The process of quark coalescence (coherent quark fusion) is described as a possible mechanism for forming cumulative particles. We evaluate and sum Feynman diagrams for the mechanism of coherent quark fusion near the cumulative thresholds. We analyze the interference between contributions due to different diagrams and the squaring of the coalescence process amplitude for one diagram and take it into account. We give closed formulas describing the process of forming cumulative particles in the coalescence mechanism.