Mineral Specimens

Libyan desert glass or Great Sand Sea Glass is an impactite, made mostly of lechatelierite, found in areas in the eastern Sahara, in the deserts of eastern Libya and western Egypt. Fragments of desert glass can be found over areas of tens of square kilometers.
Egyptians discovered Libyan Desert Glass thousands of years ago. It was prized for jewellery and amulets.

The origin of desert glass is uncertain. Meteoritic origins have long been considered possible, and recent research links the glass to impact features, such as zircon breakdown, vaporized quartz and meteoritic metals, and to an impact crater.

The glass is nearly pure silica which requires temperatures above 1,600 °C to form – hotter than any igneous rock on Earth. However, few mineral relics survived from whatever caused the melting, including a form of quartz called cristobalite, a rarely occurring high-temperature mineral; and grains of the mineral zircon, although most have reacted to form a higher-temperature mineral called zirconia.

Ideas about how the glass formed include melting during meteorite impact, or melting caused by an airburst from an asteroid or other object burning up high in Earth’s atmosphere.
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Abstract– Dark streaks and different types of inclusions in Libyan Desert Glass (LDG) collected from the LDG strewn field in Egypt were investigated. Rare transparent spherules enclosed in the glassy matrix are characterized by concentric cracks, irregular internal cracks, intense twinning, and considerable amounts of Ti and Al. Raman spectra show that the spherules are α-cristobalite. Their occurrence together with lechatelierite indicates quick heating of the source rock to at least 1550°C, followed by rapid quenching leading to crystallization of β-cristobalite, which upon cooling inverted into α-cristobalite. Brownish inclusions are irregularly shaped, elongated objects with smooth contacts to the surrounding glass.

They contain small roundish to elliptical droplets, and a few larger angular grains, which compositionally and according to their Raman spectra most closely resemble low-Ca, Al-rich orthopyroxene. Composition and texture of the orthopyroxene suggest that the brownish inclusions formed by incomplete melting of an Al-rich orthopyroxene bearing precursor, e.g., mafic phases present in desert surface sands or also of orthopyroxene-bearing granulite dykes in the LDG target.

Experimental data on Ca-poor enstatite also support that the inclusions were heated to about 1550 °C. Analyses of dark streaks in LDG reveal high abundances of Al, Ti, Mn, Cr, Fe, and Ni and a pronounced correlation between the abundances of Cr, Mn, Fe, and Ni. As the Fe/Ni, Mn/Ni, and Cr/Ni ratios are all clearly nonchondritic, the source of this material is most likely terrestrial and the dark streaks studied here represent a different type of schlieren compared to those which contain a meteoritic component. These findings suggest LDG formation during a short high-temperature event. Melting of Al-rich orthopyroxene bearing target material seems to suggest an asteroid impact rather than a near-surface airburst.

Libyan Desert Glass (LDG) is an unusual natural glass that occurs in large quantity between the sand dunes of the southern region of the Great Sand Sea of southwest Egypt. The irregularly shaped, millimeter- to several decimeter-sized, and often wind-eroded fragments are scattered over almost 6500 km2, although the main distribution is limited to about 2000 km2. The largest samples recovered so far weigh up to 26 kg. The fragments generally appear light-yellow to greenish in color, although some pieces are white due to abundant tiny bubbles, and, in a few rare cases, contain some dark streaks. The angular fragments were used over thousands of years during prehistoric and dynastic Egyptian times to produce a variety of artifacts, such as the yellow/green scarab of Tutankhamen’s pectoral (De Michele 1998).
Ref: Wiley Online Library