  In a neutron polarimetry experiment the mixed state relative phases between
spin eigenstates are determined from the maxima and minima of measured
intensity oscillations. We consider evolutions leading to purely geometric,
purely dynamical and combined phases. It is experimentally demonstrated that
the sum of the individually determined geometric and dynamical phases is not
equal to the associated total phase which is obtained from a single
measurement, unless the system is in a pure state.
