We have studied the growth of silica-sheathed 3C-Fe7S8 products on silicon substrates with FeCl2 and sulfur precursors at the temperature region of 600-800 degrees C. On the basis of the crystal structure of Fe7S8, we have proposed a model including the kinetic competition of the adsorption of silica species on Fe-2-Fe-3-Fe-4 units at the 4Fe layer and on the Fe2-Fe3-Fe4-Fe5 units parallel to the c-axis. Using this model, we have not only explained all the experimental phenomena but also especially prepared Fe7S8 nanowires at 650 degrees C by introducing water into the reaction system.