Benitoite structure

The most convincing case of zirconate luminescence is the emission observed for BaZrSi3O9 5). This compound has the benitoite structure with Zr in octahedral coordination. This emission is situated in the ultraviolet the emission band has its maximum at 285 nm. Excitation is only possible with X < 220 nm. The compound ZrP2O7 shows a similar behaviour ). No further data are available for these compounds. From... 

According to our experience all benitoite samples have intensive blue emissimis under short wave and middle wave UV, and we have never seen or heard of any that did not. On the other hand, efficient luminescence has been observed from the Ti04 , TiOe and even TiOs complexes. In addition, bazirite BaZrSisOg with the benitoite stmcture demonstrates luminescence of zirconate complexes ZrOe (Blasse 1980). The blue luminescence of impurity Ti centers has been found in synthetic BaSnSisOg-Ti" and BaZTSisOg-Ti with the benitoite structure (Konijenendijk 1981). 

The structures of two such cyclic ions, Si3096 and Si601812, are shown schematically in figure 7.1. The general formula for any such ion is Sin03n2o The ion Si3096 forms the structure of benitoite (BaTiSi309). 

This structure is analogous to P3O93 and S3O9 (a form of solid SO3 having the structure (S03)3). A substance that contains the Si3096- ion is the mineral benitoite, BaTiSi309. 

The next higher order of complexity consists of the so-called melasilicate anions, which are cyclic structures-" of general formula (SiOj) " occurring in benitoite, BaTiSi-,0,. catapleitc, Na2ZrSij0, H20. dioplasc, (Tuj.SihOix fiHjO. and beryl, Be,AI,Si,0. This is the mo.st important mineral source of beryllium, and also may form gem-quality stones (see Problem 16.9). 

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