The structures of marialite (Me_6) and meionite (Me_(93)) in space groups P4_2 and I4/m, and the absence of phase transitions in the scapolite series

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_2ln, and similar results were obtained. The Me_6 sample has a formula Ca_(0.24)Na_(3037)K_(0.24)[Al_(3016)Si_(8.84)O_(24)]Cl_(0.84)(CO_3)_(0.15), and its unit-cell parameters are a= 12.047555(7), c=7.563210(6) A, and V= 1097.751(1) A~3. The average = 1.659(1) A in I41m. The (M-O) [7] = 2.754(1) A (M site is coordinated to seven framework O atoms) and M-A=2.914(l) A; these distances are identical in both space groups. The Me_(93) sample has a formula of Na_(0.29)Ca_(3.76)[ Al_(5.54)Si_(6.46)O_(24)]Cl_(0.05)(SO_4)_(0.02)(CO_3)_(0.93), and its unit-cell parameters are a = 12.19882(1), c=7.576954(8) A, and V=1127.535(2) A~3. A similar examination of the Me_(93) sample also shows that both space groups give similar results; however, the C-O distance is more reasonable in P4_2ln than in I41 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_2ln) to be equivalent and form the T2' site (with multiplicity 16 in I4lm), but (T2-O) is not equal to (T3-O) in P4_2. Using different space groups for different regions across the series implies phase transitions, which do not occur in the scapolite series.