Comparison between the polarimetric properties (P_min, h, α₀ and α_min) of dark asteroids and their spectral properties associated with the hydrated minerals (${\widehat{R}}_{\text{UV}},{\widehat{R}}_{0.7}\text{, and }{\widehat{R}}_{2.7}$). Blue cross markers indicate asteroids with R_0.7+σ-R_0.7 < 1 (i.e., Ch-type asteroids, DeMeo et al. 2009), while black circles indicate dark asteroids other than Ch-type ones. The filled markers mean the asteroids whose P_min, h, and α₀ were all obtained. The empty makers indicate asteroids with only one parameter out of three reported. The dashed lines are the linear lines fitted by using filled markers. In panel (b), the orange square encloses the asteroids that have large |P_min| (> 1.5%) values but distribute on or above the line of ${\widehat{R}}_{0.7}=1$. In panels c, f, i, and l, ${\widehat{R}}_{2.7}$ values from Takir & Emery (2012); Takir et al. (2015) are represented as plus and square markers for Ch-type and other type asteroids, respectively.