Step 1: Understanding variance in a three-component system.
In a three-component system (A-B-C), the formula for variance (\( \nu \)) is given by:
\[
\nu = C - F + 2
\]
Where:
- \( C \) is the number of components (3 in this case, A, B, and C),
- \( F \) is the number of phases (in this case, four minerals P, Q, R, and S, so \( F = 4 \)).
Step 2: Applying the formula.
Substituting the values into the formula:
\[
\nu = 3 - 4 + 2 = 1
\]
Step 3: Conclusion.
Therefore, the variance (degrees of freedom) of the equilibrium mineral assemblage at X is 1.