The Use of End-Member Charge-Arrangements in Defining New Mineral Species and Heterovalent Substitutions in Complex Minerals

The characteristics of an end-member formula are defined as follows: (1) the chemical formula must be fixed; no variable chemical components are possible; (2) the end-member formula must be compatible with the crystal structure of the mineral (or putative mineral); (3) the chemical composition at each site in the crystal structure must be fixed; an end-member formula may have two types of cation or anion (in a fixed ratio) at one site in the structure if required for electroneutrality; two cations or anions at more than one site are not allowed. Combining these characteristics of end-member formulae with aspects of their crystal structures can lead to unambiguous definition of end-member compositions of complex minerals, and can give considerable insight into coupled heterovalent substitutions. Several examples are given. The end-member formula of the tourmaline-group mineral povondraite was originally written as Na Fe3+3 Fe3+6 (Si6 O18) (BO3)3 (O,OH)4, whereas the correct end-member formula is Na Fe3+3 (Fe3+4 Mg2) (Si6 O18) (BO3)3 (OH)3 O. The yttrium-rich milarite described by Černy ́ et al. (1991) is shown to be a new mineral of the milarite group, with the end-member formula K (CaY) [Be3 (Si12 O30)]. There are seven accredited [4](Li,Zn)-bearing minerals of the milarite group, and there has been some ambiguity over the end-member compositions of darapiosite, dusmatovite and sogdianite; end-member formulae for these minerals are unambiguously defined with the approach used here. The complex Be-bearing borosilicate mineral hyalotekite has been somewhat elusive with regard to unambiguous definition of its solid-solution behavior. The ideal formula of Christy et al. (1998) can be resolved into two distinct end-member formulae: (1) Ba4