Ferrimolybdite hasbeen found in the oxidized zone of the Jinduicheng por- phyry molybdenum deposit, Shaanxi province. The mineral occurs as bundles of fine fibers and encrustations in fissures of weathered rocks and molybdenite- bearing quartz veins, In surface exposures, the mineral is associated with. quartz, limonite, goethite and jarosite; in hand specimen, it occasionally co- retains residue of molybdenite and grades into surrounding limontie, The ferrimolybdite has been studied by such means as physical and optical exa- urination, chemical analysis, X-ray diffraction, Mossbauer spectra, IR spe- ctra, TG and DTA. The formula of the mineral is Fe2(Mo04)3.6.8H20, in which. iron is proved to be all in the form of Fe3+ by Mossbauer spectra, X-ray diffraction patterns of the mineral are comparable with those of syn- thetic ferrimolybdite (Kerr, P.F.et al.，1963) but show more reflection lines than other naturally occurring ferrimolybdite. According to geological environment and related mineral association of ferrimolybdite and conditions for forming synthetic ferrimolybdite as well as Titley 's experimental results, it is considered that formation and stabi- lity of the mineral are probably coatorlled by acid oxidized environment, and the presence of molybdenite and pyrite yields molybdenum and iron neces- sary to the formation of supergene ferrimolybdite. Although ferrimoiybdite fails to form supergene enrichment economic va- lue in this molybdenum deposa, it remains an indicator mineral in search for primary molybdenum deposits.