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Hollandite Supergroup

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Hollandite Supergroup Eur. J. Mineral. 2013, 25, 85–90 Published online October 2012 Nomenclature tunings in the hollandite supergroup CRISTIAN BIAGIONI, CARMEN CAPALBO and MARCO PASERO* Dipartimento di Scienze della Terra, Universita` di Pisa, Via S. Maria 53, I-56126 Pisa, Italy *Corresponding author, e-mail: [email protected] Abstract: 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 Â 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 2þ 4þ 3þ 2þ 4þ 2þ þ 4þ 3þ minerals may be written as either A [M 6M 2]O16 (more rarely, A [M 7M ]O16)orA[M 7M ]O16 (more rarely, þ 4þ 2þ 2þ þ 4þ 3þ 2þ A [M 7.5M 0.5]O16), where A ¼ Pb, Ba, Sr; A ¼ K, Na; M ¼ Mn, Ti; M ¼ Mn, Fe, Cr, V; M ¼ Fe. The hollandite supergroup is divided into two groups depending on the dominant tetravalent cation in the octahedral walls: the coronadite group (M4þ ¼ Mn), and the priderite group (M4þ ¼ Ti). Two main considerations led to the preparation of this report: (i)M3þ (or M2þ) cations, even if they share the same site as M4þ, are essential for charge-balance, therefore each combination of dominant A2þ (or Aþ), M4þ, and M3þ (or M2þ) 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-Mn3þ end-member of the coronadite group; concurrently, type hollandite is redefined as ferrihol- 3þ landite, a new name to denote the Ba-Fe 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. Key-words: hollandite supergroup, coronadite group, priderite group, nomenclature. 1. Introduction cryptomelane, priderite), a clear definition of the end- member formula is lacking. As a consequence, minerals Minerals of the hollandite supergroup are structurally char- with different compositions are mentioned in the literature acterized by octahedral walls (2Â2 octahedra wide) cross- with the same name. For the sake of consistency within the linked to each other to build up a tunnel structure. Hence whole hollandite supergroup and in agreement with the these compounds are generally referred to as tunnel oxides general guidelines on mineral nomenclature, those miner- (Pasero, 2005). als should be regarded as distinct species. On chemical grounds, the hollandite supergroup can be Therefore, a re-definition of the end-member formulae, divided into two groups, based on the dominant tetravalent based on the original description of type material, is desir- sixfold-coordinated cation: the coronadite group (VIM4þ ¼ able. In fact, the chemical formulae so far accepted for Mn), and the priderite group (VIM4þ ¼ Ti).1 The nomen- hollandite, cryptomelane, and priderite may represent a clature of these minerals depends on both the dominant source of confusion, as they do not provide a clear identi- tunnel cation (Aþ or A2þ) and the dominant charge-com- fication of the dominant tunnel cation and the dominant pensating octahedral cation (M3þ, more rarely M2þ). charge-compensating cation. The rule that each combination of dominant tunnel The purpose of this report is to provide a consistent cation and dominant charge-compensating cation (here- nomenclature of the hollandite supergroup and to list a after DCCC) corresponds to a distinct mineral species has number of potentially new (not approved at the moment) been adopted as the reason for the approval by the IMA mineral species that can be identified on the basis of the CNMNC of some recent new minerals of the supergroup. general crystal-chemical rules. This has been recently formalized with the introduction of The report has been submitted to the CNMNC and the concept of ‘‘valence-imposed double site occupancy’’ received its complete approval. (Hatert & Burke, 2008). However, for some of the grand- fathered minerals of the supergroup (e.g., hollandite, 2. Basic structural features 1The names of the groups are after the oldest mineral among manganese and titanium oxides, which are coronadite and priderite, respectively. The name The ideal topological symmetry of 2Â2 tunnel oxides is of the supergroup is after the most widespread mineral, which is hollandite. tetragonal, space group I4/m (a 10.0, c 2.9 A˚ ), which is eschweizerbart_xxx 0935-1221/13/0025-2255 $ 2.70 DOI: 10.1127/0935-1221/2013/0025-2255 # 2012 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart 86 C. Biagioni, C. Capalbo, M. Pasero Table 1. Different unit-cell parameters given for the minerals of the hollandite supergroup. Name Space group Unit-cell parameters Ref. Coronadite group Coronadite I2/ma¼ 9.938, b ¼ 2.868, c ¼ 9.834 A˚ , b ¼ 90.39 Post & Bish (1989) (M4þ ¼ Mn) Hollandite I4/ma¼ 9.96, c ¼ 2.86 A˚ Bystro¨m & Bystro¨m (1950) ˚ P21/n a ¼ 10.02, b ¼ 5.76, c ¼ 9.89 A, b ¼ 90.60 Mukherjee (1960) I2/ma¼ 10.026, b ¼ 2.878, c ¼ 9.729 A˚ , b ¼ 91.03 Post et al. (1982) Cryptomelane I4/ma¼ 9.82, c ¼ 2.83 A˚ Richmond & Fleisher (1942) I2/ma¼ 9.79, b ¼ 2.88, c ¼ 9.94 A˚ , b ¼ 90.62 Mathieson & Wadsley (1950) Manjiroite I4/ma¼ 9.916, c ¼ 2.864 A˚ Nambu & Tanida (1967) ˚ Strontiomelane P21/na¼ 10.00, b ¼ 5.758, c ¼ 9.88 A, b ¼ 90.64 Meisser et al. (1999) Priderite group Priderite I4/ma¼ 10.139, c ¼ 2.966 A˚ Post et al. (1982) (M4þ ¼ Ti) Redledgeite I2/ma¼ 10.135, b ¼ 2.95, c ¼ 10.129 A˚ , b ¼ 91.03 Gatehouse et al. (1986) I4/ma¼ 10.150, c ¼ 2.952 A˚ Foley et al. (1997) ˚ Mannardite I41/aa¼ 14.357, c ¼ 5.908 A Szyman´ski (1986) I4/ma¼ 10.071, c ¼ 14.810 A˚ Bolotina et al. (1992) Ankangite P4/ma¼ 10.126, c ¼ 41.41 A˚ Shi et al. (1991) I4/ma¼ 10.142, c ¼ 2.953 A˚ Biagioni et al. (2009) Henrymeyerite I4/ma¼ 10.219, c ¼ 2.963 A˚ Mitchell et al. (2000) displayed, for instance, in priderite. In some cases, struc- In some of the older chemical analyses, total manganese is tural refinements have been carried out in monoclinic already split between MnO2 and MnO. However, it is (pseudo-tetragonal) space groups, with a c 10.0 A˚ , accepted today that the reduced state of Mn in all minerals 3þ 2þ and b 90 , such as I2/m (e.g., cryptomelane) or P21/n of the hollandite supergroup is Mn and not Mn (e.g., (e.g., strontiomelane). The lowering of symmetry may be Post et al., 1982). Therefore, those analyses should be related to minor framework distortions and/or to the order- recalculated accordingly. If the total positive charges are ing of cations and vacancies within the tunnels, which may less than 32, all Mn and Fe will be given in their higher give rise to structures with multiple periodicities and valence states. In those cases, charge-balance can be incommensurate structures as well (Table 1). achieved by minor substitution of O2– by (OH)À. Nickel & Grice (1998) state that: However, due to the overall charge-balance requirements, the above substitution is not assumed to play an important The polymorphic forms of a mineral are regarded as different role in any of the hollandite-supergroup minerals. Another species if their structures are topologically different. possible mechanism for balancing the chemical formula However, if the crystal structure of the polymorphs have could be represented by some excess tunnel cations. Since essentially the same topology, differing only in terms of a the periodicity along the tunnel is shorter than the ideal structural distortion or in the order – disorder relationship of cation–cation distance, usually the tunnel sites are occu- some of the atoms comprising the structure, such polymorphs are not regarded as separate species. pied approximately at the 50 % level, which gives one tunnel cation pfu, or a bit more; for example, Foley et al. Therefore, structural varieties in the hollandite supergroup (1997) suggest a maximum Ba occupancy of 1.33 atoms should not be considered as distinct mineralogical species. pfu in redledgeite. The ideal crystal chemical formula of the minerals of the hollandite supergroup depends on the valence state of both 3. Chemical composition and calculation of the the tunnel and charge-compensating cations, and should be ideal formulae written in one of the following ways: (1) if the dominant tunnel cation has charge 2þ: 2þ 4þ 3þ 2þ 4þ 2þ For the recalculation of the ideal chemical formulae from A [M 6M 2]O16 (or, more rarely, A [M 7M ] electron-microprobe analyses, in the lack of any direct O16); indication of the valence state of manganese or iron by (2) if the dominant tunnel cation has charge 1þ: þ 4þ 3þ þ 4þ spectroscopic methods, we suggest to formalize the A [M 7M ]O16 (or, more rarely, A [M 2þ method that has been adopted for several minerals of the 7.5M 0.5]O16). supergroup, i.e. to recalculate the analytical data on the In the above formulae, the species-forming cations so far basis of 8 total octahedral cations and 16 oxygen atoms per known among minerals are: A2þ ¼ Pb, Ba, Sr; Aþ ¼ K, formula unit (pfu).
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