Model Answer
0 min readIntroduction
Quartz (SiO₂) is one of the most abundant minerals in the Earth’s crust and is frequently analyzed in petrographic studies using thin section microscopy. When a quartz grain is viewed under crossed polars, it exhibits interference colours due to the phenomenon of birefringence – the splitting of light into two rays with different refractive indices. Ideally, quartz, being a uniaxial mineral, should exhibit first-order yellow interference colours. However, it is common to observe a range of colours, including grey, white, and even higher-order colours. This discrepancy arises from several factors affecting the path difference of the two light rays.
Understanding Interference Colours and Birefringence
When polarized light passes through an anisotropic mineral like quartz, it is split into two rays vibrating in mutually perpendicular directions. These rays travel at different speeds, resulting in a phase difference. This phase difference, when viewed under crossed polars, manifests as interference colours. The order of interference colours is directly related to the mineral’s birefringence (the difference between the two refractive indices) and the thickness of the section. Quartz has a relatively low birefringence (0.009), typically producing first-order yellow colours.
Factors Causing Variation in Interference Colours
1. Crystal Orientation
The interference colour observed is highly dependent on the orientation of the crystal with respect to the vibration directions of the polarized light. Quartz is a trigonal mineral, and the observed birefringence changes as the section rotates under the microscope. If the section is not perfectly perpendicular to the optic axis, the observed interference colour will deviate from the expected first-order yellow. Different crystallographic orientations will present different path differences to the light rays.
2. Strain and Deformation
Quartz grains often contain microscopic fractures, dislocations, and other defects resulting from tectonic stress or diagenetic processes. These imperfections introduce strain within the crystal lattice. Strain alters the refractive indices locally, effectively increasing the birefringence. Areas of high strain will exhibit higher-order interference colours (e.g., blue, red) compared to unstrained areas. This is because strain induces anisotropic stress, changing the optical properties.
3. Compositional Variations & Inclusions
While pure quartz is SiO₂, trace amounts of other elements (e.g., Al, Fe) can substitute into the quartz structure. These substitutions, even in small concentrations, can subtly alter the refractive indices and thus the birefringence. Furthermore, fluid inclusions or other mineral inclusions within the quartz grain disrupt the homogeneity of the crystal. These inclusions act as scattering centers and can also locally alter the interference colour. For example, the presence of rutile inclusions can cause bright, anomalous interference colours.
4. Section Thickness Variations
The thickness of the thin section is a crucial factor. Even slight variations in thickness can significantly affect the observed interference colour. A thicker section will exhibit higher-order colours for a given birefringence, while a thinner section will show lower-order colours. Standard thin section thickness is 30µm, but deviations from this standard can occur during preparation.
5. Twinning
Quartz commonly exhibits twinning, particularly deformation twinning. Twinning introduces boundaries within the crystal structure, causing changes in the optical properties and resulting in variations in interference colours across the twinned domains. The different orientations of the twinned individuals contribute to the observed colour variations.
Illustrative Example: Deformation in Quartz
In shear zones, quartz grains are often intensely deformed. Under crossed polars, these grains display a characteristic “zebra” pattern of alternating light and dark bands, representing regions of varying strain and birefringence. These bands exhibit a range of interference colours, often including higher-order colours, indicating significant internal deformation. This is a common observation in metamorphic rocks.
| Factor | Effect on Interference Colour |
|---|---|
| Crystal Orientation | Changes the path difference of light rays, altering observed colour. |
| Strain/Deformation | Increases birefringence, leading to higher-order colours. |
| Compositional Variations | Subtle changes in refractive indices, affecting birefringence. |
| Section Thickness | Thicker sections show higher-order colours; thinner sections show lower-order colours. |
| Twinning | Creates boundaries with different optical properties. |
Conclusion
In conclusion, the observation of interference colours other than first-order yellow in quartz grains under crossed polars is a common phenomenon stemming from a combination of factors. Crystal orientation, strain, compositional variations, section thickness, and twinning all contribute to the observed diversity. Analyzing these variations provides valuable insights into the geological history of the rock, including its deformation and metamorphic processes. Understanding these nuances is crucial for accurate petrographic interpretation.
Answer Length
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