Crystal structure of the SFCAM phase Ca2(Ca,Fe,Mg,Al)6(Fe,Al,Si)6O20

The crystal structures of Mg-rich SFCA (SFCAM); Ca-2(Ca0.10Mg1.20Fe5.55Si1.50Al3.65)O-20 (triclinic P (1) over bar, a = 8.848(1) angstrom, b = 9.812(1) angstrom, c = 10.403(1) angstrom, alpha = 64.35(1)degrees, beta = 84.19(1)degrees, gamma = 66.27(1)degrees, V = 742.4(1) angstrom(3), Z = 2) and Ca-2(Mg2.00Fe4.45Si2.15Al3.40)O-20 (triclinic P (1) over bar, a = 8.928(2) angstrom, b = 9.823(2) angstrom, c = 10.389(1) angstrom, alpha = 64.41 (1)degrees, beta = 83.90 (1)degrees, gamma = 65.69 (1)degrees, V = 746.0 (2) angstrom(3), Z = 2) were determined by the single crystal X-ray diffraction. The structure of SFCAM is iso-structural with aenigmatite and well demonstrated by an alternating stacking of the tetrahedral and octahedral layers. The tetrahedral sites of oxygen are occupied either by Fe, Al and Si. The octahedral sites of oxygen are occupied either by Fe, Mg and Al and this feature contrasts with that of the Mg-free SFCA phase where Al prefers tetrahedral sites, only. In particular, Si4+ and Mg2+ prefer the tetrahedral T1, T2 and T4 sites and octahedral M5 and M6 sites, respectively, by producing a structural slab similar to that of aluminous diopside. Such local concentration of divalent Mg2+ and tetravalent Si4+ in the structure of SFCAM is strongly favored in order to compensate the local charge valance. The SFCAM phase indicates the superior structural flexibility for a variety of cations and this feature is promising for the chemical design of the bonding phase in the sinter ore.