The stability of laihunite—A thermodynamic analysis

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 Schreinemakers bundles and partial nets with maximum closure after the method described by H. W. Day have been constructed by taking into account the existence of this mineral in order to illustrate phase relations for the (n+3) multisystem in the system Fe₂O₃-FeO-SiO₂. Judging from Lindsley’s experimental data on ferrosilite, laihunite-ferrosilite assemblage is expected to be stable at pressures probably 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 Chen’s method and estimated heat capacity power-function coeficients allow oxygen fugacity to be plotted against temperature at different total pressure swith respect to a number of univariant reactions in which laihunite is involved. The logfo 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, i.e., an unusual high pressure, a temperature in the order of 600°C–700°C and a relatively higher oxygen fugacity, are mutually exclusive in common geological environment, which may probably account for the rare occurrence 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 the extreme case of geothermal gradient. So it is suggested that additional tectonic pressure in response to plate activity might have been involved.