Iceland spar

In short

Iceland spar is a chemically pure, colourless and highly transparent variety of calcite (calcium carbonate). It is named after the Helgustaðir mine in eastern Iceland, source of the clearest specimens. It splits light passing through it into two separate rays, so writing seen through it appears doubled. Thanks to this “double refraction” it has been used in experiments on light since the 17th century, and it was essential for making the first prisms that polarize light. During the Second World War it was treated as a strategic raw material for military gunsights.

Photos

How to recognise it

1. 1

   Placed over writing or a line, it splits the image in two: the writing appears twice, slightly offset (double refraction).

2. 2

   It is colourless and water-clear; its perfect cleavage in three directions lets it break easily into slanted box shapes (rhombohedra).

3. 3

   As a calcite it is soft (Mohs 3): a steel knife and glass scratch it, and it fizzes in dilute acid.

4. 4

   Optical-grade pieces are free of clouds, cavities, inclusions, twinning lamellae and cleavage cracks; in the 1940s a piece had to yield a flawless rhombohedron of at least 3.5 × 1.8 × 1.8 cm to count as optical grade.

Myths and misconceptions

• MythThe Vikings navigated in cloudy weather with Iceland spar as a “sunstone”; this has been proven.

  FactIcelandic sagas mention a “sunstone” that showed where the sun was in an overcast sky, and in 1967 it was suggested that this could have been Iceland spar. But no Viking-age sunstone has ever been found, and experiments have given mixed results. It remains a hypothesis.

• MythIceland spar is found only in Iceland.

  FactThe name comes from Iceland, but optical-grade calcite has also been mined in Mexico (Chihuahua, Durango, Sonora), the United States (Montana, New Mexico, California) and South Africa. The bulk of the optical calcite mined during the Second World War came from Montana, and in Mexico more than a hundred small mines were worked between 1942 and 1944.

Did you know?

Where is it found?

• Helgustaðir, Reyðarfjörður, Iceland · Type localityThe mine that gave the stone its name, worked from the mid-17th century into the 20th; the largest and clearest known Iceland spar and most museum specimens came from here. A nature reserve since 1975.
• Park and Sweet Grass Counties, Montana, USA · Major depositThe bulk of the optical calcite mined during World War II came from the large calcite veins of south-central Montana (the point is the centre of Sweet Grass County).
• Near the Harding mine, Taos County, New Mexico, USA · Known localityFound in 1931 and worked from 1939; a giant calcite crystal estimated to weigh 30 to 40 tons was exposed here.
• La Fe mine, Rodeo area, Durango, Mexico · Major depositLying 15 km south-west of Rodeo, this mine was Mexico’s most productive optical calcite deposit during the Second World War, yielding nearly 6,500 pounds (about 2.9 tonnes) of suboptical calcite in 1944 (the point is the town of Rodeo).
• Manuel Benavides area, Chihuahua, Mexico · Known localityMexico’s highest-quality optical calcite came, in 1942–1944, from deposits south-east of this town and from around Areponapuchic in south-western Chihuahua.

Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →

Family

Mineral

Calcite

The main mineral of limestone, marble and cave stalactites: it fizzes in acid, and its clear crystals show everything double.