Original paper

The crystal structure of SFCA-II, Ca5.1Al9.3Fe3+18.7Fe2+0.9O48 a new homologue of the aenigmatite structure-type, and structure refinement of SFCA-type, Ca2Al5Fe7O20. Implications for the nature of the "ternary-phase solid-solution" previously reported in

Mumme, W.G.


Determination of the crystal structure of a new calcium ferrite has proved that it has M34O48 stoichiometry, Ca5.1Al9.3Fe3+18.7Fe2+0.9O40. The crystal structure shows that this ferrite is a homologue of SFCA and has a structure which is a composite of the crystal structures of previously described homologues SFCA (M14O20) and SFCA-I (M20O28). Designated as SFCA-II, it has a triclinic cell with a = 10.338, b = 10.482, c = 17.939Å, α = 90.384, β = 89.770, γ = 109.398 °. SFCA-II contains two types of alternating layers in its structure: the first is composed of octahedral walls that are 5 octahedra wide; the second is a continuous layer composed of single ribbons of corner joined tetrahedra (T), alternately separating, single (W) and double (2W) ribbons of so-called "winged octahedra" (…T-W-T-2W-T…). The crystal structure refinement of a Si-free SFCA-type, Ca2Al5Fe7O20, formed in the C-A-F system by flux growth at 1290 °C is also presented. At ≈1200-1250 °C these three unique but structurally related homologues of SFCA (i.e. SFCA, SFCA-II, and SFCA-I) are seen to span, in that order, the designated compositional range of the so-called "ternary-phase solid-solution", Ca4Al4Fe20O40 - Ca4Al10Fe14O40, which has been regarded as a significant feature of the CaO - Al2O3 - Fe2O3 (C-A-F) phase diagram from ≈1470 °C to as low as 700 °C. "Ternary-phase solid-solution" in the C-A-F system should be replaced in part by more limited, yet to be determined, ranges of solid solution in at least three unique homologues of the SFCA structure-type.