Question

The figure shows variation in optical properties of plagioclase feldspars with composition. A feldspar from a lunar anorthosite, with anorthite mole % of 85, will have a birefringence of ........ (Round off to three decimal places) 
 

Hint:

When determining optical properties from phase diagrams, accurately track the given composition and read the corresponding property, such as birefringence, from the graph.

Answer 1

Correct Answer: 0.01

Step 1: What the chart shows.
The graph plots how the optical birefringence of plagioclase changes with its anorthite content along the x axis.

Step 2: Fix the composition.
The lunar feldspar has 85 mole percent anorthite, so we move to that point on the x axis.

Step 3: Read up to the curve.
Going straight up to the birefringence curve and across to the y axis gives the value for this composition.

Step 4: Read the value.
The curve at 85 mole percent anorthite gives a birefringence near 0.010.

Step 5: State the answer.
So the birefringence is about 0.010.
\[ \boxed{0.010} \]