Nomenclature of wöhlerite group minerals

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Marco E. Ciriotti
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Nomenclature of wöhlerite group minerals

Messaggio da Marco E. Ciriotti » lun 24 gen, 2022 11:28

Prossima pubblicazione.

Referenza:
▪ Dal Bo, F., Friis, H., Mills, S.J. (2022): Nomenclature of wöhlerite group minerals. Mineralogical Magazine, 86, (in press).

Abstract:
A 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, that 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 endmember 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 endmembers of a solid-solution series. Marianoite is discredited, as it is corresponding 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).
Marco E. Ciriotti

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Marco E. Ciriotti
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Re: Nomenclature of wöhlerite group minerals

Messaggio da Marco E. Ciriotti » lun 24 gen, 2022 12:00

La nuova specie madeiraite, Na2Ca2Fe2+2Zr2(Si2O7)2O2F2, monoclina, si aggiunge a quelle indicate a formare il wöhlerite group (Z = 2).

Baghdadite Ca6Zr2(Si2O7)2O4
Burpalite Na4Ca2Zr2(Si2O7)2F4
Cuspidine Ca8(Si2O7)2F4
Hiortdahlite Na2Ca4(Ca0.5Zr0.5)Zr(Si2O7)2OF3
Janhaugite Na3Mn3Ti2(Si2O7)2(OH)2OF
Låvenite Na2Ca2Mn2Zr2(Si2O7)2O2F2
Madeiraite Na2Ca2Fe2+2Zr2(Si2O7)2O2F2
Moxuanxueite NaCa6Zr(Si2O7)2OF3
Niocalite Ca7Nb(Si2O7)2O3F
Normandite Na2Ca2Mn2Ti2(Si2O7)2O2F2
Wöhlerite Na2Ca4ZrNb(Si2O7)2O3F

NdR: Il lavoro è interessante. Molto utili i criteri per l'identificazione delle diverse specie.
Marco E. Ciriotti

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