NB, the term ‘obsidian’ refers to a jet-black glassy rock. It usually has a rhyolitic composition, but not always.
Black rock with the characteristic vitreous lustre and conchoidal fracture of glass.
The surfaces are scattered with tiny spherical holes. These are vesicles.
Holding the thin-section up to the light, the spherical vesicles within the brown glass are clearly visible.
Under the microscope the pale brown glass is scattered with tiny opaque speckles in a weak flow texture.
Spherical vesicles are mostly filled in with mounting medium; notice its distinctive appearance under cross-polarised light. There are a small number of opaque infilled circles. These could be artefacts or black clumps of iron oxide.
Tiny crystals are scattered throughout the glass, indicating that some devitrification has occurred. Circles of devitrification, probably nucleated on oxide crystals.
Glass → Very rapidly cooled magma. Probably extrusive.
Vesicles: perfectly spherical, indicating that they were not being sheared by magma flow, i.e. the magma was not flowing.
glassy rhyolite
glass
obsidian
Cenozoic
Yellowstone, U.S.A.
NB, the term ‘obsidian’ refers to a jet-black glassy rock. It usually has a rhyolitic composition, but not always.
Black rock with the characteristic vitreous lustre and conchoidal fracture of glass.
The surfaces are scattered with tiny spherical holes. These are vesicles.
Holding the thin-section up to the light, the spherical vesicles within the brown glass are clearly visible.
Under the microscope the pale brown glass is scattered with tiny opaque speckles in a weak flow texture.
Spherical vesicles are mostly filled in with mounting medium; notice its distinctive appearance under cross-polarised light. There are a small number of opaque infilled circles. These could be artefacts or black clumps of iron oxide.
Tiny crystals are scattered throughout the glass, indicating that some devitrification has occurred. Circles of devitrification, probably nucleated on oxide crystals.
Glass → Very rapidly cooled magma. Probably extrusive.
Vesicles: perfectly spherical, indicating that they were not being sheared by magma flow, i.e. the magma was not flowing.
glassy rhyolite
glass
obsidian