THE STRUCTURE OF MARIALITE (Me6) AND MEIONITE (Me93) IN SPACE GROUPS P42/n AND I4/m, AND THE ABSENCE OF PHASE TRANSITIONS IN THE SCAPOLITE SERIES

The structure of marialite (Me6) from Badakhshan, Afghanistan and meionite (Me93) from Mt. Vesuvius, Italy was obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements. Their structure was refined in space groups I4/m and P42/n, and similar results were obtained. The Me6 sample has a formula Ca0.24Na3.37K0.24[Al3.16Si8.84O24]Cl0.84(CO3)0.15, and the unit-cell parameters are a = 12.047555(7), c = 7.563210(6) Å, and V = 1097.751(1) Å. The average distance is 1.599(1) in I4/m, and 1.600(2) Å in P42/n, indicating that the T1 site contains only Si atoms. In P42/n, = 1.655(2) and = 1.664(2) Å, these average distances are distinct and are not equal to each other. However, the mean = 1.659(2) Å in P42/n, and is identical to the = 1.659(1) Å in I4/m. The [7] = 2.754(1) Å (M site is coordinated to seven framework O atoms) and M-A = 2.914(1) Å; these distances are identical in both space groups. A similar examination for the Me93 sample also shows that both space groups give similar results; however, the C-O distance is more reasonable in P42/n than in I4/m. Refining the scapolite structure near Me0 or Me100 in I4/m forces the T2 and T3 sites (both with multiplicity 8 in P42/n) to be equivalent and form the T2’ site (with multiplicity 16 in I4/m), but is not equal to in P42/n. Using different space groups for different regions across the series implies phase transitions, which do not occur in the scapolite series.