Stacking Variations and Nonstoichiometry in the Bixbyite-braunite Polysomatic Mineral Group

Bixbyite, braunite, neltnerite, and braunite-II are members of a mineral group based on the cation arrangement of the fluorite structure type, with general formula M₈O₁₂ (M: Mn, Fe, Ca, Si). They consist of layer modules of (Mn,Fe)-O octahedra stacked in various sequencesth at depend on the coordination ofinterlayered M cation.

The structures of 2l possible layer assemblagesh ave been generated using stacking vectors derived from the known mineral varieties. These structures have been used to simulate the images obtained by electron microscopy.

Nonstoichiometry has been observed in braunites from several localities and is manifested by variations in their M²⁺ and Si contents. High-resolution transmission electron microscopy (HRTEM)revealed the presence of several layer assemblages of different thicknesses. In addition to the 2-module bixbyite, the 4-module braunite, and the 8-module braunite-Il, new 3-, 5-, and 8-module assemblagehsa ve been observed.T hesea ssemblages have calculated SiO₂, contents that vary between 0 and 10 wt/%.

Fine intergrowths of bixbyite (free of silica) and braunite-Il (5 wt%SiO₂) have also been observed by Hnrelr. These intergrowths consist of different amounts and thicknesses of the two components and account for nonstoichiometric variations in the M²⁺ and Si contents lower than those for braunite-II.