Crystal structures of monoclinic variants of two SFCA structure-types containing significant Fe2+: crystal structure of monoclinic SFCA-II, Ca2.6Fe8.03+Fe3.42+Al4O24; and proposed crystal structure of monoclinic SFCA-I, ideally, Ca2Fe82+Fe83+Al4O28

摘要

Determination of the crystal structure of a new calcium alumoferrite has proved it to be a monoclinic variant of the previously determined SFCA-type. triclinic SFCA-II. It has formed during firing mixtures of ternary phase compositions in the CaO-Fe2O3-Al2O3 system (CF(2)A) under reducing conditions. Monoclinic SFCA-II has the cell dimensions a = 10.335, b = 17.987, c = 10.488 A, beta = 109.50 degrees, V = 1838 A(3), Z = 8, and space group symmetry P2/n. Its composition, determined from the crystal structure refinement, is Ca1.2Fe4.03+Fe1.82+ A1(2)O(12), (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 monoclinic variant of triclinic SFCA-I is proposed. The unit cell dimensions of this phase are a = 10.34, b = 21.44, c = 10.50 A, beta = 109.96 degrees, P2(I)/n symmetry. The ideal composition of this phase is Ca2Fe82+Fe3+ (8)A1(4)O(28), (M11O14). In this case the structure may be represented by the polysomatic formulation of pyroxene (P) and spinel (S) modules as SSPSSP.