Crist obalite and Triclymite. 425 



The furnace and set-up used in the determination of 

 the melting point of cristobalite were available and were 

 used without change in this work. 



Results. 

 1. Natural tridymite, kindly furnished by G. P. Merrill from 

 the collection in the New National Museum. Occurrence : Lan- 

 der Co., Nevada, associated with "wood tin," described by 

 A. Knopf; 12 aggregate having radiating structure and no vis- 

 ible impurities except minute amounts of granular quartz ; 

 refractive index measured 0-003 higher than the other four sam- 

 ples here described. 

 Temperature Sample Quenched 



Results. 

 The quartz alone had fused. 



(corrected). 



after : 



1655° 



4min. 



1659° 



10 min. 



(1659-82°) 



5 min. 



1667° 



10 min. 



1675° 



10 min. 



1675° 



20 min. 



Practically all melted. 

 Quartz only melted. 

 Practically all melted. 

 A few grains partly melted, but- 

 many entirely melted. 



2. Artificial tridymite. After heating some pure quartz 

 crystals at approximately 1350° for 144 hours the quartz has 

 inverted mostly to cristobalite but a few grains inverted to tridy- 

 mite and these grains were used in these experiments. (See 

 results under No. 7 above.) 



Temperature Sample Quenched 

 (corrected). after: Results. 



1667° 10 min. The tridymite in both cases in- 



1677° 6 min. verted to dense grains of cristo- 



balite without melting. 



3. Natural tridymite. Locality: Cerro San Cristobal, near 

 Pachuca, Mexico. National Museum No. 51,288. Clear, faceted 

 crystals, having the same refractive index as (1) the artificial 

 tridymite above, (2) the tridymite from the glass furnace. 

 (3) some natural tridymite crystals obtained through the kind- 

 ness of Mr. E. S. Larsen, Jr., of the U. S. Geological Survey. 



Sample Quenched 

 Temperature after : Results. 



1677° 5 min. Many grains all glass, some few 



partly melted. 

 1667° 10 min. A clear crystal which showed no 



sign of inversion or melting. 



12 U. S. Geol. Survey, Bull. 640, 133, 1916. Schaller's values of the 

 refractive indices of this material are about 0-004 higher than Fenner's for 

 pure tridymite. 



