Double Trouble: Navigating Birefringence.
Skalwold, Elise Ann and William Akers Bassett. (2015) 20 pages. ISBN 978-0-939950-02-7
Abstract: Optical mineralogy has many fascinating though often complex concepts which underlie common effects observed in minerals and lapidary specimens fashioned from them. Doubling of images such as seen through a calcite rhomb is perhaps one of the most readily observed of these properties and could well have been one put to use centuries ago in a very practical way. The intriguing theory of the Viking’s use of a coveted stone to find their way in arctic waters has its roots in the ancient Viking Sagas, optical mineralogy, and in practical application by modern navigators. The proposed minerals thought to be the Viking “sunstone” are excellent models for understanding the optical phenomena of birefringence and pleochroism; the very properties which make them useful for navigation are also those which make them valuable to mineral and gem enthusiasts today (see Skalwold 2008). There are several candidates for the stone. Among them are “Iceland Spar” calcite of which a coveted optical-quality variety was found abundantly in eastern Iceland, and the blue variety of the mineral cordierite, found in Norway and popularly known as “Viking’s Compass” and as the gem “iolite.” While the latter’s extraordinary pleochroism is explored in the authors’ article “Blue Minerals: Exploring Cause & Effect” (Skalwold and Bassett 2016), the more likely candidate, Iceland spar, is the classic model for demonstrating the phenomenon of birefringence and doubling in optically anisotropic minerals. However, whether one’s adventures with minerals are land-bound or at sea, before venturing far there is some trouble with doubling to untangle first.
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