  We report a study of selenospinel Cu$_{6-x}$Fe$_{4+x}$Sn$_{12}$Se$_{32}$ ($x$
= 0, 1, 2) single crystals, which crystalize in a cubic structure with the
$Fd\overline{3}m$ space group, and show typical semiconducting behavior. The
large discrepancy between the activation energy for electrical conductivity
$E_\rho$ (32.3 $\sim$ 69.8 meV), and for thermopower $E_\textrm{S}$ (3.2 $\sim$
11.5 meV), indicates a polaronic transport mechanism between 350 and 50 K. With
decreasing temperature, it evolves into variable-range hopping conduction.
Furthermore, the heat capacity shows a hump around 25(5) K and diverges from
the Debye $T^3$ law at low temperatures, indicating the observation of
structural glassy features in these crystalline solids.
