Rectorite crystals [[4](Si6.81Al1.19)[6](Al3.26Ti0.04Fe0.553+Mg0.18Mn0.01)[12](Na0.02K0.88Mg0.16Ca0.01)O20(OH)4·0.75H2O] found in micaschist of the Sesia-Lanzo Zone (NW Italy) were studied using a variety of techniques including microprobe analysis, infrared spectroscopy, single crystal and powder X-ray diffraction, atomic force microscopy and thermal analysis. Chemical data and exchangeable cation determination indicate that K+ is the dominant non-exchangeable interlayer cation, and thus is believed to occupy the mica interlayer site; Mg2+ together with small amounts of Ca2+, Na+ and K+ represent the exchangeable cations and can therefore be related to the smectite-like component. The coefficient of variation, CV, of d(00l) values (CVnatural = 0.47; CVglycolated = 0.43) demonstrates the regularity of the mica-smectite interstratification, whereas the unit-cell parameters obtained by single crystal methods suggest different layer-stacking models.

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