Question

Which one of the following optic signs is CORRECT for a mineral with the given centered optic axis figure?

• A. Uniaxial positive
• B. Biaxial positive
• C. Uniaxial negative
• D. Biaxial negative

Hint:

To identify optic sign in interference figures:
1. Insert the accessory plate.
2. Note the quadrants showing color addition (blue) and subtraction (yellow).
3. If addition lies along the slow direction of the plate → \textbf{Biaxial positive}.
4. If subtraction lies along the slow direction → \textbf{Biaxial negative}.

Answer 1

The Correct Option is B

Step 1: Understanding the figure.
The given diagram is an optic axis interference figure obtained under crossed nicols in a conoscopic setup. The key features are:
- The presence of an isogyre (dark curved band).
- The compensator plate (gypsum plate) is inserted with the slow direction indicated.
- Color addition (blue) and subtraction (yellow) regions are marked.
Step 2: Determining the optic sign.
- In biaxial minerals, the optic sign is determined by observing the quadrants where color addition (blue) and subtraction (yellow) occur when the accessory plate is inserted.
- For a biaxial positive mineral, the addition (blue) appears in the same quadrant as the slow direction of the plate, and the subtraction (yellow) appears opposite.
- In the figure, the slow direction of the plate is toward the bottom right, and in this quadrant we see addition (blue) in the top left and subtraction (yellow) in the bottom right, matching the criterion for biaxial positive.
Step 3: Eliminating incorrect options.
- Uniaxial positive/negative: Not possible, as the figure clearly shows a biaxial interference pattern (isogyres moving in biaxial fashion).
- Biaxial negative: Would show the opposite arrangement of addition and subtraction colors.
\[ \therefore \, \text{The mineral with this optic axis figure is } \boxed{\text{Biaxial positive}}. \]