On a new monoclinic SFC polytype

Dark red crystals of a silico-ferrite of calcium (SFC) with composition Ca5.34Fe20.55Si2.12O40 have been obtained from synthesis experiments at 1250°C. The SFC phase crystalizes in space group C2/c with the following lattice parameters: a= 9.981(4) Å, b= 15.283(6) Å, c= 5.303(2) Å, β = 99.88(4)°, Z=1.

The structural formula of the new phase can be written as VIIA4 VI(M1M2M3M4)Σ12O4[IV(T12T22)Σ12O36], where A represents a seven coordinated Ca-site, M=Fe,Ca are octahedral sites and T=Fe,Si are tetrahedral positions. The structure is composed of spinel(S)- and pyroxene(P)-type modules.

The SFC structure is closely related to that of the minerals of the rhönite group (sapphirine supergroup) and synthetic so-called SFCA phases (Silico-ferrite of calcium and aluminum), where different polytypes composed of the same fundamental S- and P-modules have been observed.

The new monoclinic SFC phase exhibits disorder in the spinel module, representing a layer with edge-sharing zig-zag octahedral chains connected via tetrahedra. Two of three octahedral sites in these chains and the connecting tetrahedra are half occupied only. This enables two (left and right) orientations of the chains along [-1 0 -1]. The same arrangement is observed in equivalent layers of a triclinic SFCA phase reported by Liles et al. (2016). However, in this compound two possible configurations of these layers allow for stacking faults.

Liles, D.C., de Villiers, J.P.R. & Kahlenberg, V. (2016): Miner Petrol 110:141–147.