Flotation is the most known beneficiation method for the separation of complex and refractory iron ores. As a typical iron-containing silicates, it is difficult to separate chlorite from specularite, because of the similar surface physicochemical properties. In this study, the selective depression effect of sodium hexametaphosphate (SHMP) was conducted via the cationic micro-flotation. The surface adsorption mechanism between SHMP and the two mineral surface was explored through surface adsorption amount tests, Zeta-potential measurements, Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) analyses. The micro-flotation results indicated that SHMP could selectively depress around 90% of chlorite, while its effect on the floatability of specularite was negligible (<20% depressing). The surface adsorption amount tests, Zeta-potential measurements analysis demonstrated that SHMP selectively adsorb on chlorite surface while on the surface of specularite is feeble. The further surface adsorption analysis via FT-IR and XPS proved that SHMP selective adsorption occurred on the chlorite surface mainly by chemisorption mainly through the chelation reaction between O in the phosphate groups of SHMP molecular and metal ions on surface of chlorite.