What is CCZN-armalcolite? A crystal-chemical discussion and an ad-hoc incursion in the crichtonite-minerals group

Abstract

The status of CCZN-armalcolite, commonly still believed to be a variety of armalcolite s.s., is questionable in view of partial evidence suggesting that it would represent a distinct phase, as initially claimed by its discoverers. Because of the rarity of the mineral, combined with its habitual small grain-size, no successful structural investigation could be undertaken so far. Therefore we attempted a chemical overview of existing and original data pertaining to CCZN-armalcolite. With the view of systematizing structure-composition relationships, we analyzed the topology of large cations-bearing close-packed oxides, while extending the nomenclature in use in order to accommodate further topologies. By using relevant chemical plots, a fair compositional match between CCZN-armalcolite and the crichtonite group minerals was demonstrated, as well as a chemical incompatibility with armalcolite. Stoichiometric crichtonite compositions and matching optical properties allowed identification of at least part of "CCZN-armalcolites" with the mineral loveringite of the crichtonite group. A detailed inspection of structure-composition relationships in crichtonites allowed an insight in their trends and range of chemical variation, as well as a comparison between them and a group of "CCZN-armalcolites" slightly differing from known crichtonites. These "CCZN-armalcolites" departing from normal chemical trends in crichtonites either represent an ordered variety of non-stoichiometric crichtonite or a new group of close-packed oxides. Their composition clustering around formula AM 16 O 30 is temptingly consistent with an hypothetical structure intermediate between magnetoplumbites and crichtonites, based on close-packed stacking of layers made up by triangular clusters of octahedra, stuffed with large cations. A model of such a structure, not encountered so far in minerals, is outlined, displaying a hexagonal symmetry P#, with a ≈ 7.45Å and c ≈ 9.2Å.