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Series of structural studies of systematically selected local portions of the same crystal of ilvaite CaFe 2  2 +  Fe 3 +  OOHSi 2 O 7  with moderate size have revealed a trend that the monoclinic and apparently orthorhombic structures co-exist in the crystal regardless of their mode of occurrence. The diffraction-intensity profiles of high angle reflections such as 24,0,0 and 008 were then studied by means of step-scanning on a four-circle diffractometer, using graphite monochromatized Mo K α   radiation: the former was scanned along  c * and the latter along  a *. The result has brought out that either reflection from the crystals of high monoclinicity ( β  ≈ 90.2° ∼ 90.3°) splits into a double peak, suggesting that those crystals are most likely made up of two kinds of domains, each having a monoclinic structure with  β  = 90.45°, one twinned on (001) and the other twinned on (100). In the case of crystals of low monoclinicity ( β  ≈ 90.0°), those reflections tend to exhibit single broad profiles with fine structure. A profile-fitting procedure has shown that they are in general composite of three peaks, suggesting that they consist of complex composites of fine monoclinic components twinned on (001) and those twinned on (100). The distribution of Mn in the crystals studied tends to increase towards their marginal portions. Crystal portions containing more than roughly 0.25 Mn per formula unit assume orthorhombic symmetry without exception. The coexistence of the different symmetries in the same crystal of ilvaite is thus originated in the variation in scale or mode of twinnings incorporated mainly with inhomogeneous distribution of Mn in the crystal. The occurrence of orthorhombic ilvaite is unlikely.

Submicroscopic twinning and chemical inhomogeneity of ilvaite, a mixed-valence iron sorosilicate HCaFe 2 2 + Fe 3 + Si 2 O 9