Fluorite packing fraction
= Fluorite packing fraction
{title2=$\eta$}
With <anion> <radius> $r_a$, <cation> <radius> $r_c$ and unlike-neighbour contact, the conventional cell gives $\eta=(\pi\sqrt3/4)(r_c^3+2r_a^3)/(r_c+r_a)^3$. At the eight-coordinate limiting contact ratio this is $\pi(3\sqrt3-4)/6$. The cation-contact tetrahedral-hole limit is a different geometric ideal.