Aerinite: distribution of the highly distorted BAl site along the column of face-sharing octahedra at (x = 2/3, y = 1/3). For a given BAl position the refined distances to the six corners of the octahedron are 1.64 (3), 1.68 (3), 2.13 (4), 2.24 (4), 2.59 (4) and 2.60 (4) Å. The only possible arrangement explaining this result is the formation of Al domains where the O ligands labelled with 1 in the figure are bound to one Al (bond strength ∼1 v.u.) and to one Si of the pyroxene-type chain (not reproduced), whereas those denoted by 2 are linked to two Al atoms (ideally 0.5 v.u. each one) and one Si. This arrangement produces the largest separation, 2.77 (2) Å, between consecutive Al atoms and confers to each Al atom a distorted tetrahedral coordination (the two longest Al⋯O distances plotted as thin lines are considered as too weak interactions). According to the existence of the ternary symmetry axis in P3c1, each BAl site is averaged at three positions.