Abstract

The behavior of molybdenite in the system Na 2 Si 2 O 5 -K 2 Si 2 O 5 -MoS 2 -H 2 O has been investigated from 340 bars to 1.36 kb in the temperature range 450 degrees to 850 degrees C. Phase relations in the system show enhanced molybdenite solubility in the alkali silicate liquids and coexisting vapors. MoS 2 solubility is up to 10 wt percent in vapor-saturated Na 2 Si 2 O 5 melt and 12.5 wt percent in vapor-saturated K 2 Si 2 O 5 melt at 680 bars and 650 degrees C. There is a positive pressure dependence of solubility, but retrograde solubility is indicated as a function of temperature above about 650 degrees C at the pressures investigated.The experimental data combined with published data on structure of silicate melts (Mysen and Virgo, 1980) suggest that Mo silicate and/or Mo-hydroxylated silicate complexes such as MoSi 2 O (super +2) 5 , MoSi 2 O 6 , and MoSi 2 O 4 (OH) (super +3) may account for the observed molybdenite solubility. If these results can be extrapolated to nature, it seems likely that geologically significant quantities of molybdenum may be transported in hydrous alkalic silicic melts as Mo silicate complexes for eventual fractionation into the vapor phase during vapor generation. Such a transport mechanism would be consistent with the commonly observed association of Mo with silica-rich rocks.

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