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Mineralogical Magazine; December 2007; v. 71; no. 6; p. 671-682; DOI: 10.1180/minmag.2007.071.6.671
© 2007 Mineralogical Society of Great Britain and Ireland
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Crystal chemistry of leucite from the Roman Comagmatic Province (central Italy): a multi-methodological study

G. D. Gatta1,2,*, N. Rotiroti1,2, F. Bellatreccia3 and G. Della Ventura3

1 Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, I-20133 Milano, Italy
2 CNR-Istituto per la Dinamica dei Processi Ambientali, Milano, Italy
3 Dipartimento di Scienze Geologiche, Università Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Roma, Italy

* E-mail: diego.gatta{at}unimi.it

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 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 (Formula10) with the two individuals having parallel crystallographic axes with a and b interchanged; pseudo-merohedric twins: on the tetragonal planes (101), (011), (Formula01), (0Formula1), 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 (~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.

KEYWORDS: leucite, crystal-chemistry, single-crystal X-ray diffraction, FTIR spectroscopy, transmission electron microscopy, structurally incorporated water






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