Atomistic-scale structural and dynamic properties of almandine-spessartine solid solutions: an X-ray absorption fine structure study at the Fe and Mn K-edges

X-ray absorption fine structure (XAFS) measurements at the Fe and Mn K-edges were undertaken to investigate the Fe2+-O and Mn2+-O bonding and Fe2+- and Mn2+-vibrational properties for a series of synthetic almandine-spessartine, (Fe, Mn)3Al2Si3O12, garnet solid solutions. The two end members almandine, Fe3Al2Si3O12, and spessartine, Mn3Al2Si3O12, and three solid solutions of composition Alm75Sps25, Alm50Sps50 and Alm25Sps75 were studied at different temperatures between 77 K and 423 K. The spectra show that the state of alternating bonds provides an appropriate description for the two crystallographically independent X-O(1) and X-O(2) bonds in the solid solution. An exchange of X-site cations in garnet is associated with measurable structural relaxation. The compositional and temperature dependence of the XAFS Debye-Waller factors for Fe2+ and Mn2+ in the plane of the X-O(1) and the X-O(2) bonds were also determined. Their values in the plane of the X-O(2) bonds are greater compared to those for X-O(1) and they do not vary greatly as a function of garnet composition. In the case of end-member spessartine, the XAFS Debye-Waller factors for Mn2+ are compared to the Debye-Waller factors measured from single-crystal X-ray diffraction. An analysis shows correlation in the atomic displacements along the shorter Mn-O(1) bond.