Original paper

Crystal chemistry of borates and borosilicates with mullite-type structures: a review

Fischer, Reinhard X.; Schneider, Hartmut


Boron compounds are included in the family of mullite-type structures if they contain chains of edge-sharing MO6 octahedra (M are the octahedrally coordinated cations in the chains) similar to those in mullite. The crystal structures of all boron compounds in the mullite family are derived from a tetragonal aristotype and should obey the criteria defining the limits for the deviation from the root structure. The compounds complying with these criteria for mullite-type crystal structures have an observed maximum deviation from orthogonality by Δγ′ = 4° (ominelite: γ′ = 86.0°, mullite: 90°), a range of octahedral tilting angles ω between 61.5° (boromullite) and 90° (for 2:1 mullite it is 59.8°), a range of distances between neighboring chains normalized to the ionic radii of the central atoms between Qr = 8.2 % (PbCrBO4) and 13.8 % (PbMnBO4, for 2:1 mullite it is 9.9 %), and a maximum deviation from tetragonal metric (a = b, Qa = 1) with Qa = 0.78 (PbMnBO4; for 2:1 mullite Qa = 0.986). The average value of all mean B-O distances in BO3 groups is 1.383 Å, which compares well with known trigonally coordinated B-O distances. It is recommended to designate all boron compounds with the characteristic mullite-type MO4 chains of MO6 octahedra as "mullite-type boron compounds" and to use the term "boron-mullite" or "B-mullite", initially introduced by Werding & Schreyer (1984) (Geochim. Cosmochim. Acta, 48, 1331-1344) for the subgroup of Al borates and Al borosilicates with mullite-type structures. The name "boromullite" is reserved for a natural mineral with sillimanite-like and Al5BO9-like modules.


mullite-type structuresboratesaluminum borates"boron-mullites"al2o3-b2o3-sio2