Polarized light passing through a supercooled binary liquid mixture sample is analyzed during the moment of the nucleation of the crystal phase to determine whether the stable equilibrium crystal is nucleated, or whether a transient phase of different composition or broken-symmetry is formed. This experiment is performed for the particular case of heterogeneous nucleation of a supercooled clathrate-forming liquid mixture, tetrahydrofuran (THF)/water, compared with ice nucleating from pure supercooled water. The new experimental results are consistent with the hypothesis that the equilibrium clathrate hydrate crystal is nucleated directly, with no transient phase detected on the time scale of these experiments.