Keywords:silicate glass, X-ray diffraction, neutron diffraction, amorphous structure
Structure determinations of basaltic glass have been conducted under high pressure using X-ray diffraction (XRD) and neutron diffraction (ND) techniques. The total structure factor, S(Q) suggests that the position of the first sharp diffraction peak (FSDP) shifts to higher-Q region with increasing pressure. This shift indicates the intermediate-range structure of glass becomes compact. The radial distribution function, G(r) shows the shrinkage of the average T-T length, and no detectable change of the average T-O length with increasing pressure (T means tetrahedrally coordinated cations, such as Si4+ and Al3+). This result implies that drastic shrinkage of network structure involving a decrease in the mean T-O-T angle is the dominant structural evolution under experimental pressure conditions. Moreover, the second sharp diffraction peak, SSDP (Elliot, 1995) was observed in S(Q) from the ND experiment. The intensity of SSDP enhanced, while that of FSDP weakened with increasing pressure. These trends indicate the disordering of the intermediate-range order (Salmon, 1994) and the ordering of the extended-range order (Salmon et al., 2005), respectively. G(r) of ND also indicates no extension of the T-O bond in the present study. Considering the T-O extension reported in basaltic liquid (Sakamaki et al., 2013), this difference seems to be due to thermal effect. Also, G(r) of ND represents the Mg-O and Fe-O distances show increase two times at about 2.0 and 6.0–6.9 GPa. These changes might be caused by the increase in their coordination numbers due to the polymerization of TO4 tetrahedron. On the other hand, Ca-O and Na-O distances are less sensitive to the pressure. Since the Mg/Fe-O distance is shorter than the Ca/Na-O distance, the Coulomb force between Mg/Fe and O ions is larger than that of Ca/Na and O ions. Hence, Mg and Fe cations are easier to combine with non-bridging or isolated O anions than Ca and Na cations.