MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 619 



The inversion from a-quartz into jS-quartz at 575=^ 2°.' 



According to the statement made by A. L. Day, R. B. Sosman, 

 and J. C. Hostetter^ the specific volume of j8-quartz is at 20° 0.3775 

 ( = density 2.649), but rises by heating, and at 561°, or just below 

 the inversion point, it has risen to 0.3922. At the inversion point 

 there is a sudden rise, which, however, is not precisely stated, and 

 then we pass on to a-quartz at 585° with a specific volume of 0.3972. 



According to the statements of F. E. Wright and Esper L. 

 Larsen the inversion heat, when iS-quartz is transformed into 

 a-quartz, amounts to 4.3 =t i cal. If we set the volume difference at 

 the point of inversion at 0.003 and the inversion heat at 4.3 cal, 

 there is a rise of 0.015° per atmosphere, which makes at a pressure 

 of 1000 atmospheres, corresponding to a depth of about 3.7 kilo- 

 meters, a rise of about 15°. This affords a measure for the order 

 or quantity which has to be taken into account. Accordingly the 

 difference between a-quartz and /3-quartz, if depths of more than 

 5 kilometers are not involved, can, at any rate with only a little 

 modification, be employed as a geological thermometer. 



As to the relation of inversion point and pressure between 

 a-quartz and tridymite the case is quite different. At room tem- 

 perature tridymite has a specific volume of 0.4329 (density = 2.31, 

 medium of 2.282 and 2.326) and jS-quartz a specific volume of 0.3775 

 (density = 2.649); Vi— V2 accordingly amounts to 0.0554, which is a 

 very high value. By heating, j8-quartz, as has just been mentioned, 

 expands its volume considerably and then changes into a-quartz, 

 which, according to the statements of Day, etc., has at 850° a specific 

 volume of 0.3957. According to the law of the expansion of bodies 

 by heating, tridymite also must expand its volume at higher temper- 

 ature, but how much is not stated. If we estimate its specific 

 volume at 875° at 0.445, the volume difference Vi— V2 at 875° will 

 amount to 0.05 (or perhaps a little more). 



The inversion heat is not stated, but must be supposed to be 

 tolerably low. A rough estimate, rating the inversion heat at 10 

 and 5 cal., respectively, gives a rise of inversion temperature per 



'F. E. Wright and E. S. Larsen, "Quartz as a Geologic Thermometer," Amer. 

 Jour, of Scl, XXVII (1909). 



2 Amer. Jour, of Sci., XXXVII (19 14). 



