Earth Optics Video 1: Plane Polarized Light

Summary

This video covers the key concepts of color and relief in plane polarized light using a transmitted light microscope. It explains how to distinguish between transparent and opaque minerals, observe pleochroism (color change under rotation), and assess relief through the Becke line and grain boundary thickness. The video delves into understanding how minerals interact with light, with emphasis on positive and negative relief, as well as high, medium, and low relief. Additionally, it introduces isotropy, where light behaves differently along various planes in a mineral. This insightful guide equips viewers with essential techniques for mineral identification under polarized light.

Takeaways

• 😀 A petrographic thin section is used to observe minerals under plane polarized light.
• 😀 Opaque minerals do not allow light to pass through and cannot be characterized further with this microscope.
• 😀 Transparent minerals allow light to pass through and can be observed under polarized light.
• 😀 Pleochroism is a property where minerals change color when the stage of the microscope is rotated.
• 😀 Relief is the difference in the index of refraction between the mineral and its surroundings, and it can be positive or negative.
• 😀 The Becke line is a faint white line that moves into the material with a higher index of refraction as the stage is lowered.
• 😀 Positive relief is when the Becke line moves into the mineral as the stage is lowered.
• 😀 Negative relief is when the Becke line moves out of the mineral as the stage is lowered.
• 😀 Relief is classified as high, medium, or low based on the thickness and distinctiveness of the grain boundary.
• 😀 Anisotropy occurs when minerals display different relief along different planes due to varying optical properties depending on the orientation.
• 😀 To observe the Becke line and determine relief, focus in plane polarized light, adjust the diaphragm, and pay attention to the direction of the Becke line.

Q & A

What is the primary distinction between transparent and opaque minerals under plane polarized light?

Under plane polarized light, transparent minerals allow light to pass through, while opaque minerals do not. Opaque minerals cannot be characterized further with this microscope setup.

What is pleochroism and how is it observed in minerals?

Pleochroism is the property where a mineral changes color as the stage of the microscope is rotated. It is specific to certain minerals, such as the brown mineral in the example provided.

How does the index of refraction relate to the concept of relief in minerals?

Relief is the result of a difference in the index of refraction between a mineral and its surroundings. The index of refraction describes how light propagates through the material, influencing how the mineral appears in relief.

What is the Becke line, and how is it used to determine relief?

The Becke line is a faint white line that appears at the grain boundary when focusing in and out on a mineral. It moves into the material with the higher index of refraction when the stage is lowered, helping to determine whether the mineral has positive or negative relief.

What is the difference between positive and negative relief in mineral identification?

Positive relief occurs when the Becke line moves into the mineral as the stage is lowered, indicating the mineral has a higher index of refraction than its surroundings. Negative relief occurs when the Becke line moves out of the mineral as the stage is lowered.

How can the relief of a mineral be categorized as high, medium, or low?

Relief is categorized based on the appearance of the grain boundary. A faint line indicates low relief, a pronounced dark line indicates high relief, and an intermediate line indicates medium relief.

What does the change in the Becke line movement reveal about a mineral's relief?

The movement of the Becke line helps to identify whether a mineral has positive or negative relief. It also indicates the mineral's relative index of refraction compared to the surrounding material.

Why does the appearance of the grain boundary change in certain minerals when the stage is rotated?

In some minerals, such as the center mineral in the grain mount example, light behaves differently depending on the orientation, causing the grain boundary to shift from faint to distinct, displaying a range from medium to high relief.

What is the optical property of isotropy in minerals?

Isotropy is the property where a mineral displays different optical behaviors, such as relief, depending on its orientation under polarized light.

What are some tips for identifying relief using the Becke line under plane polarized light?

To identify relief, stay in plane polarized light, adjust the light and diaphragm for better visibility of the Becke line, and observe the movement of the Becke line as the stage is lowered. Remember that relief is relative to the surrounding materials.