Crystal structures of monoclinic variants of two SFCA structure-types containing significant Fe2+: crystal structure of monoclinic SFCA-II, Ca2.6Fe3+8.0Fe2+3.4Al4O24; and proposed crystal structure of monoclinic SFCA-I, ideally, Ca2Fe2+8Fe3+8Al4O28

Determination of the crystal structure of a new calcium alumoferrite has proved it to be amonoclinicvariant of the previously determined SFCA-type, triclinic SFCA-II. It has formed during firing mixtures of ternary phase compositions in the CaO-Fe2O3-Al2O3system (CF2A) under reducing conditions. Monoclinic SFCA-II has the cell dimensions a = 10.335, b = 17.987, c = 10.488 Å, β = 109.50°, V = 1838 Å3, Z = 8, and space group symmetryP2/n. Its composition, determined from the crystal structure refinement, is Ca1.2Fe 3+4.0Fe 2+1.8Al2O12, (M9O12). Monoclinic SFCA-II is a disordered structure composed of polysomatic sequences of pyroxene (P) and spinel (S) modules and, in this context, may be represented by the polysomatic formulation <SSPSP>. Further, a crystal structure for a monoclinicvariant of triclinic SFCA-I is proposed. The unit cell dimensions of this phase are a = 10.34, b = 21.44, c = 10.50 Å, β = 109.96°, P21/nsymmetry. The ideal composition of this phase is Ca 2Fe2+8Fe3+8Al4O28, (M11O14). In this case the structure may be represented by the polysomatic formulation of pyroxene (P) and spinel (S) modules as <SSPSSP>.