and Rock Densities at High Temperatures. 21 



The Brazilian quartz used in our experiments showed under 

 the microscope no trace of gas bubbles or inclusions. 8'37 

 grams (in large fragments) heated for a few minutes at 620° 

 lost l'8 mg or 0*022 per cent. Heated further for 2^ hours at 

 1105°, the quartz lost - 3 milligram additional, or 0*025 per 

 cent in all. If this were all water vapor at 1000°, it would 

 have a volume about 3 - 7 times that of the quartz. In contact 

 with graphite this volume might be doubled by the formation 

 of hydrogen and carbon monoxide. Such a volume of gas 

 would be sufficient to account for the phenomena observed. 

 This question was not pursued further as it would lead us too 

 far astray from our present purpose, and as it presents a large 

 problem for further work. 



Formation of cristobalite. — Fenner* has shown in this 

 laboratory that quartz is the stable form of silica under atmos- 

 pheric pressure only up to 870°. From 870° to 1470° tridy- 

 mite is stable, and above 1470°, cristobalite. Quartz in the 

 dry state will not pass directly into tridymite between 870° 

 and 1470°, but above 1000° it will be converted directly, 

 though slowly, into cristobalite. At 1300° this conversion is 

 fairly rapid. 



As will be seen from the curves of fig. 6, the volume begins 

 to increase noticeably above 1300°. In the series of 21 January, 

 1911, with a maximum temperature of 1602°, cristobalite 

 formed a surface layer over the block about half a millimeter 

 thick, and had also grown into the larger cracks, causing con- 

 siderable cracking and distortion of the block and increasing 

 its apparent volume about 15 per cent. This block was above 

 1400° for about 40 minutes. In the series of 13 February, 

 1911, considerably more cristobalite had formed, and the block 

 was easily broken to pieces ; this had been above 1400° for 65 

 minutes. In the series of 21 February the block was held at 

 1500° for about 3^ hours. The conversion to cristobalite was 

 still quite incomplete, although the chalky white growth of 

 crystals had penetrated all through the block. In every case 

 the cristobalite formed first on the exposed surfaces, and grew 

 inward, so that even in the most completely converted block 

 there were clear unaltered fragments weighing a gram or more 

 in the center of the block, and microscopic unaltered angular 

 quartz scattered all through the converted portions. f The 

 apparent volume increase in this last case amounted to 52 per 

 cent. On account of the friable and porous character of the 

 cristobalite formed, however, this is considerably larger than 



*C. N. Fenner, Jour. Washington Acad. Sci., ii, 471-480, 1912; this 

 Journal, xxxvi, 331-384, 1913. 



f This dependence of the growth of cristobalite upon the surface exposed 

 has been noted by Endell and Rieke, Zs. anorg. Chem., lxxix, 239-259, 1912. 



