obsidian - flow banded black obsidian - Unit of 5 student specimens
Obsidian is a volcanic glass with the composition of granite or rhyolite. Because it cooled and solidified almost instantaneously in the earliest stages of an explosive volcanic eruption, its atoms did not have enough time to fall into a regular crystalline pattern and are essentially an unorganized mass of atoms exhibiting the curved or conchoidal fracture typical of glass.
Flow banding is related to deformational reworking of the lava, and may be due to magma brecciation or fragmentation, which leads to variable rates of degassing of fragments, leading in turn to nonuniform crystal nucleation. Repeated fragmentation, degassing and nucleation leads to bands in the glass.
The color of obsidian is due to impurities. Black obsidian results when the impurities are hornblende, magnetite, biotite or a pyroxine. Brown obsidian forms when the impurity is hematite or limonite. Inclusions of gas or tiny inclusions of water result in a silvery reflectance that forms the bands here and which is desirable in lapidary. Brown and black are commonly swirled together in some obsidian flows.
Almost every obsidian source in North America was known to native Americans. Since obsidian from each source is geochemically distinct, the origin of the obsidian used in stone tools can be traced. Many surprisingly obscure sources, such as obsidian float in alluvium where the original flow has weathered away, were utilized for cutting tools or was collected as trading material.
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