Nomenclature tunings in the hollandite supergroup
Biagioni, Cristian; Capalbo, Carmen; Pasero, Marco
published: Feb 1, 2013
Open Access (paper can be downloaded for free)
The hollandite supergroup includes a number of manganese (IV) and titanium oxides, often referred to as tunnel oxides due to their structural features, i.e. octahedral walls, 2 × 2 octahedra wide, cross-linked to each other to build up a tunnel structure. Tunnels host mono- and divalent cations, and water molecules. Based on the nature of the tunnel cation, the generic formula of these minerals may be written as either A2+[M4+6M3+2]O16 (more rarely, A2+ [M4+7M2+]O16) or A+ [M4+7M3+]O16 (more rarely, A+ [M4+7.5M2+0.5]O16), where A2+ = Pb, Ba, Sr; A+ = K, Na; M4+ = Mn, Ti; M3+ = Mn, Fe, Cr, V; M2 + = Fe. The hollandite supergroup is divided into two groups depending on the dominant tetravalent cation in the octahedral walls: the coronadite group (M4 + = Mn), and the priderite group (M4+ = Ti). Two main considerations led to the preparation of this report: (i) M3+ (or M2+) cations, even if they share the same site as M4+, are essential for charge-balance, therefore each combination of dominant A2+ (or A+), M4 +, and M3 + (or M2 +) cations corresponds to a distinct species; (ii) the presence/absence of “zeolitic water in the tunnels should not represent the discriminant between two species. Based on these guidelines, our main actions have been the following: hollandite is redefined as the Ba-Mn3 + end-member of the coronadite group; concurrently, type hollandite is redefined as ferrihollandite, a new name to denote the Ba-Fe3+ end-member; ankangite is discredited, as a H2O-free variety of mannardite; the ideal end-member formulae of all known minerals of the hollandite supergroup are defined; six potentially new mineral species in the hollandite supergroup are envisaged. This report has been approved by the IMA CNMNC.