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

The volume compressibility of natural oligoclase (Na_0.86K_0.02Ca_0.12Mg_0.01(Fe_0.01Al_1.12Si_2.87O₈)) was investigated 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 further phase transition from C2 to C2/m in monoclinic symmetry at about 10 GPa with increasing pressure. The bulk modulus of the triclinic phase was calculated to 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, respectively. The stiffness of the T-O-T angle, the strength of the M-O bond and bending of Si-O-Al angle are as a function of the chemical compositions of feldspars. The substitution of mingled ions for main ions in the crystal structure of the oligoclase modified T-O-T angle and the strength of the M-O bonds, resulting in variation of high pressure behavior of the oligoclase. Unit cell compression of triclinic phase oligoclase is obviously anisotropic. The results indicate that oligoclase may probably contribute to the deep recycle of alkali and alkaline-earth elements in the cool subduction zone.