Red micas from basal ignimbrites of Mt. Vulture (Italy): interlayer content appraisal by a multi-methodic approach.

The crystal chemistry of red phlogopites from Mt. Vulture (Italy) ignimbrites has been studied by electron microprobe, secondary ion mass spectrometry (SIMS), single crystal structural investigation and Fourier transform infrared (FTIR) spectroscopy. The analysed phlogopite has Fe/( Fe + Mg) similar to 0.35, TiO(2) (wt%): 2.8-5.0 and H(2)O ( wt%): 1.24-3.37. Infrared spectra revealed the presence of bands due to the NH(4)(+) and H(2)O stretching and bending vibrations. The samples belong to the 1M polytype. The bimodal behaviour of several structural parameters allows red micas to be clustered into two distinct groups: K(+) <-> NH(4)(+), H(2)O and M(3+)- vacancy substitutions dominate in the first group; M(3+,4+)-oxy, in the second group. It has to be pointed out that quantitative analysis of hydrogen (via SIMS) together with the characterization of the local environment of the anionic site (via FTIR) are fundamental in assessing the correct structural formula and the substitution mechanisms in micas.