  Context. Ch-type asteroids are distinctive among other dark asteroids in that
they exhibit deep negative polarization branches (NPBs). Nevertheless, the
physical and compositional properties that cause their polarimetric
distinctiveness are less investigated. Aims. We aim to investigate the
polarimetric uniqueness of Ch-type asteroids by making databases of various
observational quantities (i.e., spectroscopic and photometric properties as
well as polarimetric ones) of dark asteroids.Methods. We conducted an intensive
polarimetric survey of 52 dark asteroids (including 31 Ch-type asteroids) in
the R$_\mathrm{C}$-band to increase the size of polarimetric samples. The
observed data are compiled with previous polarimetric, spectroscopic, and
photometric archival data to find their correlations. Results. We find
remarkable correlations between these observed quantities, particularly the
depth of NPBs and their spectroscopic features associated with the hydrated
minerals. The amplitude of the opposition effect in photometric properties also
shows correlations with polarimetric and spectral properties. However, these
observed quantities do not show noticeable correlations with the geometric
albedo, thermal inertia, and diameter of asteroids. Conclusions. Based on the
observational evidence, we arrive at our conclusion that the
submicrometer-sized structures (fibrous or flaky puff pastry-like structures in
phyllosilicates) in the regolith particles could contribute to the distinctive
NPBs of hydrated asteroids.
