Quick sand is not a special type of soil . It is ahydraulic condition. It is the condition prevailed when effective stress reduces to zero, if the head causing upward flow is increased. It occurs in cohesionless soils only . it is most likely ti occur in silt and fine sand.
it is a hydraulic condition when the effective stress is zero ... in this state , soil (sand) cant resist any load and gets distorted on load application and the thing placed on it , sinks !
a condition of sandy soil in which soil does not posses shear stress . and a matter which dont carry shear stress can flow like water so sand will behave like fluid.