The crystal structure of non-stoichiometric Eu-anorthite: an explanation of the Eu-positive anomaly

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Abstract
Synthetic Eu-anorthite of the alkali feldspar structure type has been refined toRw= 4.7% using 3-D counter diffractometer data and full-matrix least-squares methods. The chemical composition of the feldspar is Eu0.92Al1.76Si2.24O8, based on both occupancy refinement of the Eu atom site and estimation of the Al/Si distribution calculated from the meanT-Obond length. The unit cell parameters area= 8.373(1),b= 12.959(1),c= 7.124(1) Å, and β = 115.51(1)° and the symmetry is enhanced toC2/m. Mean bond lengths areT(1)-O = 1.677 Å,T(2)-O = 1.668 Å, and Eu-O = 2.721 Å. The average Al/Si distribution over theT(1) andT(2) sites calculated from the meanT-Obond length is in fairly good agreement with an estimate of the Al content from the bond strength calculation; the Al partition is calculated ast1= 0.47 andt2= 0.41 respectively. Summing the bond strengths of these Eu and partially disordered Al/Si cations approximates to electrostatic neutrality for the anion content of the feldspar structure, indicating that this synthetic Eu feldspar can be non-stoichiometric, signifying vacancies on the alkali cation site.Plagioclase and melilite generally show a positive Eu anomaly. A fair insight into the driving force of this anomaly can be afforded by the crystallo-chemical affinities of Eu2+and Eu3+cations to the crystal structures of their host minerals; (1) ioinic radius, (2) electrostatic charge balance and (3) tolerance for non-stoichiometry of the crystal structure.

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