The entrainment of fine quartz particles into magnetite coagulates in a uniform magnetic field was studied through in situ observation using a video microscope. It is shown that the mechanisms for this entrainment are attributed to the magnetic coagulation of locked quartz-magnetite particles with free magnetite particles, the entrainment of free quartz particles into magnetite pearl chains, and wrapping of magnetite circular chains round free and locked quartz particles. (C) 2011 Elsevier Ltd. All rights reserved.