- An Unusual Collector’s Mineral
Authors: Gagan Choudhary and Chaman Golecha
(This article was first appeared in Australian Gemmologist, 23, 314-315)
Anglesite – the mineral having wide occurrence all over the world is one of the least known gemstones in the gem trade, however, the authors had a chance to study this collectors' mineral at the Gem Testing Laboratory, Jaipur which was submitted for identification.
The Submitted Specimen
The submitted unidentified light gray and transparent crystal weighed 29 carats (figure 1). It was remarkably dense and a hydrostatic Specific Gravity determination gave a value of 6.36, consistent with either anglesite, i.e. lead sulphate, (6.3-6.4, usually 6.38) or cerussite, lead carbonate (6.4-6.6, usually 6.55) [E.g. Webster & Read, 2002. Gems- Their Sources, Description and Identification, 5th edn, Butterworth-Heinnemann, Burlington, USA and Joel Arem, Color Encyclopedia Of Gemstones, 1977, Van Nostrand Reinhold & Co.].
Figure 1: The tested specimen of Anglesite weighed 29.56 carats. Note the crystal habit and tiny green coloured associated crystals
The crystal had associated small green hexagonal crystals (figure 2), displaying instances of incipient cleavage, visually consistent with dioptase, a copper silicate that may often be associated with oxidised secondary lead minerals. Numerous minute colourless crystals consistent with quartz were also observed.
Figure 2: Associated green crystals displaying hexagonal habits consistent with dioptase, together with minute colourless “drusy” crystals consistent with quartz. Magnified 30x
The measurement of the crystal’s optical properties including its refractive index and birefringence was not feasible, although definite double refraction could be directly observed. A weak yellow fluorescence reaction to short-wave ultra-violet irradiation was detected. It is recorded in “The Handbook of Chemistry” (Tenth Edition, ed. N. Lange, pub. McGraw Hill) that both anglesite and cerussite may be photoluminescent, and Arem (ibid.) mentions that anglesite may fluoresce weakly yellow in both SWUV and LWUV while cerussite may fluoresce pinkish orange or yellow shades in LWUV but pale blue or shades of green under SWUV. No characteristic features were discernible through a spectroscope.
Figures 3: Etch patterns on the crystal surface displaying orthogonal pits (left) and "heart" shaped patterns (right). Magnified 45x (both)
Figure 4: Elongated tapering tubular inclusions containing dual phase liquid-gas contents (arrowed). Magnified 45x
Qualitative EDXRF (energy dispersive X-ray fluorescence) analysis revealed the presence of lead, although evidence for sulphur was inconclusive. An FTIR (Fourier Transform Infra-Red) spectrum exhibited an absorption pattern in the region of 400 to 2800 cm-1 rather similar to those seen in some phosphate and especially sulphate minerals (figure 5), but was not conclusive without a comparative reference spectrum for anglesite.
Figure 5: Infra red absorption spectra of Anglesite. Note the pattern of absorption in the region 400 to 2800 cm-1 which is similar to as seen in minerals/ gemstones of the phosphate and sulphate group.
A minute drop of concentrated hydrochloric acid was placed on an inconspicuous part of the specimen and no reaction was observed suggesting it to be anglesite, while cerussite being a carbonate would be expected to effervesce.
Although such a specimen is primarily of interest more to mineral collectors than to gemmologists, it has the potential for being faceted into an unusual fragile and brittle but optically impressive gem. This single submitted example of an unusual crystal was not even fashioned into a faceted stone, but it is possible that this material might occasionally be encountered in the rare-gem category. In view of the extreme brittleness and the difficulty in polishing of either of these lead minerals, anglesites and cerussites are considered to be amongst the rarest of collectable faceted gems (Arem, ibid.). Anglesite is not an especially uncommon component in the suite of the oxidised secondary lead minerals but as single macro-crystals it is unusual and has a high refractive index and dispersion in common with most lead compounds and could be confused with other relatively dense gem materials.
Neither FTIR nor EDXRF in our hands served to unambiguously identify the material but in conjunction with a simple acid test and the crystal’s specific gravity value its identity was revealed.
All photographs and photomicrographs by Gagan Choudhary