Al,Si order in the crystal structure of α-eucryptite (LiAlSiO4)

The crystal structure of α-eucryptite, LiAlSiO4, is reinvestigated by combining 29Si NMR spectroscopy and single-crystal X-ray diffraction. The silicon coordination in α-eucryptite of three different samples (two of natural, one of synthetic origin) is shown to be identical on a local scale by means of 29Si MAS NMR spectroscopy. This method also suggests a well ordered arrangement of Si and Al within the crystal structure of α-eucryptite and the presence of two symmetrically non-equivalent sites for silicon. The refinement of the structure of a natural crystal using single-crystal X-ray diffraction confirms these observations and demonstrates the presence of long-range Al-Si order and the acentric space group R3.