When volcanoes erupt, obsidian often forms.
In some volcanic eruptions, hot material bursts forth from the crater, cools quickly from exposure to air and hardens into igneous rocks called obsidian and pumice. In other eruptions, lava flows down a mountainside in a more tranquil fashion. This kind of eruption might also result in the formation of some obsidian. As soon as the obsidian forms, it begins to weather. Various chemical reactions alter its composition.
Composition of Obsidian
Unlike most igneous rocks, obsidian does not contain crystals in its composition. Its chief component is silicon dioxide, the same material of which quartz, glass and siliceous sand are composed. Obsidian also contains considerable amounts of magnesium oxide and ferric oxide. Other oxides may also be present, such as oxides of aluminum, calcium and potassium.
Hydration
When formed, obsidian does not have much water in its composition. However, it subsequently becomes exposed to moisture in the form of rain and atmospheric humidity. As a result, obsidian undergoes a process called hydration. In this process, the H+ and OH- ions of water molecules become loosely attached to molecular components of obsidian. Obsidian thus changes into a hydrated material called perlite.
Devitrification
The obsidian does not contain silicon dioxide in its most stable form. For this reason, the silicon dioxide gradually changes from its amorphous glassy texture to organized quartz crystals. Since the resulting product no longer resembles glass, the process is known as "devitrification." Obsidian samples which have undergone partial devitrification have a number of white spots on a dark background. Such samples are called "snowflake obsidian," according to Jim Miller, a practicing geologist from Oregon State University.
Acid Reactions
Various acids attack obsidian. When it rains, the water absorbs some atmospheric carbon dioxide as it falls. Carbon dioxide dissolved in water converts to carbonic acid, which has a weathering effect on obsidian. For example, it attacks the magnesium oxide component in obsidian, converting it to magnesium carbonate. Acid rain also weathers obsidian. Sulfur dioxide of natural or industrial origin forms sulfuric acid when dissolved in rainwater. This powerful acid attacks such obsidian components as iron oxide to form sulfates. According to a study conducted by the Department of Environmental Sciences and Technology of Kagoshima University, obsidian samples subjected to hydrochloric acid solution at high temperatures turned obsidian into boehmite, allophane, halloysite and kaolinite.
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