The line connecting rare earth elements (REE) in chondrite-normalised plots can be represented by a smooth polynomial function using λ shape coefficients as described by O'Neill (2016). In this study, computationally generated λ combinations are used to construct artificial chondrite-normalised REE patterns that encompass most REE patterns likely to occur in natural materials. The dominant REE per pattern is identified, which would lead to its inclusion in a hypothetical mineral suffix, had this mineral contained essential REE. Furthermore, negative Ce and Y anomalies, common in natural minerals, are considered in the modelled REE patterns to investigate the effect of their exclusion on the relative abundance of the remainder REE. The dominant REE in a mineral results from distinct pattern shapes requiring specific fractionation processes, thus providing information on its genesis. Minerals dominated by heavy lanthanides are rare or non-existent, even though the present analysis shows that REE patterns dominated by Gd, Dy, Er and Yb are geologically plausible. This discrepancy is caused by the inclusion of Y, which dominates heavy REE budgets, in mineral name suffixes. The focus on Y obscures heavy lanthanide mineral diversity and can lead to various fractionation processes to be overlooked. Samarium dominant minerals are known, even though deemed unlikely by the computational model, suggesting additional fractionation processes that are not well described by λ shape coefficients. Positive Eu anomalies only need to be moderate in minerals depleted in the light REE for Eu to be the dominant REE, thus identifying candidate rocks in which the first Eu dominant mineral might be found. Here, I present an online tool, called ALambdaR that allows interactive control of λ shape coefficients and visualisation of resulting REE patterns.

Synthetic Eu-anorthite of the alkali feldspar structure type has been refined to Rw = 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 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 t1 = 0.47 and t2 = 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.