Experimental and Ab Initio Investigation of the Formation of Phosphoran Olivine

With the use of both experimental and numerical methods, the conditions required for phosphoran olivine formation as well as its structural features and thermodynamic stability have been studied. Olivine containing up to 35 wt % P2O5 was synthesized in evacuated quartz glass ampoules at temperatures of 1373, 1473, and 1573 K using San Carlos olivine or an oxide mixture blended with either magnesium phosphate Mg3(PO4)2 or iron phosphide FeP and studied by scanning electron microscopy/energy-dispersive spectrometry, wavelength-dispersive spectroscopy analysis, X-ray powder diffraction, and Raman spectroscopy. The inhomogeneity of the composition of synthesized phosphoran olivines as well as the results of first-principles calculations unequivocally indicates their metastable crystallization. The absence of the shift of phosphoran olivine X-ray diffraction reflections with respect to phosphoran-free olivine might indicate that the entry of phosphorus in olivine causes only a slight distortion of the structure, which is consistent with the results of first-principles calculations. The position of the Raman bands associated with the most intensive internal vibration modes of [SiO4]4- tetrahedra at 824 and 857 cm-1 and [PO4]3- tetrahedra at 981 cm-1 does not change with respect to phosphoran-free olivine and farringtonite, respectively. Their relative intensities in phosphoran olivine may reflect the Si/P ratio. The phosphoran olivine formation energy is dependent on the distance between [PO4]3- tetrahedra in the structure. With the use of first-principles calculations, it has been shown that neither pressure in the range from 0 to 8 GPa nor temperature in the range from 0 to 1500 K favors the equilibrium entry of phosphorus into olivine at concentrations equal to or exceeding 6.3 wt %.