DOUBLE REFRACTION). In addition, each beam is refracted differently for different colours of light.

2. In engineering geology, anisotropy refers to a rock whose engineering properties vary with direction. For example, schist, a highly anisotropic rock, has a compressive strength that varies depending upon the orientation of the foliation to the applied load (see COMPRESSIVE STRESS).

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