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Title: The structures of marialite (Me[subscript 6]) and meionite (Me[subscript 93]) in space groups P4[subscript 2]/n and I4/m, and the absence of phase transitions in the scapolite series

The crystal structures of marialite (Me{sub 6}) from Badakhshan, Afghanistan and meionite (Me{sub 93}) from Mt. Vesuvius, Italy were obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements. Their structures were refined in space groups I4/m and P42/n, and similar results were obtained. The Me{sub 6} sample has a formula Ca{sub 0.24}Na{sub 3.37}K{sub 0.24}[Al{sub 3.16}Si{sub 8.84}O{sub 24}]Cl{sub 0.84}(CO{sub 3}){sub 0.15}, and its unit-cell parameters are a = 12.047555(7), c = 7.563210(6) {angstrom}, and V = 1097.751(1) {angstrom}{sup 3}. The average (T1-O) distances are 1.599(1) {angstrom} in I4/m and 1.600(2) {angstrom} in P4{sub 2}/n, indicating that the T1 site contains only Si atoms. In P4{sub 2}/n, the average distances of (T2-O) = 1.655(2) and (T3-O) = 1.664(2) {angstrom} are distinct and are not equal to each other. However, the mean (T2,3-O) = 1.659(2) {angstrom} in P4{sub 2}/n and is identical to the (T2-O) = 1.659(1) {angstrom} in I4/m. The (M-O) [7] = 2.754(1) {angstrom} (M site is coordinated to seven framework O atoms) and M-A = 2.914(1) {angstrom}; these distances are identical in both space groups. The Me{sub 93} sample has a formula of Na{sub 0.29}Ca{sub 3.76}[Al{sub 5.54}Si{sub 6.46}O{sub 24}]Cl{sub 0.05}(SO{sub 4}){sub 0.02}(CO{sub 3}){sub 0.93}, and itsmore » unit-cell parameters are a = 12.19882(1), c = 7.576954(8) {angstrom}, and V = 1127.535(2) {angstrom}{sup 3}. A similar examination of the Me{sub 93} sample also shows that both space groups give similar results; however, the C-O distance is more reasonable in P4{sub 2}/n than in I4/m. Refining the scapolite structure near Me{sub 0} or Me{sub 100} in I4/m forces the T2 and T3 sites (both with multiplicity 8 in P4{sub 2}/n) to be equivalent and form the T2' site (with multiplicity 16 in I4/m), but (T2-O) is not equal to (T3-O) in P4{sub 2}/n. Using different space groups for different regions across the series implies phase transitions, which do not occur in the scapolite series.« less
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Publication Date:
OSTI Identifier:
Resource Type:
Journal Article
Resource Relation:
Journal Name: Powder Diffr.; Journal Volume: 26; Journal Issue: (2) ; 2011
Research Org:
Advanced Photon Source (APS), Argonne National Laboratory (ANL), Argonne, IL (US)
Sponsoring Org:
USDOE Office of Science (SC)
Country of Publication:
United States