Lapis Lazuli

Lapis lazuli is made up not of a single mineral but an accumulation of minerals; it is mostly composed of lazurite (Na,Ca)8(AlSiO4)6(S,SO4,Cl)1-2), typically 30-40%. Lapis gemstones also contain calcite (white veins), sodalite (blue), and pyrite (gold flecks of colour). Dependent on metamorphic history and protolith chemistry, other common minerals in lapis include; augite, diopside, enstatite, mica, haüyanite, hornblende, and nosean. Some specimens also contain trace amounts of the sulphur-rich mineral lollingite (var. geyerite). Lazurite is a member of the sodalite group of feldspathoid minerals. Feldspathoids have chemistries that are close to those of the alkali feldspars but are deficient in silica. If free quartz were present at the time of formation, it would react with any feldspathoid precursor to form a feldspar. For more detail on this and other gemstones see Warren, 2016, Chapter 14).



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Lapis lazuli, Sar-e-Sang, Afghanistan

Natural lazurite contains both sulphide and sulphate sulphur, in addition to calcium and sodium, and so is sometimes classified as a sulphide-bearing haüyne. Sulphur gives lazurite its characteristically intense blue colour and comes in three polysulphide units made up of three sulphur atoms having a single negative charge. The S3- ion in the sulphur has a total of 19 electrons in molecular orbitals, and a transition among these orbitals produces a strong absorption band at 600 nm, giving the stone its intense blue colour with yellow overtones. The intensity of the gem’s blue is increased with increasing sulphur and calcium content, while a green colour is the result of insufficient sulphur.

Other members of the sodalite group include sodalite and nosean. Sodalite is the most sodium-rich member of the sodalite group and differs from the other minerals of the group in that its lattice retains chlorine. Interestingly, sodalite can be created in the laboratory by heating muscovite or kaolinite in the presence of NaCl at temperatures of 500°C or more. In the literature, the commonly accepted origin of lazurite is through contact metamorphism and metasomatism of dolomitic limestone. Such a metasedimentary system also requires a source of sodium, chlorine and sulphur; the apparent cause is interbedded evaporites in the protolith, as is seen in plots of its molecular constituents. 

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Components of lapis lazuli.

Lapis at Sar-e-Sang, Afghanistan

Scapolite and magnesian whiteschists are typically saline mineral phases in the classic deposits of the Sar-e-Sang District (Figure a; Faryad, 2002). There, the lapis is composed of a combination of lazurite, diopside, calcite and pyrite and occurs in beds and lenses up to 4 meters thick within a scapolitic magnesian-marble skarn near the centre of the Hindu Kush granitic massif. It is typically interlayered with or forms veins and lenses within a gneissic and pegmatitic host. Lens-shaped lodes generally are hosted in orthoclase–microcline–perthite hornfels containing albite and quartz. Lazurite bodies at the Sar-e-Sang deposit are associated with diopside metasomatites bearing nepheline, pale blue haüyne, and blue lazurite, and some lazurite-rich zones can contain up to 40-90 vol% lazurite (Figure b). The rocks also contain diopside, haüyne, afghanite, and nepheline, as well as disseminated pyrite replaced by pyrrhotite. Pockets of near pure lapis lazuli can be up to 40m across and occasionally up to a meter (for more details see Salty Matters, Nov. 13, 2015).

Schreyer and Abraham (1976) concluded that chemical variations in the metamorphic fluids at Sar-e-Sang were generated by progressive metamorphism and mobilisation of an evaporite deposit. Relict anhydrite and gypsum(rehydrated anhydrite) still occur in the Sar-e-Sang area. Whiteschists and the associated lapis lazuli deposits of the region are part of a highly metamorphosed evaporitic succession. Salts have largely vanished due to ongoing melting and volatilisation. The preservation of the three-stage succession of mineral assemblages, across such small scales and yet related to each other through isochemical reactions, means that the main factors governing the metamorphic history of this whiteschist were compositional changes of the coexisting fluids with time. Under this scenario any pressure-temperature variations were subordinate, and the chemistry of the fluids evolved as the evaporites underwent metasomatic alteration.

The sedimentary pelitic layers of this precursor evaporitic sequence first underwent a period of metamorphism in which fluid pressures approached lithostatic (stage 1). Subsequently at higher metamorphic grades, with the beginning of mobilisation of the salts, the metamorphic fluids became increasingly enriched in ions such as Na, Mg, Cl, SO4, BO3, etc., so that water fugacity dropped considerably. This period is represented by stage 2 of the whiteschist metamorphism and was characterised by strong metasomatism that led, for example, to the growth of dravite and the amphibolite, gedrite. The physical and chemical character of stage 3 is less clearly defined. Kyanite/sillimanite inversion requires an increase in temperature or a decrease in pressure, or both, but changes in the composition of a coexisting gas phase may have played an additional role in the formation of cordierite.

Unlike traditional metamorphic associations, the meta-evaporite-derived assemblage in Afghanistan may in a single thin section entrain mineral assemblages that conventionally would be assigned to the greenschist facies, the hornfels facies, and a high pressure (amphibolite) regime. The assemblages are in effect mosaic equilibria that reflect changes in fluid composition generated from a metamorphosing evaporite pile over time and only to a lesser degree by regional evolution of total temperature and pressure. Once again, evaporites make for unusual responses compared to the general reactions of metasediments.

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Lapis Lazuli at Sar-e-Sang, Afghanistan. A) Geological detail of the lapis occurrence (after Aleksandrov and Senin, 2006). B) Lapis lazuli sample from the mine.

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