438 Wright and Larsen — Quartz as a Geologic Thermometer. 



grains are present only potentially, and appear so distinctly on 

 etching that an apparently clear plate of quartz which has 

 been heated above 575° may crumble down in the etching acid 

 and break up into a number of small grains, while the purely 

 a-quartz remains intact and is etched with much greater 

 uniformity. — The fact that thin plates of quartz may warp 

 and bend temporarily at the inversion temperature, thus find- 

 ing relief from the strains set up on the change, while thicker 

 plates bend less easily and tend to fracture more readily, is a 

 factor which should be considered in any particular case. 

 Small grains, being thus less liable to- fracture, may not show 

 the phenomena of shattering as clearly as might be expected. 



Crystallographically, the difference in crystal class between 

 the a- and /3-forms finds expression in the crystal habit. In 

 the /3-form, the pyramid faces are equally developed ; trigonal 

 trapezohedrons are absent, the habit of the crystals being 

 usually that of. the simple dihexahedrons observed in quartz 

 porphyries and allied rocks. Crystals of the low temperature 

 a-form, on the other hand, are usually prismatic in habit and 

 often show marked differences in the size and character of the 

 rhombohedral faces. Trigonal trapezohedrons may occur and 

 stamp the crystal on which they do appear at once as a low 

 temperature form. 



Briefly stated, the four criteria which can be used to distin- 

 guish, at ordinary temperatures, quartz which was formed 

 above 575° from quartz which has never been heated to that 

 temperature, are : (1) Crystal form, if crystals be available, 

 the presence of trigonal trapezohedrons and other evidence of 

 tetartohedrism, irregular development of the rhombs and the 

 like, being indicative of the a-form. (2) Character of twinning, 

 as shown by etch figures on the basal pinacoid. In the a-form, 

 which crystallized from solutions at comparatively low tempera- 

 tures, the twinning is usually regular and sharply marked, 

 while in quartz plates originally of the /3-form and now a by 

 virtue of inversion in the solid state, the lines are usually 

 irregular, and the twinning patches are small and bear no 

 relation to the outer form of the crystal. (3) Intergrowths of 

 right- and left-handed quartzes are more frequent and more 



Table 6. In this table are assembled the results of the examination of 44 

 different quartzes occurring in nature. A number of basal plates of each of 

 these quartzes were cut and polished and etched for 75 minutes in cold 

 commercial hydrofluoric acid. The quartzes were examined particularly 

 with respect to: (1) the occurrence and character of intergrowths of right- and 

 left-handed individuals of quartz ; (2) the frequency and character of the 

 twinning of the quartz plates as brought to light by means of the etch figures 

 obtained by immersion of the plates in hydrofluoric acid ; (3) the character 

 of the plates themselves, whether clear and comparatively free from cracks 

 or much shattered and often crumbling after immersion in hydrofluoric acid. 

 Wherever crystals were used these were examined still further for evidences 

 of tetartohedrism. 



