The radii of a+2 and bions are 1.12a and 1.31a, respectively. the coordination number of a+2 in mineral ab2 is

As per radius ratio lies in the range 0.732-1.000 , then Coordination number of A2+ is 8.....So answer is 8 ... Here I'm attaching radius ratio and respective coordination number t...

radius ratio of cation/Anion= A^+2/B^- (1.12/1.31=0.854) = it comes in (0.731 to 1) so 8 fold coordination. Coordination no= 8