Beneficiation process in the Kirka Borax Concentrator in Turkey generates a tailings effluent containing 3-10% solids that are composed mainly of colloidal particles of clay-rock-type gangue minerals and some unrecoverable borax fines. These colloidal particles form very stable aqueous suspensions in the tailings, hindering solid/liquid separation and clean water recovery. This leads to difficulties in the sustainable and environmentally acceptable operation of the concentrator. Flocculation studies on these colloidal suspensions had shown that the particles of the gangue minerals presented significant resistance to destabilization and remained in suspension forming high-turbidity supernatants in settling tests. For this reason, this study was undertaken to characterize the nature of such gangue particles with the intention of understanding the reason for their extreme colloidal stability and poor performance in polyethylene oxide (PEO)-induced flocculation tests. Particle size and zeta potential measurements elucidated the suspension stability with d(80) <= 5 mu m and zeta-potential <= -70.1 mV. XRF and XRD studies showed that the problematical particles were rich in Mg-minerals (dolomite and trioctahedral smectite). A plausible explanation of the insufficient destabilization might be that the surface Mg ions have very high hydration energy and hold the water molecules very tightly by blocking the surface for the polymer adsorption and particle-particle interaction. FTIR spectra indicated the lack of isolated hydroxyl groups on the particle surfaces. Due to this vital drawback, the polymer chains could not be adsorbed through hydrogen bonding mechanism and this highly turbid suspension could not be flocculated. (C) 2015 Elsevier B.V. All rights reserved.