Calcite, Cleavage, and Double Vision

Calcite is calcium carbonate, one of the most common minerals in the crust and the main constituent of limestone, marble and chalk. It is soft, hardness 3, and splits cleanly in three directions into rhombs, so a broken fragment is itself a tidy rhombohedron. Its optical importance is real: clear calcite, the transparent variety known as Iceland spar, is strongly birefringent, splitting a single image into two, and it was used in nineteenth- and twentieth-century polarising instruments; some historians have argued, on disputed evidence, that a spar sunstone could have helped Norse navigators locate the sun through cloud. Calcite occurs in many colours through trace impurities and is a frequent subject of the modern crystal trade, where the different colours are assigned to different chakras, a twentieth-century scheme, not a traditional one.

Much of what is called Egyptian alabaster in museum collections and in the vessels and canopic jars of Egyptian burial practice is in fact calcite, specifically the banded travertine form long quarried at Hatnub in Middle Egypt, and not true alabaster, which is gypsum; the two very different minerals share only a milky translucence and a shared trade name that has caused confusion since antiquity. Pure white calcite marble was likewise the material of choice for Mediterranean sculpture, quarried above all at Carrara in Italy, a quarry still active and still the source most closely associated with the tradition running from Roman statuary to Michelangelo.