科研之友
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摘要
The crystal structures of marialite (Me-6) from Badakhshan, Afghanistan and meionite (Me-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 P4(2)/n, and similar results were obtained. The Me6 sample has a formula Ca0.24Na3.37K0.24[Al3.16Si8.O-84(24)]Cl-0.84(CO3)(0.15), and its unit-cell parameters are a=12.047555(7), c=7.563210(6) angstrom, and V=1097.751(1) angstrom(3). The average < T1-O > distances are 1.599(1) angstrom in I4/m and 1.600(2) angstrom in P4(2)/n, indicating that the T1 site contains only Si atoms. In P4(2)/n, the average distances of < T-2-O >=1.655(2) and < T-3-O >=1.664(2) angstrom are distinct and are not equal to each other. However, the mean < T-2,3-O >=1.659(2) angstrom in P4(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 0 atoms) and M-A=2.914(1) angstrom; these distances are identical in both space groups. The Me-93 sample has a formula of Na0.29Ca3.76[Al5.54Si6.46O24]Cl-0.05(SO4)(0.02)(CO3)(0.93), and its unit-cell parameters are a =12.19882(1), c=7.576954(8) angstrom, and V=1127.535(2) angstrom(3). A similar examination of the Me93 sample also shows that both space groups give similar results; however, the C-O distance is more reasonable in P4(2)/n than in I4/m. Refining the scapolite structure near Me-0 or Me-100 in I4/m forces the T2 and T3 sites (both with multiplicity 8 in P4(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(2)/n. Using different space groups for different regions across the series implies phase transitions, which do not occur in the scapolite series.
- 出版日期2011-6
