REFLECTION AND REFRACTION AT CRYSTAL SURFACES IN THIN SECTION

The interpretation of Becke line displacement where two crystals meet in thin section is not as straightforward as it is where a birefringent crystal at extinction, is immersed in an isotropic liquid of known refractive index. For example, the Becke lines bordering the traces of the composition planes of an albite twin individual move in the same direction. Sloping cleavage surfaces, crystal faces, or the composition planes of normal and parallel twins often separate crystals or parts of a crystal in thin section. Light passing through the lower crystal reflects and refracts at these surfaces. Crystals at the surfaces are commonly birefringent and light follows two different ray paths on both sides of the surface. Light from both ray paths in the incident medium will excite light following the two ray paths on the other side of the surface. Light associated with these ray paths will have different refractive indices; consequently, refraction can be in response to the relative magnitudes of four different combinations of refractive indices. Cleavage surfaces and the composition planes of normal twins have the same refractive indices on both sides of the surfaces; light will pass across such surfaces without refraction. Crystals meeting at crystal faces and twin individuals meeting at the composition planes of parallel twins will have different refractive indices on either side of the surface and light can be refracted. Light can be reflected from all four types of surfaces. Light normally incident on the lower surface of the thin section will intercept the surface between crystals with an angle of incidence equal to the slope of the surface. The steeper the surface, the greater is the angle of incidence. More light is reflected than refracted at steep surfaces than at shallow ones. Consequently, at steep surfaces, refractive indices play a smaller role than at shallow ones unless critical phenomena occur. If the crystal below the surface has a larger refractive index than the crystal above the surface, total reflection can occur where the dip of the surface exceeds the critical angle. Those who look at crystals in thin section generally concentrate their observations on steeply dipping surfaces. Consequently, more often than not they will see effects caused by reflection rather than by refraction.