Instances are described in which Dauphiné twinning has been formed artificially in quartz by sawing. The secondary twinning occurs as a very thin surface layer, usually on one side of the saw cut only, and seems to have been produced by frictional heat developed at the cutting edge of the saw blade. The twinning often is restricted to particular growth zones in the quartz. The tendency to twin is related to the tendency of quartz to become smoky in color when irradiated with x-rays, easily twinned quartz being relatively little affected.

The boundaries of some natural Dauphiné twins in quartz coincide with natural smoky color zones. In these specimens the original boundary of the twin is returned more or less exactly when the crystals are re-inverted at 573°C., and a differential smoky coloration opposite to that existing initially is affected across the original twin boundary by radiation. Brazil and natural Dauphiné twins of colorless quartz sometimes also may be differentially pigmented by radiation, but Dauphiné twins produced artificially in originally untwinned quartz are not so affected.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.