Perfect rhombohedral cleavage in three directions at 74° 55'
Core Principles & Analysis
Calcite is the most thermodynamically stable polymorph of calcium carbonate (CaCO₃) at ambient pressure, whereas orthorhombic aragonite and hexagonal vaterite convert to calcite upon heating.
Transparent rhombohedral crystals of 'Iceland spar' exhibit strong birefringence (nω = 1.658, nε = 1.486 at 590 nm), splitting transmitted light into two polarized rays — an effect first described by Rasmus Bartholin in 1669.
Unlike most minerals, calcite exhibits retrograde solubility, dissolving more readily in cold, carbon-dioxide-rich water than in warm water, which drives both karst cave dissolution and speleothem precipitation.