ICELAND SPAR: THROUGH THE LOOKING-GLASS

In Through the Looking-Glass, the sequel to Alice's Adventures in Wonderland, author Lewis Carroll used the title phrase as a metaphor for peering into a world where nothing was quite what it seemed. When it came to understanding the mysterious nature of light, the "looking glass" for early scientists was optical calcite or Iceland spar.

- STEVE VOYNICK

Specimens of the Iceland spar variety of calcite are popular items in rock shops and at gem-and-mineral shows. Looking like miniature ice cubes, these colorless, transparent, cleaved rhombohedrons sell for only a few dollars each. But the commonplaceness of these little rhombohedrons belies a fascinating history that has a profound impact on science and technology.

Calcite (calcium carbonate, CaCO3) crystallizes in the trigonal system and is slightly denser than quartz. Pure calcite is colorless, but impurities impart a range of soft colors. With its weak ionic bonding, calcite is rather soft (Mohs 3.0) and cleaves easily into rhombohedrons.

Birefringence is calcite's most notable property. This ability to polarize light and refract it in two slightly different directions creates the double images seen when objects are viewed through Iceland spar crystals.

SUN STONE

Centuries before the magnetic compass was introduced to Europe, Viking mariners were demonstrating an uncanny ability to navigate the northern seas, even when cloudiness and fog obscured the sun or when the sun was below the horizon.

Icelandic monastery texts dating to 1250 mention a sólarsteinn (sun stone) that the Vikings used to determine the sun's position without direct solar sightings. When a sun stone was held against the eye while scanning the horizon, polarized light cast two shadowy images upon the retina. The horizontal deflection at which their darkness equalized accurately indicated the position of the unseen sun.