The arrojadite enigma: II. Compositional space, new members and nomenclature of the group.

A systematic chemical and structural investigation of arrojadite-group minerals has provided new data allowing us to design a consistent nomenclature scheme, approved by the IMA CNMMN (vote 05-D). The cornerstones of this scheme are: (1) the fundamental structural formula for the arrojadite group is A2 B2 Ca Na2+x M13 Al (PO4)11 (PO3OH1.x) W2, where A are either large divalent cations (Ba, Sr, Pb) plus vacancy, or monovalent (K, Na) cations, B are either small divalent cations (Fe, Mn, Mg) plus vacancy, or monovalent (Na) cations. (2) The dominant cation at the M sites deÞ nes the rootname: Fe2+, arrojadite; Mn2+, dickinsonite; Mg (if any), name to be proposed. (3) Two sufÞ xes are added to the root-name according to the dominant cation of the dominant valence state at the A1 and B1 sites (the mono- or divalent nature of the cation used implicitly speciÞ es the dominant occupancy by Na or the dominant vacancy, respectively, at the A2 and B2 sites, according to two heterovalent substitutions). (4) A third sufÞ x is added in case the sum of non-(P,Al) cations exceeds 20.5 apfu [which implies that the Na3 site is more than half occupied and the total (OH,F) content is less than 2.5 apfu]. (5) PreÞ xes may be added to the root-name in the case of dominance of F over OH at the W site or of Fe3+ over Al at the Al site. The compositional range explored is quite large, with either K, Sr, Ba, or Pb as the dominant cation at A1; Na, Fe, or Mn dominant at B1; Na or vacancies dominant at A2 and B2; Na or vacancy at the Na3 site; and F or OH at the W site. Lithium can amount up to 1 Li pfu and is partitioned into the M sites, preferentially M1. As a consequence of this new nomenclature scheme, the mineral name .sigismundite. is abolished and the corresponding composition must be referred to as arrojadite- (BaFe). In addition, arrojadites.dickinsonites from classic localities are identiÞ ed as arrojadite-(KFe) (Nickel Plate Mine), dickinsonite-(KMnNa) (Branchville), ß uorarrojadite-(BaFe) (Sidi-bou-Kricha), and we deÞ ne the new members arrojadite-(KNa) (Rapid Creek), arrojadite-(PbFe) (Sapucaia), and arrojadite-(SrFe) (Horrsjöberg).