The development of efficient collectors is key to achieving the flotation separation of brucite and quartz. This study systematically investigated the separation performance and mechanism of action of the cationic surfactant octadecyl trimethyl ammonium chloride (STAC) as a novel collector for brucite and quartz. Under conditions of 30 mg/L STAC concentration, pH 7.0, and a flotation cell agitation speed of 2000 rpm, efficient separation of the two minerals was successfully achieved. Contact angle and zeta potential measurements indicate that STAC exhibits significant selectivity toward brucite and quartz. After STAC treatment, the changes in the contact angle and zeta potential of quartz were 7.82 and 57.1 times those of brucite, respectively, indicating that STAC selectively adsorbed onto the quartz surface while exerting only a weak effect on brucite. Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) analysis further revealed that the positively charged quaternary ammonium head groups in the STAC molecules preferentially bind to oxygen atoms on the quartz surface, with a binding energy change 3.2 times greater than that observed for brucite. These results confirm that STAC selectively acts on the quartz surface via an electrostatic adsorption mechanism, while exhibiting only negligible adsorption on brucite, thereby achieving efficient flotation separation of the two minerals.