Abstract
Five samples of differently colored sodalite-group minerals from gem lazurite deposits were studied by means of electron microprobe and wet chemical analyses, infrared, Raman, electron spin resonance (ESR) and UV-Visible spectroscopy, and X-ray diffraction. Various extra-framework components (SO42−, S2− and Cl− anions, S3•−, S2•− and SO3•− radical anions, H2O, CO2, COS, cis- as well as trans- or gauche-S4 neutral molecules have been identified. It is shown that S3•− and S4 are the main blue and purple chromophores, respectively, whereas the S2•− yellow chromophore and SO3•− blue chromophore play a subordinate role. X-ray diffraction patterns of all samples of sodalite-group minerals from lazurite deposits studied in this work contain superstructure reflections which indicate different kinds of incommensurate modulation of the structures.
Highlights
The crystal structures of sodalite-group minerals and related compounds are based on the aluminosilicate framework (Alx Si1-x O2 )x– built by alternating layers of six-membered rings of (Si,Al)O4 tetrahedra around the 3-fold axes, [0 0 z], [1/3 2/3 z], and [2/3 1/3 z] and hosting large cavities
Metasomatic rocks enriched in blue sodalite-group minerals are used as a decorative stone with the general name “lapis lazuli” or “lazurite”
The empirical formulae of most sodalite-group minerals from gem lazurite deposits, being calculated under the assumption that all sulfur belongs to the SO4 2− anion, are not charge balanced
Summary
The crystal structures of sodalite-group minerals and related compounds are based on the aluminosilicate framework (Alx Si1-x O2 )x– built by alternating layers of six-membered rings of (Si,Al)O4 tetrahedra around the 3-fold axes, [0 0 z], [1/3 2/3 z], and [2/3 1/3 z] and hosting large cavities (so-called β-cages). Metasomatic rocks enriched in blue sodalite-group minerals are used as a decorative stone with the general name “lapis lazuli” or “lazurite”. The latter name is applied to minerals of the sodalite group containing sulfide sulfur. Sodalite-group minerals from gem lazurite deposits have metasomatic origin and formed at temperatures below 600 ◦ C [17]. The oldest Sar-e Sang deposit was discovered in ancient times in the Koksha valley, Badakhshan, Afghanistan, and Afghan lapis lazuli is considered the best in the world. Its color varies from light blue and turquoise to deep blue and blue–violet. The most expensive stone is bright blue with a violet tint
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