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,
Trang 1topics 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
re-flected the image as our looking glasses do The first
kind was called Ociano& apo tZ& oceo&, which
ex-pressed 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
frac-ture 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
some-times passes to blue, green, brown or gray, even
yellow or red The darkest colours often discover a
tinge of green by translucent light’’
Composition
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;
conse-quently, at that temperature the melt is more viscous
than elsewhere on the curve, and points near the
eutectic tend to reach solidifying point before
reach-ing the crystallizreach-ing 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
so-lidified, obsidian is quite hard and its conchoidal
fracture results in sharp, even cutting edges to the
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 7 The chemical compositions of various obsidians are given inTable 1; also shown in the lower part of the table are the CIPW norms (named after the pet-rologists Cross, Iddings, Pirsson, and Washington,
in 1931) A norm is a means of converting a chemical composition of an igneous rock to an ideal min-eral composition In this way, similarities in rocks with contrasting mineral assemblages can be noted Some of the factors considered are temperature, pres-sure, and mineral content; in the CIPW norm calcula-tion, 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 con-tent 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)
Occurrences Worldwide 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 obsid-ian samples from the site are given in Table 1 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
268 IGNEOUS ROCKS/Obsidian