Nomenclature of wöhlerite-group minerals

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 X 8 (Si 2 O 7 ) 2 W 4 , where X = Na + , Ca 2+ , Mn 2+ , Ti 4+ , Zr 4+ and Nb 5+ ; and W = Fand O 2-. In addition, they may incorporate significant amounts of Mg 2+ , Fe 2+ , Y 3+ and REE 3+ , 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 Na 2 Ca 4 (Ca 0.5 Zr 0.5 )Zr(Si 2 O 7 ) 2 OF 3 , with one cationic site characterised by a valency-imposed double site-occupancy. These changes have been approved by the IMA-CNMNC (Proposal 20-D).

Pilanesbergite: a new rock-forming mineral occurring in nepheline syenite from the Pilanesberg Alkaline Complex, South Africa

“…Pilanesbergite is a new member of the wöhlerite group (Dal Bo et al, 2022). Its crystal structure is characterized by the so-called "octahedral walls" of four columns' width.…”

“…Pilanesbergite and related Fe-Mn-Ti-Zr silicate mineral. The four minerals pilanesbergite, normandite, madeiraite and låvenite are structurally and chemically related (Dal Bo et al, 2022). They have extensive mutual solubility, controlled by the two simple substitution mechanisms Fe 2+ −1 Mn in the X3 site and Ti −1 Zr in X1 (Fig.…”

“…Pilanesbergite is a new member of the wöhlerite group (Dal Bo et al, 2022) and is isomorphic and chemically related to låvenite (Na 2 Ca 2 Mn 2 Zr 2 (Si 2 O 7 ) 2 O 2 F 2 ) (Mellini, 1981), normandite (Na 2 Ca 2 Mn 2 Ti 2 (Si 2 O 7 ) 2 O 2 F 2 ) and madeiraite (Na 2 Ca 2 Fe 2 Zr 2 (Si 2 O 7 ) 2 O 2 F 2 ) (Mills et al, 2021). The name, pilanesbergite, is based on the name of the type locality, the Pilanesberg Complex.…”

Minerals, crystal structures and geochemistry

“…The minerals of the wöhlerite-group 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. The schematic and idealised representation of the cationic distribution in the walls of the minerals belonging to this group is presented (Dal Bo et al ., 2022).…”