Each S is bonded to 3 Fe and 1 S. The S-S distance is
    ||[x,x,x] - [1-x,1-x,1-x]||.
The Fe-S distance is
    ||[x-1/2,x-1/2,x]||.
Assume all bond lengths are equal.
       3*(2x-1)^2 = 2*(x-1/2)^2 + x^2
      18x^2-20x+5 = 0
		x = [10-sqrt(10)]/18
		  = 0.37987