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Revised nomenclature of högbomite, nigerite, and taaffeite minerals

Laboratorium für chemische und mineralogische Kristallographie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland,

e-mail: thomas.armbruster{at}krist.unibe.ch

Minerals of the högbomite, nigerite, and taaffeite groups form polysomatic series composed of spinel (S) and nolanite (N) modules. The idealised formula of a spinel module is T₂M₄O₈ where T and M represent tetrahedrally and octahedrally coordinated cations. The nolanite module in nigerite and högbomite minerals has the formula TM₄O₇(OH) whereas the nolanite module in the Bebearing taaffeite minerals is modified (N’) and has the formula BeTM₄O₈. The modules are normalised to two closest-packed oxygen layers situated parallel to (001) in hexagonal setting. The composition of these minerals thus depends: (1) on the composition of the nolanite module, (2) on the composition of spinel module, (3) on the number of spinel and nolanite modules forming the structure. Högbomite- and nigerite-group minerals are distinguished by the dominant tetravalent cation in the nolanite module. If the molar concentration of Ti > Sn, the group name is högbomite. If the molar concentration of Sn > Ti, the group name is nigerite. Taaffeite-group minerals have a nolanite module without OH groups but with additional tetrahedral Be.

In the new nomenclature, approved by the IMA CNMMN, the subgroup name is chosen according to the composition of the aluminium-spinel module. If the spinel module is dominated by the gahnite component, ZnAl₂O₄, the prefix ‘zinco’ is used. If the spinel module is dominated by the hercynite component, FeAl₂O₄, the prefix ‘ferro’ is used. If the spinel module is dominated by the spinel component, MgAl₂O₄, the prefix ‘magnesio’ is used.

To characterise the various polysomes found for each subgroup a hyphenated suffix composed of the total number of nolanite (N) and spinel (S) modules is attached. The module symbols (N, S) are italicised and given in the sequence first N and then S. According to this new nomenclature pengzhizhongite-6T and pengzhizhongite-24R are replaced by magnesionigerite-2NIS and magnesionigerite-6N6S, respectively. Furthermore, magnesiotaaffeite-6N’3S and ferrotaaffeite-6N’3S replace musgravite and pehrmanite, respectively.

Key-words: högbomite, nigerite, taaffeite, polysomatism, crystal structure, chemistry, nomenclature.

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