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



full} 7 carried out which appear to clear up the situation, even 

 though the inversion temperature cannot be determined with 

 any great accuracy. 



First of all, we heated a large charge of finely ground 

 quartz and followed the temperature curve carefully from 400° 

 to 1600°. It was found after the experiment that the quartz 

 crystals had for the most part gone over into tridymite aud 

 the change in the volume accompanying the inversion had 

 generated enough pressure to completely shatter the open 

 platinum crucible which contained the charge. The change 

 was so gradual, however, that no record of it appeared upon 

 the thermal curve. Subsequent experiments in which we 

 endeavored to change crystalline quartz into tridymite at lower 

 temperatures were successful as far down as 1100°. To be 

 sure, entire crystals showed no change whatever after six 

 hours' exposure at 1400°, but powdered quartz is completely 

 changed into tridymite after a few hours at that temperature. 

 On the other hand, if finely divided amorphous silica, i. e., 

 fused (" quartz glass "), or better, precipitated silica, be allowed 

 to remain for a short time at any temperature above 1000°, it 

 changes promptly to tridymite — the precipitated material 

 very rapidly, the quartz glass much more slowly. Neither the 

 glass nor the precipitated silica ever crystallized as quartz at 

 temperatures above 1000°, nor is there any difference in the 

 optical properties of the tridymite obtained at the different 

 temperatures, either from the quartz crystals or the amorphous 

 silica. The rate of change is much influenced by the fineness 

 of the powder, although there is no difficulty in recrystallizing 

 large blocks of solid quartz glass at the higher temperatures. 

 In our experiments in the preparation of quartz glass,* we 

 frequently obtained isolated spherulites of tridymite several 

 millimeters in diameter, even with rapid cooling, which 

 appeared to have been started by a grain of graphite or car- 

 borundum powder accidentally falling into the melt. On one 

 occasion the entire block was coated with tridymite to a depth 

 of a millimeter or more. 



We have therefore succeeded, by direct experiment upon 

 pure silica, in establishing . the fact that tridymite, and not 

 quartz, is the stable crystalline form of silica for all tempera- 

 tures above 1000°. 



At lower temperatures than this it is impossible, in view of 

 the inertness of the substance, to obtain any further reaction, 

 even with the finest precipitated silica, within the time avail- 

 able for a laboratory experiment. A month's exposure at 900° 

 produced no change. We therefore followed the example of 

 *Day and Shepherd, "Quartz Glass," Science, xxiii, p. 670, 1906. 



