The identification of the crystal-chemical parameters controlling trace-element partitioning between minerals and silicate melts is a topic of remarkable interest. Garnets represent an important group of rock-forming minerals in this regard, able to incorporate several trace elements commonly used in geochemical modeling. For these reasons, aluminosilicate garnets have received much attention, regarding either their complex crystal-chemistry, or thermodynamic properties, or trace-element behaviour. Despite the importance of the geochemical behaviour of REE in garnets, little work has been done to decipher their site preference and local coordination in trace or minor amounts. Conventional diffraction methods cannot provide direct information at such low concentration levels. In this work natural melanite garnets from carbonatitic rocks have been studied by a multi-technique approach based on both experimental (chemical, diffractometric and spectroscopic) methods and full-multiple scattering calculations of the X-ray absorption near edge structure. In particular, the site location and geometry of trace amounts of neodimium (from 176 to 1029 ppm) and cerium (791 ppm) in natural garnets have been studied by fluorescence-detected X-ray Absorption Fine-Structure Spectroscopy (XAFS) at high energy. The measurements, done at both Nd K- (43569 eV) and Ce K- (40443 eV) edges, demonstrate that, in all the samples, the trace elements are located in the dodecahedral X site. The local geometry around the two rare earth elements is compatible with their ionic radius and is compared with that of Ca, the major element at the X site, as determined by single-crystal X-ray diffraction data. This work represents the first example of direct investigation of trace-level REE coordination in natural garnets, and confirms the great relevance for the Earth Sciences of the use of fluorescence XAFS at high energy.
Characterization of trace Nd and Ce site preference and coordination in natural garnets: a combined X-ray diffraction and high-energy XAFS study
Alberto Zanetti
2002
Abstract
The identification of the crystal-chemical parameters controlling trace-element partitioning between minerals and silicate melts is a topic of remarkable interest. Garnets represent an important group of rock-forming minerals in this regard, able to incorporate several trace elements commonly used in geochemical modeling. For these reasons, aluminosilicate garnets have received much attention, regarding either their complex crystal-chemistry, or thermodynamic properties, or trace-element behaviour. Despite the importance of the geochemical behaviour of REE in garnets, little work has been done to decipher their site preference and local coordination in trace or minor amounts. Conventional diffraction methods cannot provide direct information at such low concentration levels. In this work natural melanite garnets from carbonatitic rocks have been studied by a multi-technique approach based on both experimental (chemical, diffractometric and spectroscopic) methods and full-multiple scattering calculations of the X-ray absorption near edge structure. In particular, the site location and geometry of trace amounts of neodimium (from 176 to 1029 ppm) and cerium (791 ppm) in natural garnets have been studied by fluorescence-detected X-ray Absorption Fine-Structure Spectroscopy (XAFS) at high energy. The measurements, done at both Nd K- (43569 eV) and Ce K- (40443 eV) edges, demonstrate that, in all the samples, the trace elements are located in the dodecahedral X site. The local geometry around the two rare earth elements is compatible with their ionic radius and is compared with that of Ca, the major element at the X site, as determined by single-crystal X-ray diffraction data. This work represents the first example of direct investigation of trace-level REE coordination in natural garnets, and confirms the great relevance for the Earth Sciences of the use of fluorescence XAFS at high energy.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
