Quantifying Mg–Al cation distribution in MgAl2O4-spinel using Raman spectroscopy: An experimental calibration

Abstract

Raman spectroscopy, an ideal technique to quantify the Mg–Al cation distribution on the tetrahedral T-sites and octahedral M-sites of MgAl₂O₄-spinel, was not calibrated before. By performing 16 annealing experiments on some MgAl₂O₄-spinel crystal plates at P from 1 atm to 5.0 GPa and T from 823 to 1873 K, a series of samples with different magnitudes of cation disorder (characterized by the inversion parameter x, i.e., the molar fraction of Al cations on the T-sites) have been generated. Single-crystal X-ray diffraction analyses have been performed to all these samples, and found the x values varying from 0.146 (15) to 0.362 (17). Multiple Raman spectra have been collected and analyzed for every sample. The Raman results show that the Raman scattering capabilities of the Al cations and the Mg cations on the T-sites are different, and their ratios are dependent on the Mg–Al cation distributions (i.e., x). With our extensive single-crystal X-ray diffraction data and Raman data, correlations between the x values and the relative Raman intensities of the ∼766 and ∼722 cm⁻¹ peaks (I_∼766/I_∼722 ratio), respectively caused by the internal vibrational modes of the MgO₄ and AlO₄ groups, have been established: the equations are x = 0.121 × log₁₀²(I_∼766/I_∼722) − 0.344 × log₁₀(I_∼766/I_∼722) + 0.392 (R² = 0.927; Raman peak height data used) and x = 0.069 × log₁₀²(I_∼766/I_∼722) − 0.253 × log₁₀(I_∼766/I_∼722) + 0.379 (R² = 0.928; Raman peak area data used). With this calibration, Raman spectroscopy can now be conveniently used to determine the x values of the MgAl₂O₄-spinel of different geological origins, significantly facilitating the inference of the thermal history of relevant geological bodies.