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Devitrified Glass

Many chondrules consist of phenocrysts embedded in a glassy mesostasis. Sometimes the interstitial glass will have been annealed to such an extent that it will have become devitrified through solid state diffusion into distinctly crystalline laths. Here we will look at one of each type.

 

 


Both are in this thin section of an L3.2 ordinary chondrite.

 

 


The first, the one with the glassy mesostasis, in on the left. The annealed chondrule is on the right.

 

 


In cross-polarized light, XPL, all glass will be black.

 

 


In plane polarized light, PPL, it is still dark. Chondrule section is 2.3mm wide and 3.6mm high.

 

 


This glass is dark gray and somewhat translucent.

 

 


Outside the chondrule, lower left, the matrix is opaque. Inside the chondrule the glass passes some light. Where the mesostasis is thin, as when it lies over an olivine crystal, we see that it contains fine crystallites.

 

 


Crystallites. PPL Field of View = 0.3mm.

 

 


Crystallites. XPL FOV = 0.3mm.

 

 


Crystallites. PPL FOV = 0.3mm.

 

 


Crystallites. XPL FOV = 0.3mm.

 

 


The other chondrule in cross-polarized light shows black portions but these are simply mineral grains that are in optical extinction.

 

 


Chondrule is about 3mm in diameter.

 

 


In PPL the only black is minor opaque minerals. Some brown staining encroaches from the surrounding matrix.

 

 


Interstitial glass has become fibrous crystals. PPL.

 

 


Interstitial glass has become fibrous crystals. XPL.

 

 


Devitrified mesostasis. PPL FOV = 0.3mm.

 

 


Devitrified mesostasis. XPL FOV = 0.3mm.
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