The mixed valence compound Fe2+5Fe3+2SiO10, informally known as “iscorite”, has been investigated by high-resolution and analytical transmission electron microscopy (HRTEM & ATEM) including energy-dispersive X-ray microanalysis (EDX) and electron energy-loss spectroscopy (EELS). EDX and EELS measurements confirm the chemical composition of the investigated sample yielding concentration ratios of Fe/Si = 7.1 and Fe3+/ΣFe = 0.29 ± 0.03 which resemble the expected ratios of Fe:Si = 7:1 and Fe3+/ΣFe = 2:7. The lattice parameters of the monoclinic cell given by Smuts et al. (1969) and Modaressi et al. (1985) were confirmed: a = 2.1336 nm, b = 0.30679 nm, c = 0.58744 nm, β = 98.06°. However, space group I12/m1 (No.12) was found instead of the published space group P121/m1 (No.11).

In selected area electron diffraction (SAED) patterns in the orientation [01̄1], streaking of the basic reflections and diffuse diffraction lines located at 1/2 (b⃗* + c⃗*) parallel to the a⃗a direction are observed indicating an intense microstructural disorder. For the first time, we report the occurrence of twins on (200) in “iscorite”. These structural modulations observed in SAED patterns and in Fourier-filtering analysis of the HRTEM images result from Fe3+/Si-disorder on tetrahedral sites and Fe2+/Fe3+-disorder on octahedral sites. Therefore, the crystal structure of “iscorite” has been described anew within the non-standard centrosymmetric space group I12/m1 by the use of the single-crystal X-ray diffraction (XRD) data of (Modaressi et al., 1985) in combination with our SAED and HRTEM results.

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