Introduction
This is a natural, felsic (over 66% silica) and extrusive volcanic glass.
Formation
This forms when the rock is erupted to the surface and cools rapidly, within houres, or when the magma comes into contact with water it cools in minutes. This rapid cooling means that crystals have no time to form, creating volcanic glass.
Composition
This is made up of 66% silica making it a felsic rock. Other than this the rock obsidian has no set mineral composition. However, most obsidian has a similar composition to rhyolite and it is often found on the edge of rhyolite domes and flows.
Uses
Because it could be chipped with razor-sharp edges due to conchoidal fracture, it was used by ancient civilisations to make weapons, tools and ornaments. It was used by Greeks and Aztecs for mirrors due to its reflective properties.
Varieties
Obsidian is generally jet black, however, in the presence of hematite, it can be red and brown. If it has tiny gas bubbles a golden sheen is created.
In snowflake obsidian, the volcanic glass had devitrified, meaning that minerals crystalise from the glass. This means that spherical clusters of spherulite crystals are radially arranged in obsidian.
Pitchstone obsidian has a resinous lustre from when the rock absorbs lots of water over many years.
Obsidian from proximity to hematite.
Obsidain showing flow banding.
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