Lab Notes Gems & Gemology, Spring 2023, Vol. 59, No. 1

Libyan Desert Glass Bangle Bracelet

Ching Yin Sin

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Figure 1. A 144.67 ct Libyan Desert glass bangle measuring 66.12 × 13.10 mm. Photo by Johnny Leung.
 
Figure 1. A 144.67 ct Libyan Desert glass bangle measuring 66.12 × 13.10 mm. Photo by Johnny Leung.

Libyan Desert glass is a light green or yellow natural glass which is composed of almost 98% silica. It is a type of tektite formed by meteorite impact in the desert and the fast quenching of the silica-rich melt produced. It was first reported by Clayton and Spencer in 1934 (F. Fröhlich et al., “Libyan Desert Glass: New field and Fourier transform infrared data,” Meteoritics and Planetary Science, Vol. 48, No. 12, 2013, pp. 2517–2530) and is found in Egypt’s Western Desert. Common inclusions are bubbles, cristobalite, dark brown streaks, and black iron oxides (J.A. Barrat et al., “Geochemistry and origin of Libyan Desert glasses,” Geochimica et Cosmochimica Acta, Vol. 61, No. 9, 1997, pp. 1953–1959).

Recently, GIA’s Hong Kong laboratory examined a transparent light yellow bangle bracelet measuring 66.12 × 13.10 mm and weighing 144.67 ct (figure 1). It had a spot refractive index of 1.44 and revealed weak yellow fluorescence in short-wave UV. The chemical composition, determined by qualitative analysis using energy-dispersive X-ray fluorescence, was nearly pure silica with minor amounts of iron, strontium, potassium, titanium, and zirconium.

Figure 2. Various shapes of gas bubbles were dispersed individually and in clusters in the Libyan Desert glass bangle. The row of gas bubbles in this image resembles peas in a pod. Photomicrograph by Sze Ling Wong; field of view 2.40 mm.
 
Figure 2. Various shapes of gas bubbles were dispersed individually and in clusters in the Libyan Desert glass bangle. The row of gas bubbles in this image resembles peas in a pod. Photomicrograph by Sze Ling Wong; field of view 2.40 mm.
Figure 3. White spherulites in the bangle were identified as cristobalite by Raman analysis. Photomicrograph by Sze Ling Wong; field of view 5.98 mm.
 
Figure 3. White spherulites in the bangle were identified as cristobalite by Raman analysis. Photomicrograph by Sze Ling Wong; field of view 5.98 mm.

Based on the differences in molecular vibrations detected with Fourier-transform infrared (FTIR) analysis, it was determined that the molecular structure of Libyan Desert glass is significantly different from that of other pure silica glasses. It has a higher ratio of discontinuities and defects in the tetrahedral (SiO4) network (F. Fröhlich et al., 2013). Microscopic observation revealed common inclusions in glass such as numerous gas bubbles (figure 2) and flow structure. This bangle also presented abundant white spherulites identified by Raman spectroscopy as cristobalite (a polymorph of silica). These spherulites were small and individually dispersed throughout the glass (figure 3). The presence of this high-temperature low-pressure silica phase indicates a natural high-temperature formation. From all the gemological, FTIR, and Raman data collected, this bangle was identified as Libyan Desert glass.

Most Libyan Desert glass is tubular in shape, and some are approximately spherical or rod-like (R.A. Weeks et al., “Libyan Desert glass: A review,” Journal of Non-Crystalline Solids, Vol. 67, No. 1-3, 1984, pp. 593–619). Those fragments are usually 2 to 63 mm in size, so it is surprising to examine this material in the form of a bangle bracelet.

Ching Yin Sin is a staff gemologist at GIA in Hong Kong.