THE USE OF END-MEMBER CHARGE-ARRANGEMENTS IN DEFINING NEW MINERAL SPECIES AND HETEROVALENT SUBSTITUTIONS IN COMPLEX MINERALS

Abstract

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 Fe-3(3+) Fe-6(3+) (Si-6 O-18) (BO3)(3) (O,OH)(4), whereas the correct end-member formula is Na Fe-3(3+) (Fe-4(3+) Mg-2) (Si-6 O-18) (BO3)(3) (OH)(3) O. The yttrium-rich milarite described by Cerny et al. (1991) is shown to be a new mineral of the milarite group, with the end-member formula K (CaY) [Be-3 (Si-12 O-30)]. 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) Ba-4 Ca-2 Si-8 (BeB) (Si)(2) O-28 F, and (2) Ba-4 Ca-2 Si-8 (B-2) (SiB) O-28 F; note that one end-member has essential Be and the other end-member does not. Thus the competing views on whether hyalotekite is a Be mineral or not are both correct, and there are two distinct minerals buried in the chemical compositional data for hyalotekite. "Makarochkinite" is a non-accredited phase described by Yakubovich et al. (1990) that has been identified as identical to hogtuvaite by Grauch et al. (1994). Application of the approach developed here shows that. according to the published chemical data. "makarochkinite" and hogtuvaite have distinct end-member formulae. Ca-2 (Fe-4(2+) Fe3+ Ti) (Si-4 Be Al) O-20 and Ca-2 (Fe-4(2+) Fe-2(3+)) (Si-5 Be) O-20, respectively. These examples show that the very simple approach developed here, using the characteristics of an end-member formula with aspects of the crystal structure, can simply and easily resolve many quite complicated problems in mineral chemistry.