Schur sigma-Groups of Scholz-Taussky Type F

For finite metabelian 3-groups M with elementary bicyclic commutator quotient M/M' = C3*C3, coclass cc(M) in {4,6}, and transfer kernel type F, the smallest Schur sigma-groups S with second derived quotient S/S" = M are determined. Evidence is provided of arithmetical realizations of these groups by second 3-class groups M = Gal(F(3,2,K)/K), respectively 3-class field tower groups S = Gal(F(3,infty,K)/K), of imaginary quadratic number fields K=Q(sqrt{d}).