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American Mineralogist; February 1999; v. 84; no. 1-2; p. 92-101
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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

Thomas Armbruster

Universitaet Bern, Laboratorium fuer Chemische und Mineralogische Kristallographie, Bern, Switzerland

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 Remigny (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 Remigny was collected on a three-circle 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) Aa] to R1 = 3.6% based on 4275 reflections. These results confirm complete Si. Al ordering. Mean Si-O distances range between 1.615 and 1.629 Aa; mean Al-O distances between 1.734 and 1.742 Aa. 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 (super -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.

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