Wright and Larsen — Quartz as a Geologic Thermometer. 437 



arrangement. If, furthermore, a regularly twinned crystal of 

 quartz be heated above 575° and then etched on the basal pina- 

 coid, the twinning lines are as a rule no longer straight and reg- 

 ulai\ but the field appears divided by small patches of irregularly 

 twinned material. In passing from the tetartohedral a-form 

 to the hemihedral /3-form, the bivalent common axes lose their 

 polarity and the tendency in the latter form to form twins, 

 therefore, is much less strong than in the a-form. On revert- 

 ing later to the a-form, the common axes of the /3 form become 

 again polar and the tendency during the molecular rearrange- 

 ment is again to form twins, and, in this instance, twins with 

 irregular boundary lines, since the change takes place rapidly 

 and in the solid state. — The form and character of twinning 

 on basal sections of quartz can therefore be used as one of the 

 criteria in determining whether or not quartz has been formed 

 above or below 575° C. 



Still a second fact of observation can be used to advantage 

 in ascertaining the original temperature of formation of 

 quartz.* Quartz is circularly polarizing and may rotate the 

 incident plane polarized light waves either to the right or to 

 the left. Experiments on the crystallization of circularly 

 polarizing bodies have indicated that a slight change in the 

 mother solution is often sufficient to change the character of the 

 rotation of the crystal being precipitated. In quartzes formed 

 at low temperatures, vein quartzes and the like, one might 

 expect intergrowths of right- and left-handed crystals more 

 frequently than in magma quartzes where rapid changes in the 

 composition of the solutions are less likely to occur. — In the 

 low temperature quartzes crystallizing out of quietly circulating 

 solutions, moreover, the conditions are less violent than in a 

 magma above 575° and the processes of precipitation might 

 well be considered to proceed with more regularity and uniform- 

 ity at the lower temperatures than above the inversion point. 

 The tendenev of intergrowths of right- and left-handed crystals 

 of the low temperature phase should accordingly be toward 

 regularity of outline of the intergrowths and toward hexagonal 

 symmetry. — The fact of intergrowths of left- and right-handed 

 quartz and the character of such intergrowths is a second factor 

 to be considered in the investigation of any particular quartz. 



A third feature which is of service in this connection is the 

 shattering and cracking of quartz crystals on passing the inver- 

 sion temperature as a result of the abrupt change in the coeffi- 

 cient of expansion. This occurs both on heating and on 

 cooling. It is safe to assume, therefore, that large clear quartz 

 plates free from fractures have in all probability never reached 

 the inversion temperature. The fracture cracks in many small 



Am. Jour. Sci.— Fourth Series, Vol. XXVII, Xo. 162.— June, 1909. 

 30 



