Model Answer
0 min readIntroduction
Optical mineralogy is a crucial branch of petrology, utilizing the interaction of light with minerals to identify and characterize them. Refractive index, a fundamental optical property, describes how light bends when entering a mineral. Minerals can exhibit one or two refractive indices, leading to the concept of birefringence – the difference between the maximum and minimum refractive indices. This difference is a key indicator of a mineral’s crystal system and its optic nature, whether it is uniaxial or biaxial. Determining birefringence is essential for accurate mineral identification using a polarizing microscope.
Calculating Birefringence
Birefringence (Δ) is calculated as the difference between the maximum and minimum refractive indices. In this case, we are given ω (ordinary ray refractive index) = 1.548 and δ (extraordinary ray refractive index) = 1.634.
Therefore, Δ = δ - ω = 1.634 - 1.548 = 0.086
Optic Nature Determination
The calculated birefringence value of 0.086 allows us to comment on the mineral’s optic nature. Minerals are classified as either uniaxial or biaxial based on their birefringence and the number of optic axes.
Uniaxial Minerals
Uniaxial minerals belong to the tetragonal, hexagonal, or trigonal crystal systems. They have one optic axis and exhibit a single refractive index for light vibrating parallel to that axis. Birefringence in uniaxial minerals is generally lower, typically ranging from 0 to 0.020. Examples include calcite and aragonite.
Biaxial Minerals
Biaxial minerals belong to the orthorhombic, monoclinic, or triclinic crystal systems. They have two optic axes and exhibit two refractive indices for all directions of light vibration. Birefringence in biaxial minerals is generally higher, ranging from 0.020 to 0.090 or even higher. Examples include augite and hornblende.
Since the calculated birefringence (0.086) falls within the typical range for biaxial minerals (0.020 - 0.090), we can conclude that the mineral in question is biaxial.
Further Implications
The relatively high birefringence suggests a complex crystal structure with varying atomic arrangements that cause significant differences in the speed of light propagation in different directions. This also implies that the mineral will exhibit strong interference colors when observed under a polarizing microscope. Determining the exact biaxial angle (2V) would require further optical measurements, such as determining the optic sign (positive or negative).
Table Summarizing Optic Nature and Birefringence
| Optic Nature | Crystal System | Optic Axes | Typical Birefringence Range | Examples |
|---|---|---|---|---|
| Uniaxial | Tetragonal, Hexagonal, Trigonal | One | 0 - 0.020 | Calcite, Aragonite |
| Biaxial | Orthorhombic, Monoclinic, Triclinic | Two | 0.020 - 0.090+ | Augite, Hornblende |
Conclusion
In conclusion, the mineral with refractive indices ω = 1.548 and δ = 1.634 exhibits a birefringence of 0.086. This value strongly indicates that the mineral is biaxial, belonging to one of the orthorhombic, monoclinic, or triclinic crystal systems. Further optical analysis, including determining the optic sign and biaxial angle, would be necessary for complete mineral identification. Understanding birefringence is fundamental to interpreting the optical properties of minerals and their geological significance.
Answer Length
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