Tektites

In "tektites' at
http://lists.drizzle.com/pipermail/rockhounds/2011-December/036494.html
Steve Kelland asked,

"Does anyone know if tektites ever devitrify?"

Tektites can devitrify over geological periods of time. 
However, the glass composing them typically reacts to 
weathering by either complete dissolution or etching instead
of devitrifying into clay. The devitrifcation of tektites into 
clays is currently only known to occur under conditions 
of limited water contact. 

Overall, tektites are exceptional for their high chemical 
resistance and long-term stability against devitrification.
In part, the presence of MgO in a simple silica melt 
containing CaO significantly suppresses the devitrifcation
of tektites.

For example, go read,

Mazer, J. J., J. K. Bates, J. P. Bradley, C. R. Bradley, and C. M. 
Stevenson, 1992, Alteration of tektite to form weathering 
products. Nature. vol. 357, no. 6379, pp. 573 - 576
http://www.nature.com/nature/journal/v357/n6379/abs/357573a0.html

Heide, K., G. Heide, and G. Kloessa, 2001, Glass chemistry of 
tektites. Planetary and Space Science. vol. 49, no. 8, pp. 839-844.
http://www.sciencedirect.com/science/article/pii/S0032063301000332

Happy New Year,

Paul Heinrich