Coexistence of Rutile and Trirutile Phases in a Natural Tapiolite Sample

We report X-ray powder diffraction (XRD), electron probe microanalysis (EPMA), and Mössbauer spectroscopy (MS) measurements performed on a natural tapiolite with composition Fe_0.57Mn_0.37Ti_0.10Ta_1.27Nb_0.67O₆. XRD and MS suggest that besides being partially ordered the as-collected sample is a mixture of trirutile (P4₂/mnm, a=4.7532(9) Å, c=9.228(7) Å) and Nb-rich rutile (P4₂/mnm, a=4.856(2) Å, c=3.098(1) Å) structures. The Mössbauer spectra of the rutile (Fe, Mn, Ta, Nb)O₂ were fitted to Δ=1.72±0.05 mm/s and δ=1.10±0.03 mm/s at 300 K and to Δ=2.10±0.06 mm/s and δ=1.18±0.03 mm/s at 80 K. The present results suggest that cation ordering in compounds of the tapiolite series can be easily assessed by Mössbauer spectroscopy in a way similar to that as previously demonstrated for the columbite series.