436 Wright and Larsen — Quartz as a Geologic Thermometer. 



energy involved is, however, so small that thermal methods of 

 study are relatively unproductive compared with optical 

 methods. 



It is of interest to note that iu a substance like quartz, which 

 in some respects is exceedingly inert and sluggish, certain 

 changes of equilibrium are extremely sensitive to temperature 

 differences, a difference of one tenth of one degree being 

 sufficient to cause the shift from the one form to the other. It 

 , is indeed difficult to form an adequate and satisfactory picture 

 of a mechanical system which shall satisfy the conditions of 

 such nice equilibrium and adjustment. In the case of quartz, 

 however, it is fortunate for the observer that certain physical 

 properties which can be determined with great accuracy at 

 different temperatures are extremely sensitive to the inversion, 

 since the actual change in energy content, or amount of heat 

 involved in the transformation, is extremely slight, and too 

 small, in fact, to be detected by ordinary methods for meas- 

 uring temperatures. Notwithstanding the comparatively insig- 

 nificant amount of energy required in the transformation, it is 

 still sufficient to cause a readjustment of the crystallographic 

 forces, such that the low temperature a-quartz and the high 

 temperature /3-quartz crystallize in all probability in different 

 subdivisions of the hexagonal system, and at the same time, 

 intricate twinning phenomena may be set up, the effects of 

 which are in general sufficient to enable the observer to distin- 

 guish a-quartz from yQ-quartz. The criteria which have been 

 developed for accomplishing this distinction will now be con- 

 sidered briefly and these in turn applied to different quartzes 

 as they occur in nature. 



In a recent paper, Mugge* has proved that /3-quartz crystallizes 

 in all probability in the trapezohedral-hemihedral division of 

 the hexagonal system, and that its axial ratios are practically 

 identical with those of a-quartz. — The chief crystallographic 

 change which takes place at the inversion point is a molecular 

 rearrangement such that the common divalent axes of the high 

 temperature /3-form become polar in the a-form stable at low 

 temperature, and a tendency to restore crystallographic equi- 

 librium in the a-form by twinning after the prism is therefore 

 active on the inversion from the high /3-form. This twinning 

 phenomenon is best studied by means of etch figures (obtained 

 by immersing for 1J hours plates of quartz in cold commercial 

 hydrofluoric acid) on the basal pinacoid. Miigge found that 

 if a plate of untwinned quartz after (0001) be heated above the 

 inversion point and, after cooling, etched, it is no longer a 

 simple crystal, but an intricate complex of twins after (1010), 

 the twinning lines being as a rule irregular and without definite 



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



