The selective flotation of quartz and feldspar under weakly alkaline conditions was investigated, with carboxymethyl starch sodium (CMS-Na) employed as a depressant and polyether amine (PEA) as a collector. Micro-flotation tests demonstrated that CMS-Na exhibited strong selective depression of feldspar while maintaining high quartz recovery. Optimal separation conditions were achieved at pH 9.0 with 30 mg/dm3 CMS-Na and 4.0×10-4 mol/dm3 PEA, resulting in feldspar recovery decreasing sharply to 12.51% while quartz recovery remained high at 92.17%. The selectivity index (SI) increased significantly from 3.03 to 9.07, highlighting enhanced separation efficiency. Spectroscopic analyses, including FTIR, Raman, and XPS, revealed that CMS-Na selectively adsorbed onto feldspar surfaces mainly via coordination with Al-sites, with a small portion additionally forming hydrogen bonds with -Al-OH or -Si-OH, effectively suppressing its flotation. Conversely, CMS-Na exhibited limited interaction with quartz, enabling subsequent PEA adsorption. XPS data further confirmed that PEA adsorption on quartz and feldspar occurs via electrostatic and hydrogen bonding interactions, with CMS-Na selectively inhibiting PEA adsorption on feldspar. These results underscore CMS-Na's role as an environmentally friendly and effective depressant, enabling efficient separation of quartz and feldspar without hazardous chemicals like hydrofluoric acid. This study offers a sustainable approach to mineral resource utilization in industrial applications.