A fast and easy-to-use approach to cation site assignment for eudialyte-group minerals

摘要

Eudialyte-group minerals (EGM) are typical constituents of agpaitic varieties of peralkaline rocks. In their complex structure (N_(15_l6)M(l)_6M(2)_3Z_3M(3)M(4)Si_(24)O_(66_73)(OH)_(0_9)X_2), many cations (e.g. Na~+, Ca~(2+), Fe~(2+), Mn~(2+), REE~(3+), Zr~(4+), and Si~(4+)) as well as different hydrogen-bearing species (H_2O, OH~-, H_3O~+) may occupy different structural sites. Also, two potentially vacancy bear-ing positions are present. Thus, various methods of calculation of mineral formulae for EGM in the literature are inconsistent and in some cases not charge-balanced. We present an extended and improved scheme for site assignment using IMA-approved end-members and taking into account the different structural units of EGM. This method is based on electron microprobe analyses alone not considering different valence states of Fe and Mn and undetermined H_2O-contents. However, comparison with structural refinement data from the literature re-veals major agreement and significant improvement compared to earlier proposed methods. The instruction given here can easily be transferred to a table calculation spread sheet (e.g. EXCEL0), which is available from the corresponding author on request.