obsidian - teaching hand specimen of lithophysal obsidian
Lithophysae are hollow bubble-like spherules that form in silica rich extrusive rocks such as rhyolite and obsidian. Typically the cavities are lined with cristobalite, orthoclase, tridymite or fayalite, or a combination of these. The mineralogy of this obsidian has not been examined, so the identification of the white mineral is uncertain. Roots: Greek lithos = rock, physa = bubble.
Lithophysae grow in place on the order of days to months at temperatures above the glass transition, taking crystal-forming components from the surrounding melt and expelling water into the surrounding matrix. They can also form, though more slowly, at temperatures below the glass transition, which, for anhydrous rhyolite is between 620˚ and 757˚C.
This obsidian was collected from a mass that was extruded into a body of pumice and rhyolite in a pumice cone. For the most part the obsidian body is not lithophysal and has begun to devitrify, so is losing its characteristic glassy black sheen. Occasionally interesting specimens like these are uncovered in a small area where the obsidian is still fresh.
Pumice cones are unusual. Normally cinder cone volcanoes are composed of basalt, which is silica poor and not viscous when melted. The lava fountains easily and the cone builds up from cooled basalt ejecta, scoria, blocks and bombs, collectively referred to as cinder. A pumice cone forms from a silica rich melt. Silica makes lava viscous. In the eruption of silica rich magma, instead of the release of pressure through fountaining, pressure builds up and is typically released in a violent blast. The eruption of Mt. St. Helens was a good example. Small cinder cones of pumice are rare. A chain of five cones, where this was collected, extends southward from Mono Lake in Mono County, California.
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