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HAlSi 4 O 10  was prepared from natural petalite (castor), LiAlSi 4 O 10 , by Li/H exchange in H 2 SO 4  (300°C). The crystal structure was determined by X-ray methods from a twinned crystal [ a  = 5.867(3),  b  = 4.927(3),  c  = 7.524(4) Å,  α  = 93.0(l),  β  = 113.1(1),  γ  = 90.1(1)°, space group  P 1,  Z  = l,  R  = 0.053 for 1011 reflections and 133 variables]. The topology of the Si 4 O 10  layers of petalite is not affected by ion exchange. The Al atoms are rearranged, however: Half of the Al atoms are on positions corresponding with those in petalite, the other Al atoms are on former Li positions. The remaining tetrahedral positions are empty. As a consequence the  a  translation in HAlSi 4 O 10  is only half as long as in petalite. The SiO 4  and AlO 4  tetrahedra are rotated during site exchange: one of the former O – O edges in AlO 4  resp. LiO 4  tetrahedra (now a vacant tetrahedral site) is shortened to 2.567(2) Å. Neutron powder patterns were recorded on HAlSi 4 O 10  and DAlSi 4 O 10  at 5 K and 293 K. Rietveld refinement allowed the location of the hydrogen atom in the short O … O contact.

Crystal structure and hydrogen bonding in Li/H-exchanged petalite, HAlSi 4 O 10