268 IGNEOUS ROCKS/Obsidian topics in about 300 bc, that ‘‘the antients had two or three of these dark marbles, of fine texture, of great use amongst them They took a polish, were transparent to some degree when cut into thin plates, and reflected the image as our looking glasses The first kind was called Ociano& apo tZ& oceo&, which expressed its property of reflectivity and was afterwards written in the Latin as opsidianus or obsidianus’’ This early derivation of the name does seem to agree better with the known fact that Greeks and Romans used obsidian as a gemstone and obtained it from the Island of Melos in the Aegean, where quarries have yielded it for 12 000 years In 1773, the German mineralogist UFB Bruckmann wrote that obsidian was probably a black lava and geologist Leopold von Buch in 1809 noted that it flowed out, and was not cast out, from volcanoes In 1822, the American geologist Parker Cleaveland wrote: ‘‘This variety has a strong resemblance to glass Its fracture is distinctly conchoidal, with large cavities and strongly shining with a lustre more or less vitreous The surface of the fracture often exhibits a striated or wavy appearance, and its appearance is a little unctuous It scratches glass, gives fire with steel, but is brittle, and falls into sharp-edged fragments Most commonly it is translucent at the edges, or opaque, but some varieties are translucent or in thin scales transparent Its colour is black, either deep or pure, or tinged with brown, green, blue or grey, and sometimes passes to blue, green, brown or gray, even yellow or red The darkest colours often discover a tinge of green by translucent light’’ brown fragments In many cases, the rock is spotted or banded Spherulites and lithophysae occur in some obsidians, and may be abundant, also concentrated in certain layers Normally obsidians are natural glasses of rhyolites, but any acid (siliceous) volcanic rock may solidify as similar glass by rapid cooling, and thus the terms ‘trachyte’ and ‘dacite-obsidian’ in common use, though strictly obsidians are of rhyolite composition The specific gravity of obsidian ranges from 2.30 to 2.58 The refractive index ranges from n ¼ 1.48 to 1.53 The hardness on Moh’s scale ranges from 5.6 to The chemical compositions of various obsidians are given in Table 1; also shown in the lower part of the table are the CIPW norms (named after the petrologists Cross, Iddings, Pirsson, and Washington, in 1931) A norm is a means of converting a chemical composition of an igneous rock to an ideal mineral composition In this way, similarities in rocks with contrasting mineral assemblages can be noted Some of the factors considered are temperature, pressure, and mineral content; in the CIPW norm calculation, the magma is considered to be anhydrous and at low pressure Chemically, obsidian has a low water content, but even so, this is an order or more greater than is the case for tektites, which resemble obsidian and were once referred to as obsidianites For example, water content of moldavites from Central Europe ranges from 0.006 to 0.010 Tektites also contain lechatelierite, an amorphous form of quartz that is never found in volcanic glasses (see Tektites) Composition Occurrences Worldwide The Norwegian geologist and petrologist JHL Vogt in 1923 wrote that ‘‘compositions of eutectic or nearly eutectic proportions promote the formation of glass, since the eutectic has the lowest melting point; consequently, at that temperature the melt is more viscous than elsewhere on the curve, and points near the eutectic tend to reach solidifying point before reaching the crystallizing point With relatively quick cooling the crystallization will be entirely or nearly restrained Thus it is no accidental circumstance that by far the most obsidians have nearly the chemical composition of the granitic eutectite’’ As now used, the term ‘obsidian’ is applied to massive, usually dense, but often slaggy glasses of deep brown or black, grey, red, or mottled red and black colour The viscosity of obsidian as a flow stems from branching and tangled chains of tetrahedral silicon and aluminium combined with oxygen When solidified, obsidian is quite hard and its conchoidal fracture results in sharp, even cutting edges to the Obsidian Cliff, Yellowstone National Park Obsidian Cliff in Yellowstone National Park (Wyoming, USA) is considered a typical occurrence The chemical compositions of red and black obsidian samples from the site are given in Table The composition is rhyolitic The cliff forms a giant flow 120–160 m thick The rock is locally columnar, and at the lower part is traversed by bands or layers of small grey spherulites, but cavities or lithophysae are almost absent Higher up, the obsidian is less massive and contains large lithophysae (concentric shells of flattened fine material with a central cavity) parallel to the plane of flow Eolian Islands Three recent obsidian flows from the island of Lipari have been described, being the youngest (from the sixth to eighth centuries ad) eruptives on the island (Figure 1) The Rocche Rosse flow is of obsidian