X-ray study on polymorphism of ilvaite, HCaFe2 2+Fe3+O2[Si2O7]

Abstract The crystal structure of the monoclinic form of ilvaite has been investigated. A crystal from Tsumo, Japan, was characterized by space group P21/a, with a = 13.009(1) Å, b = 8.8008(3) Å, c = 5.8589(2) Å, β = 90.324(9)°, whereas a crystal from Techukhe, Siberia, had P21/a, a = 13.013(3) Å, b = 8.8197(6) Å, c = 5.8424(5) Å, β = 90.03(1)°. In the structure, the octahedra formed by oxygen atoms about Fe(11) and those about Fe(12) are alternately linked together, sharing edges to form a chain of octahedra parallel to c. The monoclinic symmetry of ilvaite is characterized by ordering of Fe2+ and Fe3+ which are distributed over these two Fe sites with a ratio of 1:1. The atomic contents in the Fe(11) site, estimated from mean Fe(11) – O length, are: Tsumo ilvaite 0.85 Fe2+ and 0.15 Fe3+, and Techukhe ilvaite 0.53 Fe2+ and 0.47 Fe3+. From these atomic occupancies, the order parameter, σ, of Tsumo ilvaite is calculated to be 75%, while that of Techukhe ilvaite 6%. From the two cases of monoclinic ilvaites, a relation between β and σ has been deduced; the result permits predicting β ≈ 90.45° for an ilvaite crystal having σ = 100%. The orthorhombic form of ilvaite may be defined as a form of ilvaite having σ = 0.