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The  29 Si MAS NMR spectrum of armenite from Wasenalp (Valais, Switzerland) indicates complete Si, Al ordering. The same chemical shifts (- 82.3, - 95.0, and - 101.8 ppm) were also measured for armenite from Rémigny (Quebec, Canada), however, the intensity ratios of the NMR bands for the latter sample differed. A full sphere of X-ray single-crystal data on an optically homogeneous domain of armenite from Rémigny was collected on a threecircle diffractometer equipped with a CCD area-detector. The crystal structure was refined in the acentric space group Pnc2 [a = 18.660(2), b = 10.697(1) c = 13.874(2) Å ] to R1 5 3.6% based on 4275 reflections. These results confirm complete Si, Al ordering. Mean Si-O distances range between 1.615 and 1.629 Å ; mean Al-O distances between 1.734 and 1.742 Å . Calcium is sevenfold coordinated by six framework O atoms and one H 2 O molecule. Barium is 12-fold coordinated by framework O atoms. Polarized IR spectra in the region of OH absorptions (between 5700 and 1300 cm -1 ) were recorded on polished slabs of Wasenalp armenite and structurally related milarite from Val Giuf. In general, milarite and armenite show similar anisotropy of H 2 O related absorptions.

Si, Al ordering in the double-ring silicate armenite, BaCa 2 Al 6 Si 9 O 30 ·2H 2 O: A single-crystal X-ray and 29 Si MAS NMR study