Structural investigations of natural and synthetic chlorite minerals by x-ray diffraction, Mössbauer spectroscopy and solid-state nuclear magnetic resonance

The structures of one synthetic and two natural chlorites of the chlinochlore type were explored using X-ray diffraction, magic-angle spinning nuclear magnetic resonance (NMR) and Mössbauer spectroscopy. Rietveld refinements indicated that all structures are of the trioctahedral ordered IIb polytype. Mössbauer spectra provided the ratio Fe/Fe but gave no evidence for the presence of Fe in the brucite-like sheet. We also report unit-cell parameters, Mössbauer isomeric shifts, Si NMR chemical shifts as well as Al isotropic shifts and quadrupolar coupling parameters. Very broad Si NMR peaks from the natural samples prevented us from obtaining accurate information on the Si-Al ordering in the tetrahedral sheets; the limitations of Si NMR as applied to natural chlorites are discussed. High-resolution 3QMAS NMR resolved the Al signal of the M4 octahedral site in the brucite-like sheet from the other three Al signals of crystallographically inequivalent octahedral positions