Day and Shepherd — Lime-Silica Series of Minerals. 275 



several distinguished predecessors in this field, and tried vari- 

 ous catalyzers. 



Formation of Quartz. — Hautefeuille thought he had pro- 

 duced quartz crystals by fusing amorphous silica with sodium 

 tungstate at 900°, but the methods of high-temperature meas- 

 urement commonly employed in his time were very imperfect, 

 and the temperature is undoubtedly too high. He has also 

 stated that he obtained it by fusion with lithium chloride. 

 Both Hautefeuille and Margotet have recorded the fact that 

 in the presence of lithium chloride amorphous silica changes 

 to tridymite at high temperatures and to quartz at low tem- 

 peratures. TTe obtained quartz crystals from glass by the use 

 of a mixture of 80 per cent KC1 with 20 per cent LiCl at all 

 temperatures below 760°, while at temperatures of 800° and 

 higher only tridymite crystals appeared. The same results 

 were obtained with vanadic acid and with sodium tungstate. 

 The inversion point 



quartz < > tridymite 



therefore occurs at about 800°. This conclusion is subject to 

 the assumption that the inversion temperature is not lowered 

 by the catalyzing agent — an assumption which seems to be 

 justified by the fact that the quartz crystals obtained in this 

 way (judged by the optical properties) appear to hold none of 

 the reagent in solid solution. 



The situation is then, briefly, this : Both quartz and amor- 

 phous silica at high temperatures change to tridymite. Quartz 

 is consequently the unstable form of silica from 800° upward, 

 and will go over into tridymite whenever conditions favorable 

 to the change are present. The melting temperature of silica 

 is therefore properly the melting temperature of tridymite and 

 not of quartz as it is commonly described. We have once or 

 twice succeeded, by extremely rapid heating, in melting quartz 

 as such, or more correctly speaking, in carrying a quartz charge 

 past the melting temperature of tridymite, melting a portion 

 of it and tin ding a residue of quartz afterward which had 

 neither inverted nor melted. It would hardly be possible by 

 any known method, however, to obtain a separate melting tem- 

 perature for quartz independently of tridymite. 



The reverse operation, showing that tridymite inverts to 

 quartz at temperatures below 760°, cannot be carried out in 

 the laboratory without the use of catalyzers on account of the 

 extreme slowness of the change. In the presence of the mix- 

 ture of 80 per cent KC1 and 20 per cent LiCl, quartz began to 

 appear from tridymite in quantities sutiicient for positive identi- 

 fication after an exposure of five or six days, at about 750°. 



