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Posted: Mon Mar 31, 2008 5:33 pm Post subject: Obsidian Rocks
We have just finished the topic of rocks in school and a question that my science teacher couldnt answer was why Obsidian rock was so smooth compared to other igneous rocks.
Obsidian is actually glass, as a result, why is it classified as Igneous?
Hope you can help...
Man-made glass is smooth and shiney because of its microcrystaline nature and high proportions of silica present. The same is true of natures volcanic glass, geological term obsidian.
The crystaline form of obsidian is very fine grained, known as microcrystaline or cryptocrystaline and crystals can only be viewed with a high powered microscope.
The way the mineral breaks, its curved fracture, is called concoidal fracture and obsidian shows the smooth, shiney glassey concoidal surfaces just the same as any glass can fracture. _________________ onestepatatime
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