Laihunite,a mineral of ferric and ferrous iron silicate discovered in China in recent years,occurs in high grade metamorphic BIF coexisting with magnetite,quartz,fayalite,ferrosiliteand almandinc.Complete nets based on Schreinemakers bundles and partial nets with maximumclosure after the method described by H.W.Day have been constructed by taking into accountthe existence of this mineral in order to illustrate phase relations for the(n+3)multisystemin the system Fe_2O_3-FeO-SiO_2.Judging from Lindsley’s experimental data on ferrosilite,laihu-nite-ferrosilite assemblage is expected to be stable at pressures probably higher than 15 kb.Two-pyroxene and garnet-clinopyroxene geothermometers gave the temperatures of formationbetween 600—700℃.The free energy of laihunite calculated according to Chon’s method and es-timated heat capacity power-function coeficients allow oxygen fugacity to be plotted againsttemperature at different total pressure swith respect to a number of univariant reactions inwhich laihunite is involved.The log fo_2-T diagram demonstrates that to a first approximationthe stability field of laihunite overlaps the upper part of the magnetite field,but its upperlimit is a little beyond the hematite-magnetite equilibrium.The three major requirements for laihunite stability,i.e.,an unusual high pressure,atemperature in the order of 600℃—700℃ and a relatively higher oxygen fugacity,are mutu-ally exclusive in common geological environment,which may probably account for the rareoccurrence of this mineral in nature.For the BIF in which laihunite occurs,the pressure causedby the weight of overlying strata is not high enough to give rise to its formation even in theextreme case of geothermal gradient.So it is suggested that additional tectonic pressure in res-ponse to plate activity might have been involved. Laihunite, a mineral of ferric and ferrous iron silicate discovered in China in recent years, occurs in high grade metamorphic BIF coexisting with magnetite, quartz, fayalite, ferrosilite and almandinc. Complete nets based on Schreinerkers bundles and partial nets with maximumclosure after the method described by HWDay have been constructed by taking into account the existence of this mineral in order to illustrate phase relations for the (n + 3) multisystemin the system Fe_2O_3-FeO-SiO_2.Judging from Lindsley’s experimental data on ferrosilite, laihu-nite-ferrosilite assemblage is expected to be stable at pressures may be higher than 15 kb. Two-pyroxene and garnet-clinopyroxene geothermometers gave the temperatures of formation between 600-700 ° C. The free energy of laihunite calculated according to Chon’s method and es-timated heat capacity power-function coeficients allow oxygen fugacity to be plotted againsttemperature at different total pressure swith respect to a number of univariant reac tions inwhich laihunite is involved. log fo_2-T diagram demonstrates that to a first approximation the stability field of laihunite overlaps the upper part of the magnetite field, but its upper limit is a little beyond the hematite-magnetite equilibrium. the three major requirements for laihunite stability, ie, an unusual high pressure, atemperature in the order of 600 ℃ -700 ℃ and a relatively higher oxygen fugacity, are mutu-ally exclusive in common geological environment, which may probably account for the rareoccurrence of this mineral in nature. For the BIF in which laihunite occurs, the pressure caused by the weight of overlying strata is not high enough to give rise to its formation even in theextreme case of geothermal gradient.So it is suggested that additional tectonic pressure in res-ponse to plate activity might have been involved.