CORRELATIONS BETWEEN O-17 NMR PARAMETERS AND LOCAL-STRUCTURE AROUND OXYGEN IN HIGH-PRESSURE SILICATES - IMPLICATIONS FOR THE STRUCTURE OF SILICATE MELTS AT HIGH-PRESSURE

In order to understand the correlations between O-17 NMR parameters an d the local structure around 0 in high-pressure silicates, we have obt ained O-17 MAS and static NMR spectra on several crystalline alkali si licate and silica phases, as well as on ambient pressure glasses. Our results show that 110 NMR is capable of distinguishing nonbridging O a toms (NBO), bridging atoms (BO: [4]Si-O-[4]Si), O atoms in the [4]Si-O -[6]Si linkage, and O atoms coordinated to three [6]Si ([3]O). NBO are characterized by small quadrupolar coupling constants (e2qQ/h, approx imately 2.3 MHz), but their isotropic chemical shifts (delta(i)) are v ery sensitive to the type of network-modifying cations. The [3]O (in s tishovite), on the other hand, has by far the largest e2qQ/h value (6. 5 MHz) and a larger delta(i) (109 ppm) than any BO in silicates. BO an d [4]Si-O-[6]Si both have intermediate e2qQ/h values (approximately 4. 5-5.7 MHz), and the delta(i) value of the latter (97 ppm in wadeite-K2 Si4O9) is larger than those of the former (approximately 50-65 ppm). W e have also applied O-17 NMR to study the structure of alkali silicate glasses quenched from melts at high pressure. The O-17 NMR spectra of a K2Si4O9 glass quenched from 6 GPa and of Na2Si4O9 glasses quenched from 6 to 10 GPa are consistent with the presence of O sites in the [4 ]Si-O-[6]Si (and possibly also [4]Si-O-[5]Si) linkage, supporting our previous model developed from Si-29 NMR. In addition, O-17 static NMR also reveals the development of another new type of O site, possibly o ne that is connected with two or three [6]Si or [5]Si atoms, in the Na 2Si4O9 glass quenched from 10 GPa. Thus, [6]Si and [5]Si may be largel y isolated from one another below 10 GPa, whereas at higher pressure, clustering of these Si species may become significant in Na2Si4O9 melt s.