The friction coefficient depends on the type of plastic, the surface finish of the plastic, and the surface finish of the marble or iron. It may depend on the contact pressure. Do not expect to find any usable published information.
This is a case where you need to measure it yourself. Fortunately, it is an easy test. You take a flat piece of one material, set a piece of the other material on it, tip the flat piece until the other piece sloowwly slides down, then measure the ramp angle. The tangent of that angle is your friction coefficient. Good search term for deeper discussion is measure friction coefficient with ramp.