A NEW CLOSE-PACKED STRUCTURE IN PRECIOUS OPAL FROM BRAZIL

A precious opal from Pedro II, Nordeste, Brazil, shows a rare structur e consisting of silica spheres of two different sizes regularly arrang ed, with a radius ratio gamma=r/R approximate to 0.75. From scanning e lectron microscopy images, it is possible to describe the periodic arr ay, its stoichiometry, the unit cell including the ''atomic'' position s and the symmetry group, Fd $$($) over bar 3m. This cubic packing cor responds to a new AB(2) type structure, not described previously in bi disperse opals. The spheres of radius 5 occupy tetrahedral cavities of a face-centred cubic packing of spheres of radius r, and themselves d isplay a diamond-like sublattice. The stability of the structure requi res that the large spheres become slightly flattened, in order to elim inate ''rattling'' of the structure. The large spheres are indeed flat tened.