  Recently, two consecutive phase transitions were observed, upon cooling, in
an antiferromagnetic spinel GeNi$_2$O$_4$ at $T_{N1}=12.1$ K and $T_{N2}=11.4$
K, respectively \cite{matsuno, crawford}. Using unpolarized and polarized
elastic neutron scattering we show that the two transitions are due to the
existence of frustrated minority spins in this compound. Upon cooling, at
$T_{N1}$ the spins on the $<111>$ \kagome planes order ferromagnetically in the
plane and antiferromagnetically between the planes (phase I), leaving the spins
on the $<111>$ triangular planes that separate the \kagome planes frustrated
and disordered. At the lower $T_{N2}$, the triangular spins also order in the
$<111>$ plane (phase II). We also present a scenario involving exchange
interactions that qualitatively explains the origin of the two purely magnetic
phase transitions.
