In a message dated 1/15/2010 7:18:38 P.M. Eastern Standard Time,  
[log in to unmask] writes:

It doesn't really melt, but the fire damage causes it to  loose profiles 
and surface and basically the granite most affected just  "melts" away.
 
I'm a chemist, but I can say nothing about the mechanism  involved.
 
Steve Stokowski - any  comment?



Any material containing the mineral quartz, which changes volume at 573  
degrees C, deteriorates in a fire.  This is the alpha to beta  
crystallographic transition temperature.  
 
Granites have about 25% Q, but essentially no porosity, so they crumble  
from the fire side to the interior.  Quartzites (quartz-cemented quartz  
sandstones) also perform poorly.  Brownstones don't do well either,  but the 
porosity tends to relieve the crystallographic strain better than it is  
relieved in the much less porous granite, and brownstone also can be a bit moist  
which in some conditions lowers the interior temperatures a bit, resulting in 
 slightly less fire damage, but in other conditions, especially hot fast 
fires,  results in the brownstone exploding from the built up internal steam  
pressure..  
 
The alpha-beta transition temperature is why the boulders scale and crack  
around those campfire pits.  It is also the reason why you cannot  
successfully replace the stone in your gas grill with normal river gravel.   And why 
refractory mortar contains cristobalite instead of quartz, although both  
minerals are SiO2.
 
Steve Stokowski

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