BY W. F. PETTERD, C.M.Z.S. 191 



The percentage composition of the first portion is — 



Per cent. 

 Si O2 = 31-76 

 AU O3 = 54-72 

 F = 13-52 



100-00 



If this is brought to the 107 per cent., which is the usual 

 result of a topaz analysis, we get — 



Per cent. 



Si O2 = 33-98 



Alo O3 = 58-55 



F = 14-47 



1(17-00 



This shows the effect of a deficiency of fluorine, as com- 

 pared with the analyses in Dana. 



" Assuming that topaz is a silicate of alumina, with 

 portion of the oxygen in the aluminium oxide replaced by 

 fluorine, then the composition becomes the following: — 



Per cent. 

 Insolubles — 



Si O2 = 29-6 



AI2 O = 39-78 



AI2 Fe = 18-54 



87-92 

 Admixed Ca F., = H-99 



94-91 

 H^O, HO, F, &c. = 4-80 



99-71 



The final conclusion is that, " on the whole, it may be 

 stated that the analysis presents a remarkable likeness to 

 Dana's No. 8 from the Mt. Bischoff massive topaz " ' ' 

 Dana, " System of Mineralogy," page 494). 



Alteration Product — Achlusite (Fluosilicate of Alumin- 

 ium and Sodium + Ha O). 

 This extreme homogeneous derivative of the last-men- 

 tioned alteration product of topaz is generally of a rich 

 oil-green colourj showing from somewhat pale to dark- 



