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)
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When determining optical properties from phase diagrams, accurately track the given composition and read the corresponding property, such as birefringence, from the graph.
Step 1: Identify the composition of the feldspar.
The feldspar is from a lunar anorthosite, and its composition is given as 85 mole \% anorthite. We need to locate this composition on the x-axis of the graph.
Step 2: Locate the corresponding birefringence value.
On the graph, follow the line corresponding to 85 mole \% anorthite (on the x-axis) and observe where it intersects with the birefringence curve for this composition.
Step 3: Read the birefringence value.
At 85 mole \% anorthite, the birefringence value is read from the graph, which corresponds to approximately 0.010.
\[
\boxed{0.010}
\]
