Lapis Lazuli
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Lapis lazuli has been mined in the Sar-e-Sang district in Badakhshan,
north-east Afghanistan, for many centuries. The lazurite occurs in a rock with diopside, calcite and pyrite. Lapis lazuli is localized in two horizons of magnesian marbles near the centre of the Hindu Kush granitic massif (DHZ, op. cit.). Beds and lenses of lapis lazuli at Sar-e-Sang were described as up to 4 m thick and to extend for 400 m within a skarn formed under relatively high conditions of pressure and temperature
Lapis lazuli in the Baffin Island deposit at Lake Harbour
shows a pri- mary coating of deep
blue to a paler blue and a teal green, not usually recorded for lapis lazuli. In this deposit Hogarth and Griffin (Lapis lazuli from
Baffin Island-a Precambrian meta-evaporite, Lithos
11, 37-060, 1976) and Cade et al., Abstracts of
the Eighteenth General Meeting of the International Mineralogical Association, Edinburgh, 2002. The
deposit at Lake Harbour is in a dolomitic marble.
Lapis lazuli is found in the North Italian Mountains of
Colorado, USA, where the occurrence
is in impure marble layers near their contact with Tertiary quartz monzonite and quartz diorite of the Italian
Mountain stocks (Hogarth and Griffin,
Contact-metamorphic
lapis lazuli: the Italian Mountain deposits, Colorado, Canadian Mineralogist 18, 59–70, 1980).
Lapis lazuli is being mined from a limestone-granite contact (elevation 3500 m) in the headwaters of the Cazadero and Vias rivers, Ovalle, Coquimbo, Chile, close to the border with Argentina. The lapis here is paler in colour and associated with wollastonite rather than diopside as in the Baikal deposits, Russia. The Chilean lapis contains phlogopite, sodalite, calcite and pyrite and is described by Borelli et al., Caratterizzazione del lapis- lazuli (Borelli et al., La Gemmologia 11(4), 24–27, 1986 and Coenrads and Canut de Bon, Lapis lazuli from the Coquimbo region, Chile, Gems & Gemology 36, 28–41, 2000.
The properties of lapis lazuli are helpful in identification. The hardness is usually between 5 and 6, the RI near 1.50 and the SG 2.7–2.9; if a signifi- cant amount of pyrite is included the figure will be higher. Under LWUV some specimens may show a streaky orange fluorescence (Chilean speci- mens in particular). Under SW lapis may show a pinkish response. If touched with HCl a characteristic rotten egg smell will result.
Using a ceramic technique Pierre Gilson has manufactured a product known for convenience as synthetic lapis. Specimens may or may not contain added pyrite. The material has been found to be porous with a lower SG than natural lapis, near 2.46. The RI is close to 1.50 but the shadow-edge is vague. The Gilson material is discussed in a paper in the Journal of Gemmology 19(7), 1985: the material should be regarded as an imitation though it has been offered as synthetic. Ultramarine and hydrous zinc phosphates are the main components. An imitation of lapis by Verneuil-grown spinel has higher constants (SG near 3.6, RI near 1.73) – interestingly gold flecks have sometimes been added to simulate pyrite!
Dyed howlite has also been used as an imitation though it is easily detected by an orange fluorescence after the dye has been removed. A dyed jasper with the quartz SG near 2.65 was at one time known as Swiss lapis.
A cobaltian polycrystalline material has a granular structure and shows red through the Chelsea filter. A dyed blue quartzite has also been used as a lapis imitation. In many cases of dyed specimens acetone will be found to remove the dye which has been found to penetrate about 1.5 mm from the surface. One interesting imitation was found to be dyed dolomite with RI between 1.50 and 1.68 and SG 2.85.
The best contemporary account of lapis from the classic deposits of Afghanistan is given by Bowersox and Chamberlin, Gemstones of Afghanistan, 1995 (ISBN 0945005199). The Badakhshan deposits are in the north-east of the country in a mountainous area and occur in mainly plutonic and metamorphic rocks which are cut by deep valleys. The lapis deposits are situated along the valley of the Kokcha river, on the Blue Mountain on which folding and faulting occurred in the Cretaceous period when intrusion by a diorite porphyry took place. The steep valley slopes of the upper Kokcha river in the eastern Hindu Kush are gneisses with thick intercalations of greyish white dolomitic marble. Lenses of light to dark blue lapis frequently occur. In Lapidary Journal 38(11) Emmett explains that the lapis occurs in grey lenses of calcite- dolomite skarn formed by contact metamorphism of impure limestone and high-grade metamorphic sequence of Precambrian rocks resulting from the intrusion of masses of molten granite, causing the formation of marble. The skarn lenses are more than 1–4 m thick and are underlain by gneiss. One of the classic mines is set in strata of black and white limestone. The crystalline series in which the lapis is found consists of gneiss, leptinites and cipolin marble in a heavy layer, amphibolites, pyroxenites and peridotites in which the lapis is found in disseminated veins.
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