  Surface and bulk critical properties of an aperiodic spin chain are
investigated in the framework of the $\phi^4$ phenomenological Ginzburg-Landau
theory. According to Luck's criterion, the mean field correlation length
exponent $\nu=1/2$ leads to a marginal behaviour when the wandering exponent of
the sequence is $\omega=-1$. This is the case of the Fibonacci sequence that we
consider here. We calculate the bulk and surface critical exponents for the
magnetizations, critical isotherms, susceptibilities and specific heats. These
exponents continuously vary with the amplitude of the perturbation.
Hyperscaling relations are used in order to obtain an estimate of the upper
critical dimension for this system.
