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 wave-length dispersive mode, single-crystal X-ray diffraction, transmission electron microscopy, and single-crystal Fourier-transform infra-red 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 examined crystals were found to be tetragonal (space group I41/a with a=13.076-13.103 Å and c=13.744-13.784 Å), and characterized by a complex twinning (merohedric twins: on the tetragonal planes (110) and ( 10) with the two individuals having parallel crystallographic axes with a and b interchanged; pseudo-merohedric twins: on the tetragonal planes (101), (011), ( 01), (0 1), with the two individuals having parallel a (or b) axes and the remaining two axes not parallel). The chemical analyses show that all samples contain minor Na and Fe. Infrared spectroscopy shows that all samples contain structurally bound water molecules, up to unexpectedly high amounts (~ 0.4 wt%) for a nominally anhydrous mineral, suggesting that the “analcime-like substitution” (potassium to sodium + water) occurs in the samples of leucite here investigated. The limits of detection of the “analcime-like substitution” by single-crystal X-ray diffraction are discussed.