Obsidian (Volcanic Glass)
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.
It is
produced when felsic lava extruded from a volcano cools rapidly with
minimum crystal growth. Obsidian is commonly found within the margins of
rhyolitic lava flows known as obsidian flows, where the chemical
composition (high silica content) induces a high viscosity and
polymerization degree of the lava. The inhibition of atomic diffusion
through this highly viscous and polymerized lava explains the lack of
crystal growth. Obsidian is hard and brittle; it therefore fractures
with very sharp edges, which were used in the past in cutting and
piercing tools, and it has been used experimentally as surgical scalpel
blades
Origin and properties
The translation into English of Natural History written by the elder
Pliny of Rome shows a few sentences on the subject of a volcanic glass
called Obsian, so named from its resemblance to a stone (obsiānus lapis)
found in Ethiopia by Obsius, a Roman explorer.
Obsidian is the rock formed as a result of quickly cooled lava, which is
the parent material. Tektites were once thought by many to be obsidian
produced by lunar volcanic eruptions, though few scientists now adhere
to this hypothesis.
Obsidian is mineral-like, but not a true mineral because as a glass it
is not crystalline; in addition, its composition is too complex to
comprise a single mineral. It is sometimes classified as a mineraloid.
Though obsidian is usually dark in color similar to mafic rocks such as
basalt, obsidian's composition is extremely felsic. Obsidian consists
mainly of SiO2 (silicon dioxide), usually 70% or more. Crystalline rocks
with obsidian's composition include granite and rhyolite. Because
obsidian is metastable at the Earth's surface (over time the glass
becomes fine-grained mineral crystals), no obsidian has been found that
is older than Cretaceous age. This breakdown of obsidian is accelerated
by the presence of water. Having a low water content when newly formed,
typically less than 1% water by weight, obsidian becomes progressively hydrated when exposed to groundwater, forming perlite.
Pure obsidian is usually dark in appearance, though the color varies
depending on the presence of impurities. Iron and magnesium typically
give the obsidian a dark brown to black color. Very few samples are
nearly colorless. In some stones, the inclusion of small, white,
radially clustered crystals of cristobalite in the black glass produce a
blotchy or snowflake pattern (snowflake obsidian). Obsidian may contain
patterns of gas bubbles remaining from the lava flow, aligned along
layers created as the molten rock was flowing before being cooled. These
bubbles can produce interesting effects such as a golden sheen (sheen
obsidian). An iridescent, rainbow-like sheen (rainbow obsidian) is
caused by inclusions of magnetite nanoparticles. - SOURCE |
