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After an obsidian flow has been emplaced it is subject to the atmosphere that causes weathering to the obsidian. One specific type of weathering done on to obsidian is called hydration, which occurs by the water within the atmosphere being absorbed by the obsidian thus increasing the water content within the rock. The product of a hydrated obsidian is called devitrified obsidian, (de- to remove) (vitrify- glassy), so essentially the removal of the glassy property of the obsidian. The products of devitrified obsidian can produce secondary fiberous mineral crystals that can form ball like shapes called spherulites that are embedded within the obsidian, these secondary minerlas can develope into what is called snowflake obsidian as well, which makes the obsidian look as if it is decorated with snowflackes due to the snowflake shapes the secondary minerals produce. The snowflake obsidian can be used as jewlery and many rockhounds search for it, Yellowstone caldera is home to snowflake obsidian.
This is an image of snowflake obsidian, as you can see the seconday minerals group together forming snowflake like shapes scattered across the rock surface. This image is from Rockhoundblog.com.
This is an image of what a spherulite looks like due to secondary mineralization from devitrification of obsidian.
Photo by (c) 2008 Andrew Alden, licensed to About.com
Some researchers have used the hydration rate of obsidian to acquire specific dates of the obsidian, which is based on the idea that a freshly broken obsidian surface begins to absorb water from its environment almost immediately. Absorption continues with time, generating a hydrated layer whose thickness is proportional to the time the glass surface was exposed this according to Anovitz et al (2006). This technique of dating can be unreliable due to the many factors influencing rate of hydration. Just recently Anovitz et al discovered that the natural hydration of obsidian can be used as a tool to provide paleoclimatic reconstructions. Anovitz et al (2006) article in GSA’s Geology Journal, “Obsidian hydration: A new paleothermometer,” the first successful application of this idea was done on samples from the the Chalco site in the Basin of Mexico successfully obtaining the temperature change within the region of study, for further information refer to the Anovitz (2006) article.
Anovitz, L. (2006). Obsidian hydration; a new paleothermometer. Geology [Boulder], 34(7), 517-520. doi:10.1130/G22326.1.
Francis, Peter., and Oppenheimer, Clive. (2004), Volcanoes, Second Edition. New York: Oxford University Press Inc., New York: (162-164).