Refinement of the crystal structure of osumilite

The crystal structure of osumilite, a = 10.155(1), c = 14.284(4) Å; P6/mcc, 2[(Ca_0.005 Na_0.31K_0.78)^XII(Mg_0.92Fe_0.92Mn_0.16)^VI(Si_10.22Al_4.41Fe_0.37³⁺)^lvO₃₀·H₂O] was refined to R = 0.066 using 259 nonequivalent reflections recorded with a Weissenberg counter-diffractometer.

Mean Tl-O (1.627 [2] Å) and T2–O (1.762 [2] Å) bond lengths indicate that 10.22 Si plus 1.78 A1 are restricted to the 12 tetrahedra [Tl] in the double hexagonal rings and that 2.63 A1 plus 0.37 Fe³⁺ occupy the three tetrahedra [T2] outside the rings. Edges shared between (Mg,Fe²⁺,Mn)-containing octahedra and T2 tetrahedra are significantly shorter (2.568[11] Å) than unshared ones (3.155[10] Å, 2.879[8] Å). K,Ca,Na atoms are coordinated between the double six-membered rings in a hexagonal antiprism which shares an edge (2.601 [5] Å) with the T1 tetrahedron.

The water molecule is randomly distributed within each cavity formed by the double hexagonal rings in one of six symmetrically identical positions at x = 0.031, y = 0.114, z=0.0. A short H₂O-O distance of 2.6[1] Å indicates hydrogen bonding between H₂O and the cavity-wall oxygens.

Because of structural and physical similarities with cordierite and beryl, osumilite is classified with the framework rather than with the ring silicates in agreement with Zoltai's (1960) classification of the mineral.