446 Wright and La r sen — Quartz as a Geologic Thermometer. 



deduced theoretically for /3-quartzes formed above 575°. This 

 places the temperature of final solidification of an intrusive 

 granite mass above 575°. With the quartzes examined in the 

 course of this investigation, a number of other minerals, 

 garnet, magnetite, albite, lepidolite, etc., were associated, and 

 in certain instances where, from the degree of idiomorphism 

 and similar criteria, the relative periods of precipitation of the 

 associated mineral can be ascertained, temperature limits of 

 formation of the latter can thus be established. By thus 

 determining stability ranges of certain minerals, points on the 

 geologic thermometer scale- are gained which in turn serve to 

 fix limits for the temperatures of formation of other associated 

 minerals. 



Summary. 



In the foregoing pages, attention is directed to a geologic 

 thermometer scale the points for which are to be sought in the 

 stability ranges of the different phases of rock-making minerals 

 (their melting and inversion temperatures), and also in the 

 melting temperatures of certain mineral aggregates (eutectics). 

 Quartz is well adapted to furnish at least one and possibly two 

 points for the geologic thermometer scale, since on heating at 

 575° it suffers an enantiotropic change to a second phase, called 

 /3-quartz by Miigge, while above 800° it is no longer stable at 

 ordinary pressures, but passes into tridymite. Following the 

 example of Le Chatelier and Mallard, the point of inversion 

 of a- and yS-quartz was redetermined by observing the abrupt 

 change in the birefringence, circular polarization and expansion 

 coefficient at that temperature. The most accurate optical 

 determinations place this inversion temperature at 575° ± 2°. 

 Proofs that these represent an energy change were obtained by 

 the perceptible variation in heat capacity in this region by the 

 Frankenheim method of heating and cooling curves ; and also 

 by direct determination of the specific and latent heats in this 

 region. Crystallographic proof of the change has been studied 

 in detail by O. Miigge, who finds the high temperature phase, 

 /3-quartz, to be in all probability hexagonal and trapezohedral- 

 hemihedral, while the low temperature a-quartz is hexago- 

 nal and trapezohedral-tetartohedral. This particular relation 

 between the two phases entails certain consequences which can 

 be used as criteria to distinguish quartz which has been heated 

 above 575° from quartz which has never reached that tempera- 

 ture. These criteria were in large part indicated by O. Miigge 

 and have been applied above to a number of natural quartzes 

 occurring in different kinds of rocks ; the net result of the 

 investigation being that vein and geode qaartzes and certain 

 large pegmatite quartz masses and pegmatite veins were formed 



