GEOPHYSICAL LABORATORY. 97 



pends, we may pass in rapid review some of the mineral studies now in prog- 

 ress. Of these the investigation of the various forms of pure silica and their 

 relation to each other is perhaps the oldest. 



Silica is the most common and widely distributed ingredient in all of our 

 rocks. It is therefore of first importance to know the precise conditions 

 under which it crystallizes. This is the more necessary because it occurs 

 in so many different crystal forms — at least seven, known to mineralogists 

 by the names quartz (two forms, a and ft), tridymite (a and ft), cristobalite 

 (a and ft), and chalcedony — and neither the relations between these nor the 

 conditions of their formation are known. 



In one of the earlier papers from this laboratory evidence was offered that 

 quartz, when heated to about 8oo°, goes over (without melting) into another 

 crystal form, tridymite (or cristobalite), albeit very slowly even in the pres- 

 ence of a liquid flux which greatly increased its molecular mobility. It is 

 also known that quartz itself undergoes a change of crystal form (a to ft) at 

 575 , this change being very prompt in its appearance and sharp. Both 

 changes are reversible. It was furthermore found, by reason of the extremely 

 sluggish behavior in the vicinity of 8oo°, that the low-temperature form 

 (quartz) could exist for a considerable time at temperatures higher than 8oo° 

 and that the high-temperature forms (tridymite and cristobalite) if cooled 

 quickly down past this temperature could exist indefinitely at ordinary tem- 

 peratures in spite of the tendency to change to quartz. 



Continuing this investigation, we now find further that under more favor- 

 able conditions the temperature of the sluggish change from quartz to tridy- 

 mite is very near 870 , and that tridymite and not cristobalite is the stable 

 form above 870 as far as 1470 , where tridymite (also without melting) 

 goes over into cristobalite. Further heating melts the cristobalite very slowly, 

 beginning at 1600 C. The change from tridymite to cristobalite at 1470 is 

 also reversible. From these observations it appears that it is not strictly cor- 

 rect to speak of the melting-point of quartz, as is frequently done. Quartz 

 normally* passes through three different modifications (^-quartz, tridymite, 

 and cristobalite) before reaching the melting temperature, and finally melts 

 as cristobalite. It has been previously noted that when tridymite and cristo- 

 balite are quickly cooled past the inversion temperatures they may be cooled 

 to ordinary temperatures (where quartz alone is stable) without reverting 

 to quartz. It does not follow that they do so unchanged. Although neither 

 can be persuaded to revert to the form stable at these low temperatures, 

 each undergoes a further and definite change of crystal form. Tridymite 

 inverts (ft to a) at 117 (reversible), and cristobalite (ft to a) at a tem- 

 perature (205 to 275°) variable with certain experimental conditions which 

 are not yet cleared up (also reversible). Chalcedony, on the other hand, 

 though of identical chemical composition, shows no change whatever when 



* In the comparatively rapid heating necessary in all laboratory work these steps 

 may not all occur in each instance. 



7 — yb 



