The structures of marialite (Me6) and meionite (Me93) in space groupsP42/nandI4/m, and the absence of phase transitions in the scapolite series

Abstract

The crystal structures of marialite (Me6) from Badakhshan, Afghanistan and meionite (Me93) 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 groupsI4/mandP42/n, and similar results were obtained. TheMe6sample has a formula Ca0.24Na3.37K0.24[Al3.16Si8.84O24]Cl0.84(CO3)0.15, and its unit-cell parameters area=12.047555(7),c=7.563210(6) Å, andV=1097.751(1) Å3. The average ⟨T1-O⟩ distances are 1.599(1) Å inI4/mand 1.600(2) Å inP42/n, indicating that the T1 site contains only Si atoms. InP42/n, the average distances of ⟨T2-O⟩=1.655(2) and ⟨T3-O⟩=1.664(2) Å are distinct and are not equal to each other. However, the mean ⟨T2,3-O⟩=1.659(2) Å inP42/nand is identical to the ⟨T2′-O⟩=1.659(1) Å inI4/m. The ⟨M-O⟩ [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. TheMe93sample has a formula of Na0.29Ca3.76[Al5.54Si6.46O24]Cl0.05(SO4)0.02(CO3)0.93, and its unit-cell parameters area=12.19882(1),c=7.576954(8) Å, andV=1127.535(2) Å3. A similar examination of theMe93sample also shows that both space groups give similar results; however, the C–O distance is more reasonable inP42/nthan inI4/m. Refining the scapolite structure nearMe0orMe100inI4/mforces the T2 and T3 sites (both with multiplicity 8 inP42/n) to be equivalent and form the T2′ site (with multiplicity 16 inI4/m), but ⟨T2-O⟩ is not equal to ⟨T3-O⟩ inP42/n. Using different space groups for different regions across the series implies phase transitions, which do not occur in the scapolite series.