Crystal-chemistry of leucite from the Roman Comagmatic Province (central Italy): a multi-methodological study

A multi-methodological study, based on electron microprobe analysis (in wavelength dispersive mode), single-crystal X-ray diffraction (XRD), transmission electron microscopy and single-crystal Fourier transform infrared spectroscopy was performed in order to describe the crystal chemistry of four leucite samples from different localities of the Roman Comagmatic Province (central Italy). All the crystals examined were found to be tetragonal (space group I4(1)/a with a = 13.076-13.103 angstrom and c = 13.744-13.784 angstrom) and characterized by a complex twinning (merohedric twins: on the tetragonal planes (110) and ((1) over bar 10) with the two individuals having parallel crystallographic axes with a and b interchanged; pseudo-merohedric twins: on the tetragonal planes (101), (011), ((1) over bar 01), (0 (1) over bar1), with the two individuals having parallel a (or b) axes and the remaining two axes not parallel). The chemical analyses show that all the samples contain minor Na and Fe. Infrared spectroscopy shows that all samples contain structurally bound water molecules, up to unexpectedly large amounts (similar to 0.4 wt.%) for a nominally anhydrous mineral, Suggesting that 'analcime-like' substitution (K to Na + H2O) occurs in the leucite samples investigated here. The detection limits of the 'analcime-like' substitution by single-crystal XRD are also discussed.