Titre du document / Document title

The role of the coordination defect : A new structural description of four fluorite-related sesquioxide minerals, bixbyite (Mn₂O₃), braunite (Mn₇SiO₁₂), braunite II (CaMn₁₄SiO₂₄), parwelite (Mn₁₀Sb₂As₂Si₂O₂₄), and their structural relationships

Auteur(s) / Author(s)

BEVAN D. J. M. ⁽¹⁾ ; MARTIN R. L. ⁽²⁾ ;

Affiliation(s) du ou des auteurs / Author(s) Affiliation(s)

⁽¹⁾ School of Chemistry, Physics and Earth Sciences, The Flinders University of South Australia, Bedford Park, South Australia, AUSTRALIE
⁽²⁾ School of Chemistry, Monash University, Clayton, Victoria, AUSTRALIE

Résumé / Abstract

The anion-deficient, fluorite-related structures of the manganese-based minerals bixbyite (Mn₂O₃), braunite (Mn₇SiO₁₂), braunite II (CaMn₁₄SiO₂₄) and parwelite (Mn₁₀Sb₂As₂Si₂O₂₄) are reinterpreted in terms of the coordination defect (CD) theory to gain new insights into their structural interrelationships. CDs are extended, octahedral defects centred by an anion vacancy and including its immediate atomic environment: it is represented as □ M₄O₆, where the symbol □ is the anion vacancy. The bixbyite motif is a CD dimer (two edge-sharing octahedra), and this motif repeats, by further edge-sharing, around the 2-fold screw axes of the cubic structure. These same dimers are present in each of the other structures, but the presence of Si⁴⁺ in braunite and braunite II, together with that of other foreign cations such as As⁵⁺ and Sb⁵⁺ in parwelite, leads to different juxtapositions of these motifs. Moreover, the structure of braunite, Mn²⁺(Mn³⁺)₆SiO₁₂, reflects the clustering of 12 Mn³⁺-centred octahedra (MnO₆) around a central SiO₄ tetrahedron to generate a structure for the [(Mn³⁺)₆SiO₁₂]^2- anion which is almost identical to that of the well-known cuboctahedral structure of the PO₄-centred heteropolytungstate anion, [(W⁶⁺)₁₂PO₄₀]^3-. The structure of braunite II, [Ca(Mn³⁺)₁₄SiO₂₄], is simply an intergrowth of slabs of bixbyite- and braunite-type structures, linked by the CaO₈ cubes of the latter. Our various analyses of the reported structure of parwelite in terms of the only possible vacancy assignment have led to some apparent anomalies. We report briefly on these, and have decided to seek confirmation of the reported structure as a consequence. Despite the increasing complexity of these structures, there are clear and defining relationships in the distribution of CDs. The assumption of a close relationship to the fluorite parent in all these structures is based on the observation that the cation sub-lattices are essentially face-centred cubic, with the anions in the tetrahedral sites, so there is little variation from this between one structure and another. The cation contents, however, are very different in the four structures discussed here - a single cation species in bixbyite, two in the braunites and four in parwelite. This factor, and the topology of the CD arrangements, are structure-determining and confirm the close relationships between these four minerals.

Revue / Journal Title

Journal of solid state chemistry   ISSN 0022-4596   CODEN JSSCBI 

Source / Source

2008, vol. 181, n^o9, pp. 2250-2259 [10 page(s) (article)] (44 ref.)

Langue / Language

Anglais

Editeur / Publisher

Elsevier, San Diego, CA, ETATS-UNIS  (1969) (Revue)

Mots-clés anglais / English Keywords

Bixbyite ; Cubic lattices ; Cadmium sulfide ; Manganese oxides ; Crystal structure ; Topology ; Crystallographic site ; Sublattices ; Intergrowth ; Dimers ; Atomic arrangement ; Vacancies ; Extended defects ; Cadmium ; Fluorite structure ; Crystal defects ;

Mots-clés français / French Keywords

Bixbyite ; 6166F ; Mn10Sb2As2Si2O24 ; Mn7SiO12 ; CaMn14SiO24 ; Mn2O3 ; Réseau cubique ; Sulfure de cadmium ; Oxyde de manganèse ; Structure cristalline ; Topologie ; Site cristallographique ; Sous réseau ; Intercroissance ; Dimère ; Arrangement atomique ; Lacune ; Défaut étendu ; Cadmium ; Structure fluorite ; Défaut cristallin ;

Mots-clés espagnols / Spanish Keywords

Bixbyita ; Cadmio sulfuro ; Manganeso óxido ; Sitio cristalográfico ; Intercrecimiento ; Arreglo atómico ; Estructura fluorita ;

Mots-clés d'auteur / Author Keywords

Anion-deficient fluorite-related structures ; Sesquioxides ; Bixbyite ; Braunite ; Braunite II ; Parwelite ; Coordination defect ;

Localisation / Location

INIST-CNRS, Cote INIST : 14677, 35400018566324.0200