The crystal that doubles everything may have guided Viking ships
Look through a clear piece of calcite and every letter on the page beneath splits into two. That strange doubling may have helped Norse sailors find the sun behind cloud, and Icelandic sagas mention a navigating sunstone that some believe was this very mineral.
The effect is called birefringence, or double refraction, and the Danish scientist Rasmus Bartholin first described it in calcite in 1669. A crystal bends light by two different amounts depending on its polarisation, so a single beam emerges as two images. Rotate a pure crystal until both images look equally bright while viewing the sky, and the faint pattern of polarised light around a hidden sun can be picked out even on an overcast day, giving a navigator a fixed reference point. The finest clear variety, Iceland spar, was once mined at Helgustadir in Iceland, now a nature reserve where digging is banned.
The same property proved useful long afterwards. Before plastic polarising sheets existed, calcite prisms were the standard way to polarise light, and during the Second World War high-grade optical calcite went into bomb sights and anti-aircraft gun sights. Researchers have even experimented with it for invisibility cloaks.
Calcite is the most stable form of calcium carbonate and one of Earth's commonest minerals. It sets the value of 3 on the Mohs hardness scale, fizzes in acid as it releases carbon dioxide, and makes up most limestone, much of it built from the shells of dead sea creatures, as well as marble. It grows in more than a thousand crystal forms, with fanciful names like dogtooth spar and nailhead spar. Oddly, it becomes less soluble as water warms. Groundwater carrying dissolved carbon dioxide can either dissolve it, hollowing out caves, or deposit it as stalactites and cement between rock grains.
People have long prized it. Ancient Egyptians carved it and linked it with the goddess Bast, a connection that fed into the word alabaster. Michigan's Calcite Quarry, worked for over 85 years, is the largest carbonate mine on Earth.
Source: Calcite
