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

Boron compounds are included in the family of mullite-type structures if they contain chains of edge-sharing M O 6 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 Q r = 8.2 % (PbCrBO 4 ) and 13.8 % (PbMnBO 4 , for 2:1 mullite it is 9.9 %), and a maximum deviation from tetragonal metric ( a = b , Q a = 1) with Q a = 0.78 (PbMnBO 4 ; for 2:1 mullite Q a = 0.986). The average value of all mean B–O distances in BO 3 groups is 1.383 A, which compares well with known trigonally coordinated B–O distances. It is recommended to designate all boron compounds with the characteristic mullite-type M O 4 chains of M O 6 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 Al 5 BO 9 -like modules.