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Re: question



On Wed, 16 Jul 1997, hummer marchand wrote:

> Question,
> is the presence of vesicles in a sample of material conclusive evidence of 
> non-meteoric origin?  I have been told that only very tiny vesicles have 
> occured in one or two meteorites.
> Thanks,  Hummer Marchand
> 
	The presence of vesicles in a rock does not preclude a meteoritc 
origin. The Ibitira eucrite, and Mt. Tazerzait chondrite contain 
vesicles. The Mt. Tazerzait, however may not be vesicles, as it is hard 
to produce vesicles in the current model of chondrite formation.(not 
impossible, just difficult). 
	Vesicles are defined as an area affected by the release of gas 
from a molten(igneous) rock. This means basicly that, gas bubbles formed 
and then burst leaving a hollow pocket behind. Their size is based upon 
the viscosity of the material in which the bubble formed.
	Asteroidal vulcanism would probobly be apt to produce only small 
vesicles due to a lack of liquid water in a asteroid's magma system. 
Perhaps volitile gas would increase the size in locally affected areas. 
Also the length of cooling would have some affect on the vesicle size, as 
the temperature of space is almost absolute zero, there by promoting an 
almost instantaneous cooling of the magma. It should be said, that 
vesicles are formed from only molten material extruded on the surface of 
a planetary body.( Perhaps an instance of subsurface vesicles are known, 
but not to my knowledge).
	 If vesicles are present, I would be wary. If they are 
4mm and larger, it may not be a meteorite. The smaller the vesicles the 
more chance of being a meteorite. This is due to the fact, as stated above, 
that liquid water is present in all magmas on Earth, but not as much on  
asteroids. 
	So, yes, vesicles can be present in a meteorite, however they 
would tend to be tiny. Their size is a function of viscosity, and depth 
of formation in a flowing magma. Since there is less water, and colder 
temperature of interplanetary space, the magma would be more viscous, 
promoting smaller vesicles. 

I hope this answers your question.

Frank Stroik