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

¹ Laboratoire de Géologie, UMR 8538 du CNRS, Ecole normale supérieure, 24 rue Lhomond, 75005 Paris, France
² CNR Istituto di Geoscienze e Georisorse (IGG), unità di Pavia, via Ferrata 1, 27100 Pavia, Italy

Correspondence: ^* E-mail: chopin{at}geologie.ens.fr

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 Na_2+x M₁₃ Al (PO₄)₁₁ (PO₃OH_1–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 defines the root-name: Fe²⁺, arrojadite; Mn²⁺, dickinsonite; Mg (if any), name to be proposed. (3) Two suffixes 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 specifies the dominant occupancy by Na or the dominant vacancy, respectively, at the A2 and B2 sites, according to two heterovalent substitutions). (4) A third suffix 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) Prefixes 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 identified as arrojadite-(KFe) (Nickel Plate Mine), dickinsonite-(KMnNa) (Branchville), fluorarrojadite-(BaFe) (Sidi-bou-Kricha), and we define the new members arrojadite-(KNa) (Rapid Creek), arrojadite-(PbFe) (Sapucaia), and arrojadite-(SrFe) (Horrsjöberg).

Key Words: Crystal structure • arrojadite • analysis • chemical (mineral) • new minerals • arrojadite-(PbFe) • fluorarrojadite-(BaFe) • arrojadite-(BaFe) • optical properties • XRD data

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