The crystal structure of SFCA-II, Ca_(5.1)Al_(9.3)Fe~(3+) _(18.7)Fe~(2+) _(0.9)O_(48) a new homologue of the aenigmatite structure-type, and structure refinement of SFCA-type, Ca_2Al_5Fe_7O_(20). Implications for the nature of the 'ternary-phase soli

摘要

Determination of the crystal structure of d new calcium ferrite has proved that it has M_(34)O_(48) stoichiometry, Ca_(5.1)Al_(9.3)Fe~(3+) _(18.7)Fe~(2+) _(0.9)O_(40). 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 (M_(14)O_(20)) and SFCA-I (M_(20)O_(28)). Designated as SFCA-II, it has a triclinic cell with a = 10.338, b = 10.482, c = 17.939 A, alpha = 90.384, beta = 89.770, gamma = 109.398 deg. 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, Ca_2Al_5Fe_7O_(20), formed in the C-A-F system by flux growth at 1290 deg C is also presented. At approx 1200-1250 deg 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", Ca_4Al_4Fe_(20)O_(40) -Ca_4Al_(10)Fe_(14)O_(40), which has been regarded as a significant feature of the CaO - Al_2O_3-Fe_2O_3 (C-A-F) phase diagram from approx 1470 deg C to as low as 700 deg 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.