Abstract
AbstractA nomenclature and classification scheme for wöhlerite-group minerals has been established. The general formula of minerals belonging to this group is given by X8(Si2O7)2W4, where X = Na+, Ca2+, Mn2+, Ti4+, Zr4+ and Nb5+; and W = F– and O2–. In addition, they may incorporate significant amounts of Mg2+, Fe2+, Y3+ and REE3+, where REE are the lanthanides. The main structural feature of these minerals is the four-columns-wide octahedral walls, which are interconnected through corner sharing and via the disilicate groups. The wöhlerite-group minerals crystallise in different unit-cell settings and symmetries, depending on the cationic ordering in the octahedral walls and the relative position of the disilicate groups. Different combinations of X and W constituents should be regarded as separate mineral species. In the case of coupled heterovalent substitutions at different crystallographic sites, it is advised to use the site-total charge approach to determine the correct end-member composition. Due to their structural and chemical features, wöhlerite-group minerals can easily form crystals with several micro domains, showing different crystal structures and chemical compositions. In addition, the crystallisation of polytypes is relatively common, although they should not be regarded as distinct mineral species. To date, ten minerals belonging to the wöhlerite group are considered as valid species: baghdadite, burpalite, cuspidine, hiortdahlite, janhaugite, låvenite, moxuanxueite, niocalite, normandite and wöhlerite. Låvenite and normandite are isostructural and are respectively the Zr and Ti end-members of a solid-solution series. Marianoite is discredited, as it is corresponds to wöhlerite. The ideal formula of hiortdahlite is revised as Na2Ca4(Ca0.5Zr0.5)Zr(Si2O7)2OF3, with one cationic site characterised by a valency-imposed double site-occupancy. These changes have been approved by the IMA–CNMNC (Proposal 20–D).
Highlights
The first mention of wöhlerite in the literature was made by Scheerer (1843) who studied the mineralogical paragenesis of the syenite pegmatites occurring on Løvøya island, Brevig area, Langesundsfjord, Norway
Among the wöhlerite-group minerals, cuspidine was the first to have its crystal structure solved (Smirnova et al, 1955) and the wöhlerite group is sometimes mentioned as the cuspidine group in the literature (e.g. Merlino and Perchiazzi, 1988; Chakhmouradian et al, 2008)
In order to define the correct endmember formula of hiortdahlite new chemical data was collected and a crystal structure refinement was performed on a sample from the type locality, Langodden, Langesundsfjord, Norway
Summary
The first mention of wöhlerite in the literature was made by Scheerer (1843) who studied the mineralogical paragenesis of the syenite pegmatites occurring on Løvøya island, Brevig area, Langesundsfjord, Norway. Merlino and Perchiazzi (1988) demonstrated that the nature of the crystal structure of the wöhlerite group minerals (WGM) permits the crystallisation in different unit-cell settings and the formation of polytypes. They identified ten different structure-types that are possible within the fixed cell dimension a ≈ b ≈ 10.5 Å and c ≈ 7.3 Å. The general formula proposed for WGM is similar to that of the rinkite group (seidozerite supergroup) minerals (Sokolova and Cámara, 2017), but rinkite- and wöhlerite-group minerals have different structures. The crystal structure of the borate minerals warwickite and yuanfuliite show the same type of framework, with isolated triangular BO3 groups replacing the disilicate groups (Bigi et al, 1991; Appel et al, 1999)
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