Model Answer
0 min readIntroduction
In optical mineralogy, the behavior of light passing through a mineral section under polarized light provides crucial information about its internal structure and composition. Minerals are classified as either isotropic or anisotropic based on their optical properties. Isotropic minerals possess the same refractive index in all directions, while anisotropic minerals exhibit varying refractive indices. When a mineral section appears completely dark under crossed polars, it indicates a specific optical characteristic. This response will explain the reasons for this observation and detail the procedures to determine the optic sign of the mineral, should it exhibit any subtle optical activity.
Isotropy and Extinction
A mineral section appearing dark in all positions under crossed polars signifies that the mineral is isotropic. Isotropy implies that the refractive index (n) is constant in all directions. This is typically observed in cubic minerals like garnets, halite (NaCl), and diamonds. When polarized light enters an isotropic mineral, it is split into two rays that travel at the same speed, resulting in no interference and thus, no color development. Consequently, the mineral appears dark, exhibiting complete extinction.
Why Complete Darkness?
Under crossed polars, two polarizing filters are oriented at 90 degrees to each other. If a mineral is isotropic, the two rays passing through it experience the same refractive index and travel in phase. Therefore, when they exit the mineral and encounter the second polarizer, they are blocked, resulting in complete extinction and a dark appearance. Any deviation from this – even slight birefringence – would result in interference colors.
Determining Optic Sign – When Applicable
While a completely dark mineral suggests isotropy, certain scenarios might necessitate determining the optic sign. These include:
1. Anomalous Birefringence/Strain Effects:
Sometimes, even cubic minerals can exhibit slight birefringence due to factors like internal strain, solid solution, or twinning. In such cases, subtle interference colors might be observed upon rotating the stage. To determine the optic sign, the following methods can be employed:
- Retardation Measurement: Using a Michel-Levy chart or a retardation plate, the retardation (δ) can be measured. The sign of the retardation (positive or negative) can provide clues about the optic sign.
- Rotation of the Stage: If slight interference colors appear, rotating the stage and observing the color changes can help determine if the mineral is effectively uniaxial.
2. Pseudo-Isotropic Minerals:
Some minerals, though structurally anisotropic, may appear isotropic in certain orientations due to symmetry. These are termed pseudo-isotropic. Examples include some forms of olivine and nepheline. To determine the optic sign in these cases:
- Optical Axial Angle (2V) Determination: If the mineral is uniaxial, the optical axial angle (2V) can be determined using a Wollaston prism or a quartz wedge. The sign of 2V indicates the optic sign.
- Interference Figure Analysis: Tilting the section and observing the interference figure can reveal the optic axial plane and help determine the optic sign.
3. Uniaxial Minerals with Parallel Extinction:
Certain uniaxial minerals, like some varieties of tourmaline, can exhibit parallel extinction, making them appear dark in some orientations. In such cases:
- Conoscopic Examination: Observing the mineral under conoscopic illumination (using a Bertrand lens) will reveal a circular interference figure for uniaxial minerals. The shape and characteristics of the figure can help determine the optic sign.
- Accessory Plate Method: Using a first-order red plate (tint plate) can help identify the optic sign. A positive mineral will show a yellow tint, while a negative mineral will show a blue tint.
Table Summarizing Methods
| Scenario | Method | Principle |
|---|---|---|
| Anomalous Birefringence | Retardation Measurement | Measuring the difference in optical path length between the two rays. |
| Pseudo-Isotropic Minerals | Optical Axial Angle (2V) Determination | Determining the angle between the two optic axes. |
| Uniaxial Minerals with Parallel Extinction | Accessory Plate Method | Utilizing interference colors produced by a tint plate to identify the optic sign. |
| All Scenarios | Conoscopic Examination | Analyzing the interference figure under conoscopic illumination. |
Conclusion
In conclusion, a mineral section appearing dark under crossed polars generally indicates isotropy. However, subtle optical effects or pseudo-isotropic behavior can necessitate determining the optic sign. Techniques like retardation measurement, optical axial angle determination, accessory plate method, and conoscopic examination are crucial for accurately characterizing the mineral's optical properties. A thorough understanding of these methods is essential for precise mineral identification and geological interpretation.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.