K-rich rectorite from kaolinized micaschits of Sesia-Lanzo Zone (Italy)

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 mica schists of the Sesia-Lanzo Zone (NW Italy) were studied using a variety of techniques including microprobe anal., IR spectroscopy, single crystal and powder X-ray diffraction, at. force microscopy, and thermal anal. Chem. data and exchangeable cation detn. indicate that K+ is the dominant non-exchangeable interlayer cation, and thus is believed to occupy the mica interlayer site; Mg2+ together with small amts. of Ca2+, Na+ and K+ represent the exchangeable cations and can therefore be related to the smectite-like component. The coeff. 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.