Ultramarine blues

   The soft metamorphic mineral lapis lazuri (or lazurite) was prized by the ancient Egyptians for its blue colour and was cut, carved and polished for ornamental uses. Deposits of the mineral occur in, for example, Iran and Afghanistan. Powdering lapis lazuli produces the pigment ultramarine, although for commercial purposes, synthetic ultramarine is now manufactured by heating together kaolinite, Na₂CO₃ and sulfur. Lapis lazuri is related to the aluminosilicate mineral sodalite, Na₈[Al₆Si₆O₂₄]Cl₂, which contains a zeolite framework (the sodalite or SOD lattice type). The cavities in the zeolite framework contain Na‏ cations and Cl^- anions. Partial or full replacement of Cl^- by the radical anions [S₂]^-  and [S₃]^- results in the formation of ultramarines, and the chalcogenide ions give rise to the blue pigmentation. The relative amounts of [S₂]^-  and [S₃]^- present determine the colour of the pigment: in the UV–VIS spectrum, [S₂]^- absorbs at 370 nm and [S₃]^- at 595 nm. In artificial ultramarines, this ratio can be controlled, so producing a range of colours through from blues to greens

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