276 Day and Shepherd — Lime-8ilica Series of Minerals. 



~No effort was made to invert an entire charge on account of the 

 slowness of the change and the fact that its character was now 

 fully established. The glass crystallizes to quartz below 760° 

 and to tridymite above 800°, crystalline quartz goes over to 

 tridymite above 800°, and tridymite to quartz at 750° ; the 

 change is therefore enantiotropic and not monotropic. 



Incidentally, a sufficient reason has been given for the com- 

 plete failure of experimenters to produce, quartz without catal- 

 ysis. If dry silica at 900° is so inert as to undergo no reaction 

 at all during a month's exposure under favorable conditions, 

 how can we expect reaction below 800° where the viscosity is 

 even greater ? Silica must be crystallized below 800° to pro- 

 duce quartz.* 



Density of Silica. — The density of the silica used and 

 obtained in our experiments was determined with the follow- 

 ing results, the aggregate impurity being not over one-tenth 

 of one per cent : 



Purified Natural 



Quartz. Quartz Glass. H 2 at 25° = 1. 



2-655 First preparation, 2-209 



2-653 " . " 2-215 



2-654 f< " 2-212 



Second " 2-213 



a 



2-215 



Mean, 2-654 (25°) Mean, 2-213 (25°) 



It will be noted that there is a difference of more than 16 

 per cent between the density of the glass and that of the 

 quartz crystals. , 



A charge of powdered crystalline quartz heated for several 

 days at 1200° appeared under the microscope to be homoge- 

 neous tridymite. Some observations of its density are contained 

 in the subjoined table under the heading "tridymite from 

 quartz." 



*E. Baur (Zeitschr. f. phys. Chem. xlii, p. 575, 1903) appears to have 

 obtained tridymite and quartz side by side from a mixture of 5s rms Si0 2 , 

 4 - 3s r A10 2 Na and 12 oc water heated for six hours in a closed steel bomb at 

 520°. We find it very difficult to reconcile this result with our experience. 

 That tridymite is not the stable phase at this temperature under the con- 

 ditions of the experiment appears to be established beyond reasonable doubt 

 by our own work, although we have never studied a mixture of exactly this 

 composition. We should therefore not expect it to form in such a melt, 

 certainly not in the presence of quartz. If tridymite came to be present by 

 accident as a result of some previous operation, or by the temperature in 

 the furnace having been too high, it might revert gradually to quartz and 

 thus explain the presence of both forms in such a charge. Until we are in 

 position to repeat Baur's experiment, therefore, we are unable to explain the 

 simultaneous appearance of quartz and tridymite except by assuming that 

 two operations have taken place : (1) a formation of tridymite, and (2) a 

 partial reversion to quartz, or some unchanged silica may subsequently have 

 formed a quartz at a lower temperature. 



