抄録
The crystal structure of banalsite, BaNa2A14Si4O16, has been determined based upon intensity data obtained with the use of a four-circle automatic diffractometer. The unit cell has the dimensions: a=8.496(2), b=9.983(2), c=16.755(3)Å, with four formula units per cell. Space group, Ibam, was assumed and, on refinement, turned out to be correct. The structure is built on a framework formed by the linking of rings of four oxygen-tetra-hedra around Al or Si. Although the mutual orientation of the component tetrahedra in a ring is different from that of the four-membered rings in feldspars, there is a close relationship between the banalsite structure and the feldspar structures. The mean T-O length of one of the two independent tetrahedra in the structure is 1.683(4)Å, and that of the other 1.680 (4)Å, suggesting that Si and Al are, in spite of the ratio of Al/Si=1, distributed randomly over the tetrahedral sites. Because of such an unexpected result, an attempt was made at refining the structure in space group Iba2, which the structure would possess if Al and Si were perfectly ordered. However, the refinement was not successful in this noncentric space group. Na and Ba atoms are respectively coordinated by six and ten oxygen atoms; the mean bond lengths are 2.501(6)Å for Na-O, and 2.870(4)Å for Ba-O, both are smaller than corresponding values in other alumino-silicates.
