In order to establish the history and conditions of nucleation and growth of pyrite formed inside methane-derived carbonate chimneys collected in the seafloor of the Gulf of Cadiz, we compared the size distributions, morphologies, textures and geochemical compositions of framboidal and euhedral pyrite as well as iron oxyhydroxides pseudomorphs. More than 2400 framboids and crystals were measured and four types of statistical populations were identified. Framboid size (diameter) ranges from 6 to 12.5 μm with a mean size of 10.4 μm and a maximum value of 27.9 μm, whereas euhedral crystal size (maximum dimension) ranges from 6.3 to 9 μm with a mean size of 7.2 μm and a maximum value of 21.6 μm. Crystal size distributions (CSD) of different populations of framboidal iron oxyhydroxides show strongly negatively skewed shapes, close to the universal steady-state CSD curve for Ostwald ripening, an unequivocal signal of closed-system growth. Euhedral crystal populations of pyrite and iron oxyhydroxides present CSDs with lognormal or slightly negatively skewed lognormal shapes and low size variances, indications of open-system growth. This was confirmed by simulating framboidal and euhedral crystal populations with different growth mechanisms using the GALOPER (Growth According to the Law of Proportionate Effect) program. From CSDs shapes and the values of α and β2, the populations of framboids are inferred to have grown initially in an open system (surface and then transport-controlled) and then in a closed system (Ostwald ripening) and the populations of euhedral crystals only in an open system. In this case, the initial stage of surface-controlled growth is shorter than that deduced for the framboidal populations. We suggest that: (1) the concentration and availability of reactive iron and hydrogen sulphide are the main factors controlling the morphology and growth of pyrite, and (2) the higher reactive surface of framboidal textures would govern their preferential oxidation, dissolution and recrystallization during the closed-system growth and consequently the development of higher sizes in comparison with euhedral forms.