SINGLE-CRYSTAL HIGH-PRESSURE ELECTRONIC ABSORPTION SPECTROSCOPIC STUDY OF NATURAL ORTHOPYROXENES

Abstract

Unpolarized, (X,Y)-mixed electronic absorption spectra were measured on (001) plates of two natural orthopyroxenes with 24 and 45 mole % ferrosilite and with a Ca content 0.07 atoms pfu, at pressures up to 10.3 and 8.83 GPa, using DAC techniques. Polarized spectra could not be obtained as the plane of linearly polarized light is lost due to strain-induced birefringence in the diamond anvils at pressures above 2.5 GPa. The (X,Y)-mixed electronic spectra exhibit a slightly pressure-dependent UV-edge at around 25000 cm-1. There are also two strong bands a and d at ~11000 and ~5000 cm-1, respectively, caused by spin-allowed transitions 5A1→5A1 and →5B1 of Fe2+ in the highly distorted (M2)O6 polyhedra with site symmetry 1 of the orthopyroxene structure. The corresponding Fe2+(M1) transitions expected to be observed in mixed spectra (Y,Z) of the (100) plates were not found due to their low intensity and smaller energy difference. Increasing pressure induces two effects in the electronic spectra of Fe2+ in M2 sites: (i) A high energy shift of the two bands a and d, such that the mean energy, representing the crystal field parameter 10 Dq, is increasing which indicates local compression of the Fe2+(M2)O6 polyhedra. The shift with pressure is +88 cm-1·GPa-1 for band a in the entire pressure range and +210 cm-1·GPa-1 up to ~7 GPa or +80 cm-1·GPa-1 above that pressure for band d. This indicates a decrease in M2 polyhedral distortion for the Fe2+-centered M2 sites up to ~7 GPa, but nearly unchanged distortion at higher pressures. (ii) This change of distortion is accompanied by a strong reduction of the linear as well as integral intensities in the range of applied pressures, of the two Fe2+(M2) bands to about 70% of the values at ambient conditions. This effect is ascribed to a gradual reduction of the odd crystal field components of Fe2+(M2), i.e. to a gradual development of the polyhedral geometry towards a regular octahedron or – at least – a centrosymmetric one. Distinct breaks in both dependences, (i) and (ii), at pressures of 6-7 GPa, are interpreted as a change in the compression mechanism of the orthopyroxene structure.