Wright and Larsen — Quartz as a Geologic Thermometer. 423 



above 1190°, would have been precipitated. Since the specific 

 volumes of wollastonite and pseudo-wollastonite are practically 

 identical, it is probable that the effect of pressure on this 

 inversion point is very slight, and can, therefore, be neglected. 



Melting points of minerals and of definite aggregates of 

 minerals (eutectics), melting regions of rocks, inversion tem- 

 peratures of minerals and stability ranges for different forms 

 of the same chemical compound, furnish the geologist with 

 fixed points on his geologic thermometer scale, just as the 

 freezing and boiling points of water are the two standard fixed 

 points on the ordinary thermometer scale. 



In like manner, experimental data may furnish points of 

 reference for, a geologic pressure gage, which is of equal 

 import to the geologist. 



Equipped with a satisfactory geologic thermometer scale and 

 geologic pressure gage expressed in terms of stability ranges 

 of different minerals and aggregates of minerals under dif- 

 ferent conditions, the geologist would be in a position to 

 attack many problems which, at the present time, defy all 

 solution. Exact data of this sort in turn tend to act as a gov- 

 ernor on geologic theory establishing limits of temperature 

 and pressure beyond or below which it is not safe to assume 

 certain conditions ; and at the same time they strengthen 

 materially the foundation of fact on which geologic reasoning 

 is based. 



For such geologic thermometric purposes, quartz has been 

 found by experience to be well adapted. It is plentiful in 

 nature and occurs in many different kinds of rocks. Si0 2 in 

 the form of tridymite melts at about 1625°, * while between 

 that temperature and about 800° tridymite is the stable phase ; 

 below about 800° quartz is the stable phase. From evidence 

 thus far gathered, it is probable that pressure has but slight 

 effect on raising or lowering such an inversion point, and that, 

 therefore, wherever quartz appears in nature, it was formed at 

 a temperature below about 800 °.f 



Quartz itself undergoes a reversible change at about 575°. 

 This was first observed by Le Chatelier^: in 1890, who noted 

 a sudden change in the expansion coefficients and circular 

 polarization of quarti heated to and above 575°. At the same 

 time Mallard and Le Chatelier§ found noticeable changes in 

 the birefringence at about 575°, and recently O. Mugge|| has 



*The Lime-Silica Series of Minerals, A. L. Day and E. S. Shepherd, 

 Optical Study by F. E. Wright, this Journal, xxii. 271-273, 1906. 



f Vogt has shown that for a pressure corresponding to a depth of 100 km. 

 this inversion point would probably be raised less than 135°. 



{Cornpt. Eend.. cviii, 1046, 1889; cix, 339, 1890; Bull. Soc. Min., xiii, 

 112, 1890; xiii, 119, 1890. 



SCompt. Eend., ex, 339, 1890 ; Bull. Soc. Min., xiii, 123, 1890. 



| Neues Jahrb. Festband, 181-196, 1907. 



