Question

Based on the figure, the anorthite content of first formed plagioclase crystals from a liquid composition having 50 mole % anorthite is ......... mole % (In integer). 
 

Correct Answer: 74 mole \% 
Solution:

Hint:

When analyzing phase diagrams, follow the lines to interpret compositional changes at different temperatures. The intersection points provide key insights into the crystallization process.

Answer 1

Correct Answer: 74

Step 1: 
The given composition has 50 mole % of anorthite. We need to refer to the graph showing the relationship between the composition (in mole %) and temperature.

Step 2: 
From the graph, locate the liquid composition at 50 mole % anorthite. 

Step 3: 
Read the temperature at the point where the composition of 50 mole % anorthite intersects the line of the first formed plagioclase crystals. From the graph, this corresponds to a temperature of approximately 1350°C.

Step 4: 
Next, follow the horizontal line from this temperature until it intersects with the Solid Soln + Liquid curve. This curve indicates the anorthite content of the first formed plagioclase crystals. 

Step 3: 
At the intersection point, the mole % of anorthite is approximately 74 mole %. Therefore, the anorthite content of the first formed plagioclase crystals is 74 mole %. 
\[ \boxed{74} \]