1. 17. Devitrification Dan Barker, March 2009

2. Under surface conditions, glass is a metastable liquid with a tendency to crystallize. Devitrification is this process by which glass becomes crystalline. It is promoted by time, water dissolved in the glass, by slow cooling or reheating, and by the presence of crystal nuclei and fractures. Rhyolite glass devitrifies to form silica polymorphs and feldspars. Basalt glass devitrifies to form plagiclase and chlorite, usually, and the effects are much more subtle, commonly requiring microscopic study. A few examples of Precambrian volcanic glass are known, but only from impermeable surroundings.

3. Roman glassware devitrified after 2000 years in the ground, Koln,Germany.

4. Devitrified Roman glass, Koln, Germany. The handles, which are not devitrified, are modern reconstructions.

5. Devitrified obsidian, Lipari. The white things are spherulites, made of radiating needles of cristobalite and alkali feldspar.

6. Obsidian Cliff, Yellowstone. Spherulites concentrate in bands, perhaps where the water content was higher in the glass.

7. Devitrification spherulites, unit XV of Cougar Point Tuff, ID. How could you tell these from accretionary lapilli?

8. Basketball-size spherulites (arrow) in a thick pyroclastic flow deposit, at El Salto on road from Durango to Mazatlan, Mexico.

9. Another big spherulite, ID

10. A weathered fragment of a big spherulite shows the radial structure.

11. The strange texture of this Precambrian dike is probably a result of devitrification. Ilimaussaq intrusion, SW Greenland.

12. Lithophysae are hybrids of vesicles and spherulites. They are cavities, commonly nonspherical, lined with repeated crusts of minerals, many of which appear to have grown from vapor and hot solutions. In addition to alkali feldspar and silica polymorphs, lithophysae may contain iron-rich olivine or topaz.

13. Lithophysae (arrows) are commonly found with spherulites. Obsidian Cliff, Yellowstone.

14. Lithophysae and spherulites, Obsidian Cliffs, Yellowstone.

15. Lithophysae in welded pyroclastic flow deposit, Tuff of Goose Creek, ID

16. Goose Creek lithophysae, ID.

17. Palagonite is devitrified basaltic glass, generally green or brown. Strangely, glasses of intermediate composition (andesite, latite) have pretty much been ignored by geologists and are rarely reported.

18. Bedded palagonite surge deposit around a tuff ring, Kempenich, E. Eifel district, Germany

19. Closeup of palagonite surge, Kempenich

20. Palagonite tuff, Moeraki Point, North Island, NZ

21. Remnant of a tuff cone with palagonite, Volcan Ecuador, Isabela I., Galapagos.

22. Palagonite lapilli tuff cut by dikes, Volcan Ecuador, Isabela I., Galapagos