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

Abstract Synthetic Eu-anorthite of the alkali feldspar structure type has been refined to R w = 4.7% using 3-D counter diffractometer data and full-matrix least-squares methods. The chemical composition of the feldspar is Eu 0.92 Al 1.76 Si 2.24 O 8 , based on both occupancy refinement of the Eu atom site and estimation of the Al/Si distribution calculated from the mean T-O bond length. The unit cell parameters are a = 8.373(1), b = 12.959(1), c = 7.124(1) Å, and β = 115.51(1)° and the symmetry is enhanced to C2/m . Mean bond lengths are T (1)-O = 1.677 Å, T (2)-O = 1.668 Å, and Eu-O = 2.721 Å. The average Al/Si distribution over the T (1) and T (2) sites calculated from the mean T-O bond length is in fairly good agreement with an estimate of the Al content from the bond strength calculation; the Al partition is calculated as t 1 = 0.47 and t 2 = 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 Eu 2+ and Eu 3+ 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.