Vol. 5] 



Loicderback 



. — Benitoite. 



351 



relationship of benitoite to titanite may be represented by the 

 following formulae : 



Titanite Benitoite 

 0—0 0— o o 



\ / \ / u 



Si Si — — Si 



Ca < > Ba< >0 



Ti Ti — O — Si 



A A \\ 



0—0 O— o o 



Professor Kraus has recently stated objections to this view," 

 which was expressed in our preliminary paper. He considers 

 the chemical composition of beryl and benitoite sufficiently sim- 

 ilar to consider the compounds isomorphovis. and interprets 

 the composition of the latter as a metasilicate of barium and 

 titanium of the formula Ba...Tio(Si0 3 ),i. The facts that it is 

 associated paragenetically with the basic metasilicate natrolite 

 and that the deposit is formed in basic rocks are also considered 

 of weight in deciding the metasilicate character in contrast to 

 that of an acid titano-silicate. 



A general survey of the compounds of Ti0 2 shows that it 

 normally possesses the properties of a weak acid, and its com- 

 mon formation of titanates and titano-silicates is well known. 

 That it may act as a base in combination with a strong acid is 

 probable, but as far as known to the writer, no evidence that it 

 may hold this relation to silicic acid has been found. It readily 

 forms compounds in which it acts as an acid with only mod- 

 erately basic oxides. In the present case, the temperature 

 conditions for the production of the deposit being considered 

 limited by the conditions for the formation of natrolite. Si0 2 

 may be looked upon as acting as a weak acid, while barium 

 is a strong base. Without specific evidence to the contrary, 

 then, it would be expected that the Ti0 3 would act as an acid 

 forming a titano-silicate. The intimate association of neptunite 

 with the benitoite would suggest that the TiO, played the same 

 role in each. In the latter the very strong bases soda and potash 

 are present, and its character as an acid titano-silicate is gen- 



e Science, n. s., XXVII (1908), pp. 710-711. Blasdale has discussed 

 this point in Science, n. s., XXVIII (1908), pp. 233-234; mi p. 234, line 

 19, the word "base" unfortunately appears where "acid" was intended. 



