Wright and Lav sen — Quartz as a Geologic Thermometer. 425 



considered the problem in detail, and by means of etch figures 

 combined with crystal lographic reasoning has been able* to 

 show that while the low temperature form of quartz stable 

 below 575°, and called by him a-qnartz, crystallizes in the 

 trapezohedral-tetartohedral division of the hexagonal system, 

 the high temperature /3-form, stable above 575°, is also hex- 

 agonal but in all probability trapezohedral-hemihedral in its 

 symmetry, the axial ratios of the two forms being, however, 

 very nearly identical. Practically the only crystallographic 

 change which takes place on the inversion is a molecular 

 rearrangement, such that the common divalent axes of the 

 high temperature /3-form become polar in a-form, and this 

 fact involves certain consequences which can be used to dis- 

 tinguish quartz which has been formed above 575° from quartz 

 which has never reached that temperature. At ordinary tem- 

 peratures all quartz is a-qnartz, but if at any time in its his- 

 tory a particular piece of quartz has passed the inversion point 

 and been heated above 575°, it bears ever afterward marks 

 potentially present which on proper treatment can be made to 

 appear just as an exposed photographic plate can be distin- 

 guished at once from an unexposed plate on immersion in a 

 proper developer, although before development both plates 

 may be identical in appearance. 



To corroborate the data of Mallard and Le Chatelier, and at 

 the same time to locate the inversion point more accurately, if 

 possible, the birefringence and circular polarization of quartz 

 were remeasured by means of a specially constructed thermal 

 (electric resistance) microscope.* For the measurement of the 

 birefringence, polished plates of different thickness cut parallel 

 with the principal axis were used and readings with the Babinet 

 compensator taken, both in white light and in sodium and 

 lithium lights. The results of these observations are contained 

 in tables la and Ih below, and are expressed graphically by the 

 curves I, II, III of fig. 1. In Table la the results of measure- 

 ments of the birefringence in white light at different tempera-, 

 tures on four different plates are listed. In each column the 

 readings taken while heating the plate have been combined 

 with those obtained on cooling ; these readings in every instance 

 were practically identical and indicate a remarkably rapid and 

 complete change at the temperatures of inversion and rever- 

 sion. In Table I&, column I, the measurements of the bire- 



* Described in this Journal, xxvii, 42-44, 1909. 



Table la. Birefringence measurements with Babinet compensator on 

 four different quartz plates at different temperatures in white light. In 

 each column the readings taken while heating the plate have been combined 

 with those obtained on cooling ; these readings in all four cases were practi- 

 cally identical and indicate a remarkably rapid and complete change at the 

 temperatures of inversion and reversion. 



