... objective on the microscope, a bright fringe can often be seen around the boundary. This might be inside or outside of the crystal. When the focal plane is raised, i.e. the stage is lowered, the bright fringe will move towards the medium with the higher refractive index. For example, suppose the embedding mineral is olivine, with a refractive index of about 1.7, and we are looking at a quartz grain under the microscope. When the stage is lowered, the Becke line will move towards the olivine because it has a higher refractive index than quartz (1.55). Diagram of the Becke Line Test 1 A thin section is a ground down slice of rock (to 0.03mm thick) which has been glued to a glass slide Form and Cleavage The form of a crystal is the shape it acquires as it grows. For example, crystals could be tabular, platy, scaly, rhombohedral, or rounded. Some crystals do ...

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