压力高达27 GPa下天然奥长石的XRD分析

在室温高达27 GPa压力下对天然奥长石(Na0.86K0.02Ca0.12Mg0.01(Fe0.01Al1.12Si2.87O8))粉晶进行了原位同步辐射X光衍射(XRD)测量,获得了样品的状态方程. 实验数据表明随着压力增大奥长石样品在大约3.5 GPa发生了三斜向单斜的相变(P1→C2)和在大约10.0 GPa发生了单斜对称相变(C2→C2/m). 样品三个相的体模量计算值分别为K0=73.8 GPa (K′=10.98),K(C2)=124 GPa (K′=1.05) 和K(C2/m)=272 GPa (K′=0.625). 样品的元素组成影响其T-O-T 键角的刚度、M-O键的强度和Si-O-Al键角的弯曲,从而导致奥长石样品在高压行为的特殊变化. 三斜相的奥长石晶胞压缩性具明显的各向异性. 实验结果表明在冷俯冲带奥长石可能是碱金属和碱土金属深循环的载体.

XRD Investigation of a Natural Oligoclase at Pressure Up to 27 GPa

The volume compressibility of natural oligoclase (Na0.86K0.02Ca0.12Mg0.01(Fe0.01Al1.12Si2.87O8)) was investi gated by in situ powder synchrotron X-ray diffraction (XRD) methods at pressure up to 27 GPa,and the equations of state (EoS) of the oligoclase were obtained.The experimental data indicate that the oligoclase specimen underwent triclinic to monoclinic phase transition (P1 to C2) at about 3.5 GPa and a fu rther phase transition from C2 to C2/m in monoclinic symmetry at about 10 GPa wi th increasing pressure.The bulk modulus of the triclinic phase was calculated t o be K0=73.8 GPa,and those of monoclinic phases with C2 symmetry and C2/m symmetry to be K(C2)=124 GPa and K(C2/m)=272 GPa,respecti vely.The stiffness of the T-O-T angle,the strength of the M-O bond and bend ing of Si-O-Al angle are as a function of the chemical compositions of feldspa rs.The substitution of mingled ions for main ions in the crystal structure of t he oligoclase modified T-O-T angle and the strength of the M-O bonds,resulti ng in variation of high pressure behavior of the oligoclase.Unit cell compressi on of triclinic phase oligoclase is obviously anisotropic.The results indicate that oligoclase may probably contribute to the deep recycle of alkali and alkali ne-earth elements in the cool subduction zone.