Recently, a model for oscillatory zoning in a geochemical system (plagioclase feldspar) was proposed. In that model, oscillations in the composition are generated through a Hopf bifurcation. In this paper, the effects of fluctuations of the bulk composition are studied by means of computer simulations. It is shown that environmental noise can lead to pattern formation such as oscillatory zoning, even when no deterministic periodic solutions exist. The fluctuations in the bulk composition thus lead to an enlargement of the range of system's variables values for which oscillatory zoning occurs. Coherence resonance close to the Hopf bifurcation is also observed in such a system.