Unusual Al-Si ordering in calcic scapolite, Me79.6, with increasing temperature

A scapolite sample, Me(79.6), from Slyudyanka, Siberia, Russia, has been studied using in situ synchrotron powder X-ray diffraction (XRD) and Rietveld structure refinements on heating from 26 to 900 degrees C and on cooling to about 300 degrees C. The structure was modeled and refined in space group I4/m. An accurate room-temperature structure was also obtained by using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinement. From HRPXRD, the cell parameters are a = 12.16711(2), c = 7.575466(5) angstrom, and V = 1121.461(3) angstrom(3); < T1-O > and < T2-O > are 1,643(1) and 1.672(1), respectively, so the T1 (Al(0.25)Si(0.75)) and T2 (Al(0.46)Si(0.54)) sites are partially ordered at room temperature. On heating, the < T-O > distances indicate that the T1 and T2 sites become more Si- and Al-rich, respectively, and therefore, ordering increases unusually with increasing temperature. This increase in Al-Si ordering occurs from 892 to 900 degrees C. At 900 degrees C, the T1 site becomes fully ordered with only Si atoms, while the T2 site contains Al(0.51)Si(0.49) and therefore, is fully disordered. On cooling, the sample does not fully revert back to the original partially ordered state. At 300 degrees C, all the cell parameters are smaller because of the increased Al-Si ordering that is quenched in.