“Iceland Spar” Calcite
Written by Boss Tambang Wednesday, 02 December 2009 00:07
Calcite is a very common mineral in many different geologic settings, but clear pieces like this are less common. Iceland spar is named after classic occurrences in Iceland, where fine specimens can be found as big as your head. This is not a true crystal, but a cleavage fragment. Calcite is said to have rhombohedral cleavage, because each of its faces is a rhombus, or warped rectangle in which none of the corners are square. When it forms actual crystals, calcite takes platy or spiky shapes.
If you hold a piece and look from one corner through the stone to the opposite corner, you’ll see why crystallographers classify calcite as a six-sided or hexagonal mineral. Look at how the letters behind the specimen are offset and doubled. The offset is due to refraction of the light traveling through the crystal, just as a stick appears to bend when you stick it partway into water. The doubling is due to the fact that light is refracted differently in different directions within the crystal. Calcite is the classic example of double refraction, but it’s not that rare in other minerals.
Calcite is used to define hardness 3 in the Mohs scale of mineral hardness. Your fingernail is about hardness 2½, so you can’t scratch calcite. If its appearance and hardness aren’t enough to identify calcite, the acid test, in which cold dilute hydrochloric acid produces bubbles of hydrogen on the mineral’s surface, is the definitive test. Very often calcite is fluorescent under a black light.
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