Spectroscopic and microscopic characterization of neodymium(3+) uptake by calcite

The local structure of metal impurity ions in minerals is important in determining their long-term stability in the environment. It is particularly important for understanding or predicting the migration of nuclear waste plumes in the subsurface. Calcite doped with Nd3+ ions is chosen as a model system. Calcite micro-crystals were grown from solution with single crystal dimensions up to 3 mm and doped up to ∼0.1 atomic % with Nd 3+ ions. Several methods are applied to determine the structure and doping of the crystals. Phase purity was verified by powder x-ray diffraction. The concentration of Nd3+ was measured by energy-dispersive spectrometry and Rutherford backscattering spectrometry. Micro x-ray fluorescence mapping of the calcite grains indicates uniform Nd distribution in as-grown crystal grains. X-ray absorption fine structure suggests that Nd3+ is substituted for Ca2+ with local lattice dilation. The assumption of a simple local structure in the analysis of XAFS data has lead to the conclusion that there are 7 oxygen atoms in the first coordination shell of Nd3+ in calcite. However, temperature dependent near-infrared spectroscopy of Nd3+ impurities in calcite reveals large inhomogeneous linewidths and smooth line profiles that are characteristic of glassy hosts, even though the samples are well crystallized. This indicates that the assumption of a simple local structure is poor. Comparison of absorption lines with reference Neodymium compounds confirms that Nd3+ in calcite has a local structure similar to that of amorphous materials, though the calcite is well crystallized. This comparison also reveals new information about the metal-ligand bonding. Comparison of the Nd3+:CaCO₃ covalency factor with reference compounds suggests that Nd3+ is actually nine-fold coordinated. The lattice dilation observed around the metal impurity ions in calcite suggests large local strain possibly relieved by dislocations. Both of these are associated with increased mineral solubility and decreased long-term stability. Surface adsorption properties of calcite powder and of large, natural calcite single crystals are compared to show that the abundant surface defects inherent in powder significantly increase the uptake of Nd 3+.