[S-MP38] 鉱物の物理化学

[SMP38-08] X線・中性子回折を用いた高圧下での玄武岩ガラスの構造解析

*大橋 智典¹、坂巻 竜也¹、服部 高典²、佐野 亜沙美²、舟越 賢一³、鈴木 昭夫¹ (1.東北大学大学院理学系研究科、2.日本原子力研究開発機構 J-PARCセンター 物質・生命科学ディビジョン 中性子利用セクション、3.総合科学研究機構 中性子科学センター)

キーワード：珪酸塩ガラス、X線回折、中性子回折、非晶質体の構造

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.