Model Answer
0 min readIntroduction
Optical mineralogy is a crucial branch of petrology that utilizes the interaction of light with minerals to identify and characterize them. The ‘optic sign’ of a mineral refers to whether the extraordinary ray travels faster or slower than the ordinary ray within the crystal. This property is particularly important for uniaxial minerals, which possess a single optic axis. Understanding the optic sign is fundamental to mineral identification using a polarizing microscope and provides insights into the mineral’s crystal structure and optical properties. Determining the optic sign involves analyzing the interference colours observed when rotating the stage of a petrographic microscope.
Understanding Uniaxial Minerals and Optic Sign
Uniaxial minerals are those possessing a single optic axis. This means light vibrates equally in all directions perpendicular to this axis. Examples include gypsum, calcite, and aragonite. The optic sign, denoted as positive (+) or negative (-), indicates the relative velocity of the ordinary (o) and extraordinary (e) rays. If the extraordinary ray is faster than the ordinary ray, the mineral is said to have a positive optic sign. Conversely, if the ordinary ray is faster, the mineral is negative.
Determining Optic Sign: The Michel-Levy Chart
The most common method for determining the optic sign of a uniaxial mineral is using the Michel-Levy chart. This chart is a circular diagram divided into three sections: acute bisectrix, obtuse bisectrix, and parallel. The chart helps to interpret the interference colours observed as the stage is rotated.
Procedure for Determining Optic Sign
- Mineral Preparation: A thin section of the mineral must be prepared for microscopic examination.
- Polarizing Microscope Setup: The thin section is placed on the microscope stage with crossed polars (polarizer and analyzer at 90 degrees).
- Identifying the Optic Axis: Locate the optic axis, which appears as a dark line.
- Rotation and Observation: Rotate the stage 45 degrees from the optic axis. Observe the interference colours that develop.
- Michel-Levy Chart Interpretation: Compare the observed interference colours with the Michel-Levy chart for the mineral’s birefringence.
- Determining the Optic Sign:
- If the maximum interference colour lies in the acute bisectrix zone of the chart, the mineral is negative.
- If the maximum interference colour lies in the obtuse bisectrix zone of the chart, the mineral is positive.
Diagrammatic Representation
(Image source: Wikimedia Commons - Michel-Levy chart for uniaxial minerals)
Fast and Slow Rays in Uniaxial Minerals
In uniaxial minerals, the ordinary ray (o) always travels at a constant velocity regardless of direction. The extraordinary ray (e) travels at different velocities depending on its direction relative to the optic axis. The optic sign is determined by comparing the velocities of these two rays. For example, in calcite (negative), the ordinary ray is faster, while in gypsum (positive), the extraordinary ray is faster.
Birefringence and Interference Colours
Birefringence is the difference between the refractive indices of the ordinary and extraordinary rays. This difference causes interference colours when light passes through the mineral under crossed polars. The magnitude of birefringence influences the intensity and order of the interference colours observed, which are then used in conjunction with the Michel-Levy chart to determine the optic sign.
Conclusion
Determining the optic sign of a uniaxial mineral is a fundamental skill in optical mineralogy. Utilizing the Michel-Levy chart in conjunction with careful observation of interference colours under a polarizing microscope allows for accurate identification of the mineral’s optical properties. This information is crucial for understanding the mineral’s composition, structure, and geological history. Accurate determination of optic sign contributes significantly to the broader field of petrology and geological interpretation.
Answer Length
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