Structural environments around molybdenum in silicate glasses and melts. I. Influence of composition and oxygen fugacity on the local structure of molybdenum

The coordination chemistry of molybdenum was investigated in nine series of synthetic silicate glasses, including sodium disilicate (NS2), sodium trisilicate (NS3), albite (Ab), anorthite (An), Ab(50)An(50), Ab(30)An(70), diopside (DI), rhyolite (RH), and basalt (BA), using electron paramagnetic resonance (EPR) and X-ray absorption fine structure (XAFS) spectroscopies. The Mo content of these glasses ranges from 300 ppm to 3 wt.%. On the basis of results derived from high-resolution X-ray absorption near-edge structure (XANES) spectroscopy, molybdenum is present primarily as molybdate moieties [MO(VI)O-4(2-)] in most of the glass compositions prepared at f(O-2) values ranging from 1 atm to 10(-12) atm (temperatures ranging from 1100 to 1700 degrees C, i.e., from air to IW+4). Analysis of extended XAFS (EXAFS) spectra of these glasses indicates an average Mo-O distance of similar to 1.76(1) angstrom. No evidence for second-neighbor Si or Al around Mo was found in any of the glasses, confirming that molybdate moieties are not connected to the tetrahedral framework, in agreement with Pauling bond-valence predictions. The presence of molybdate moieties in regions of these glasses enriched in network modifiers helps explain why crystalline molybdates can nucleate easily in silicate glasses (and, by extension, in the corresponding melts). In the highly polymerized glass compositions (such as Ab or RH), Mo(VI)O-6(6-) moieties also exist, but at minor levels (<20% of the total Mo). In glasses prepared at low f(O-2) (near IW), reduced species of Mo occur, such as molybdenyl [Mo(V) and Mo(IV)]. In glasses prepared at even lower f(O-2) (near IW+4), Mo is present as a metallic precipitate. The prevalence of molybdate moieties in silicate glasses until relatively low oxygen fugacities (IW) are achieved appears to be at variance with the fact that molybdenite, Mo(IV)S-2, is the dominant Mo-bearing mineral in the Earth's crust. In a companion paper, we re-examine the speciation of molybdenum in more complex systems that are closer to geochemical reality, such as high-temperature melts, densified (high-pressure) glasses, and silicate glass compositions enriched in volatiles.