In this work, we present a sulfurization apparatus, we call Russian Doll, which provides a quasi-stationary sulfur rich environment. We prepared several pyrite films using the technique to sulfurize iron oxide films. We studied the influence of the synthesis parameters in the phase composition and microstructure of the disulfide films. We found no traces of iron oxides nor of secondary iron sulfide phases, such as pyrrhotite. Nevertheless, we observed the formation of the marcasite polymorph at two different temperature ranges, due to the similarity in the thermodynamic properties of both disulfide phases. At temperatures lower than 550 degrees C, however, marcasite undergoes a transformation reaction to pyrite, which is completed after 4 hat 550 degrees C. At temperatures higher than 550 degrees C, pyrite slowly decomposes producing pyrrhotite and sulfur gas, due to the high sulfur equilibrium fugacity. During cooling, the sulfur equilibrium fugacity drops shifting the reaction inversely towards the formation of iron disulfide, resulting in a core-surface pattern of pyrite-marcasite. We also observed that oxide films with larger particles generated pyrite films also with larger particles. (C) 2016 Elsevier B.V. All rights reserved.