Silicate-silicate liquid immiscibility and graphite ribbons in Lybian desert glass

Transmission electron microscopic (TEM) investigation of the dark (brown or bluish) streaks occuring in Libyan Desert Glass reveals the common presence of small glass spherules. The spherules, mostly 100 nm in size, are homogeneously dispersed within the silica-glass matrix. The complete absence of electron diffraction effects confirms their amorphous nature. The spherules are Al-, Fe- and Mg-enriched with respect to the surrounding silica matrix and their (Mg + Al + Fe): Si ratio is close to 1. The silica-glass matrix and amorphous spherules form an emulsion texture (i.e., globules of one glass in a matrix of another glass), which originates from silicate-silicate liquid immiscibility. This texture has also been observed in other impact-derived glasses. The silica glass also contains carbonaceous inclusions consisting of 5-50 nm thick, polygonalized graphite ribbons that form closed structures up to 200 nm in diameter. These observations are in agreement with an impact origin for Libyan Desert Glass. Copyright © 2002 Elsevier Science Ltd.