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Moscow State University, Department of Crystallography, Moscow, Russian Federation
The crystal structure of synthetic end-member marialite (Ma) Na 4 Al 3 Si 9 O 24 Cl and three samples of Na, Cl-rich scapolite from Pamir (central Asia) were refined using Rietveld methods. Compositional measurements indicate meionite (Me) Ca 4 Al 6 Si 6 O 24 CO 3 contents of 0 (SYN MAR), 4.6 (PAM-1), 7.5 (PAM-2), and 7.6% (PAM-3). The crystal structures were refined in space group I4/m using ionized X-ray scattering factors: Rp 4.89-5.92%, Rwp 6.78-7.28%, R b 2.53-3.40%, R f 2.49-3.34%, s 1.26-2.09. The synthetic end-member marialite has unit-cell parameters a = 12.0396 (2) Aa, c = 7.5427(2) Aa and V = 1093.3(4) Aa 3 . A linear correlation was found between the a and c unit-cell dimensions and the Si content of these samples of marialitic scapolite. Additional electron-density maxima were found on the difference Fourier maps D(xyz), and correlation with an increase with H 2 O content suggests partial occupancy by H 2 O along the channels of the marialite framework. 27 Al satellite transition NMR spectra show that Al is in only one environment in the natural samples, and 29 Si MAS NMR spectra show that Si alone occupies the T1 site. Calculation of the numbers of Al-O-Si bonds from peak fitting to the 29 Si NMR spectra indicate that up to 80% of the A1 atoms in the T2 site are involved in one A1-O-Al bond.
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