You observe a thin section containing a colored, chemically homogeneous uniaxial

Question

You observe a thin section containing a colored, chemically homogeneous uniaxial mineral. As you rotate the stage in plane polarized light, you notice the following 3 scenarios: 9. a) Equant grains are dark blue and do not change color with rotation. Prismatic (elongate) grains vary from dark blue to dark red with rotation, showing a deep purple hue at intermediate positions Other grains show blue to red variation with rotation, but the red is a lighter shade as is the purple hue at intermediate positions. b) c) i. What is the name for the optical property observed? (2) How is it possible to have so much color variation in the same mineral? Your answer should address the cause for the specific color(s) observed in each of the 3 scenarios. You may find it helpful to use indicatrices in your answer. (9) ii.

Explanation / Answer

i) The optical property observed here is known as Pleochroism.

Pleochroism is the property by which certain anisotropic minerals under plane polarised light change their colour as the microscope stage is rotated.

ii) The main cause of Pleochroism is that the mineral absorbs light differently in different orientations. In the first case, the equant grains do not change their color with stage rotation. This means the absorption of light in the equant grains is the same in every direction and it behaves as an isotropic mineral having the same refractive index in every vibration direction. They have a circular indicatrix. In the second and third case, the grains show different shades of color with stage rotation. This is due to the variable refractive indices because in these cases, light travels through the crystal with different speeds in different direction and so the amount of light absorbed varies. The absorption of specific light wavelengths depends upon the orientation of the crystal.

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