TOPAZ. 153 



83. TOPAZ.* 



6 (3 A1 2 3 , 2 SiO 2 ) + (3Al 2 F 3 + 2 SiFJ. 



At a very intense white heat, the topaz loses all its 

 fluorine in the form of tetrafluoride of silicon. (23 per 

 cent.) 



When fused, in the state of very fine powder, with 

 4 times its weight of anhydrous carbonate of soda, it 

 is decomposed, with formation of fluoride of sodium, 

 which is extracted by water. Before filtering off the 

 residual silicate of alumina, however, the solution should 

 be digested with some carbonate of ammonia, in order 

 to precipitate any small quantities of alumina aad 

 silica which may have been dissolved. 



The residue is then filtered off, washed with dilute 

 carbonate of ammonia, and farther treated as in No. 75. 



The alkaline filtrate is concentrated and freed from 

 ammonia by evaporation, and the greater part of the 

 carbonate of soda neutralized by nitric acid, so that 

 some carbonate may still remain undecomposed. The 

 solution is then mixed with chloride of calcium, which 

 precipitates a mixture of carbonate of lime and fluo- 

 ride of calcium. When the precipitate has separated, 

 by the aid of a gentle heat, it is filtered off, washed, 

 and ignited. The carbonate of lime is then dissolved 

 in dilute acetic acid, the solution evaporated to dry ness 

 on a water-bath to expel the excess of acid, and the 

 acetate of lime extracted from the dry mass with hot 

 water. The residual fluoride of calcium is filtered off, 

 washed, dried, ignited, and weighed. If the precipitate 

 of carbonate of lime and fluoride of calcium had not 

 been ignited previously to the treatment with acetic 

 acid, the fluoride would have entered the pores of the 

 filter, and the filtrate would have been turbid. 



* Defective crystals of Brazilian topaz may frequently be ob- 

 tained at a cheap rate. 



