Abstract: Ferric-iron stilpnomelane is triclinic, space group Pl, a = 21.724 Å, b = 21.724 Å, c = 17.740 Å, α = 124.14°, β = 95.86°, γ = 120.0°, Z = 16. The structure is dominated by a subcell for which aS = ¼a, bS = ¼b, cS = ¼c, aS = α, βS = β, γS = γ. The subcell reflections have h-k = 3n, and have a trigonal intensity distribution. The structure was determined from Patterson projections and by trial and error. Three-dimensional refinement by least squares gave an R factor of 14.4 per cent for 296 reflections.
Stilpnomelane is a layer structure, having five anion and five cation planes parallel to (001). An iron-rich octahedral sheet is flanked by two tetrahedral sheets. The tetrahedral sheets differ from those found in the common layer silicates in that adjacent tetrahedra point in opposite directions. Each tetrahedral sheet has 1.25 tetrahedra pointing toward the octahedral sheet, and 0.75 tetrahedra pointing away. Detail in the tetrahedral sheet cannot be determined from the subcell reflections only.