6i8 /. E. L. VOGT 



and consequently the inversion temperature, even if Vi — V2 only- 

 amounts to a medium positive value, will have a not quite incon- 

 siderable rise on increasing pressure. And if Vi— V2 is considerable 

 (and positive) the rise of inversion temperature owing to pressure 

 will be exceedingly great} 



This has been experimentally examined for monoclinic and 

 orthorhombic sulphur (monocline S, on pressure of one atmosphere 

 having its melting point at 119.25°, density = 1.98, melting heat, = 

 12.5 cal. I inversion temperature between monoclinic and ortho- 

 rhombic S, on pressure of one atmosphere = 95.6°. inversion heat = 

 2.52 cal., density of orthorhombic S = 2.07 and melting point = 112.8), 



On pressure the inversion temperature between monoclinic and 

 orthorhombic sulphur rises more than the melting curve of mono- 

 clinic sulphur. The two curves intersect at a pressure of 1320 

 kilogrammes pr. cm^ ( = 1275 atmospheres) and at a temperature 

 of 151°, and at still higher pressure only orthorhombic sulphur may 

 be formed.'^ 



Accordingly it will not be surprising that, in the case of a rock- 

 forming mineral which at low pressure has an a-form (at high 

 temperature) as well as a jS-form (at lower temperature), it should 

 always be the j8-form,^ or the form stable at lower temperature, that 

 crystallizes in eruptive rocks formed at high pressure. 



Let us consider particularly some of the most important modi- 

 fications of SiOz : 



The inversion from a-cristobalite into a-tridymite at 1470=*= 10°.'' 



The inversion from a-tridymite into a-quartz at 870=*= 10°).'' 



' Therefore, the doctrine, advanced by me m 1908, of the inconsiderable rise of the 

 melting point of silicate minerals cannot also, as a general rule, be transferred to the 

 inversion point. I refer, on this subject, to the instructive remarks, made by V. M. 

 Goldschmidt, Die Kontaktmetamorphose im Kristianiagebiet (191 1), p. 112, and by 

 C. N. Fenner, Jour, of the Wash. Acad, of Sci. (19 12), 2. 



^ See G. Tammann Krystallisieren und Schmelzen (1903), and several publications 

 by Roozeboom reviewed in Doelter's Phys.-chem. Mineralogie (1905), p. 31. 



2 According to the terminology used by Boeke and by many other mineralogists, 

 I employ a to designate the modification stable at a higher temperature, /3 and y to 

 designate the modifications stable at lower temperatures. To avoid misconception, 

 it should be noted that some investigators, among them Wright and Larsen, have 

 employed quite the reverse terminology (o designating lower, and /3 higher temperature) . 



4 C. N. Fenner, Amer. Jour, of Sci., XXXVI (1913). 



