Re: why does quartz-stones glow when pressed together?

Westy:

I am puzzled by the first part of your question. I have never heard of a
glow resulting when two pieces of quartz are pressed together. I asked a
mineralogist and a geochemist and they also were at a loss.

The second part of your question is a different matter. When you strike two
pieces of quartz together you may get a spark. The same thing can happen
with other rocks, and also when striking certain rocks with metal. In fact,
striking pyrite against flint was one of the first methods of starting a
fire. However, this has nothing to do with the piezoelectric effect. The
sparks are formed because of sudden intense pressure and friction. If you
want to know more about striking sparks I suggest you contact a physics
professor at the nearest university.

Best regards,

David Kopaska-Merkel

Geological Survey of Alabama

PO Box 869999

Tuscaloosa AL 35486-6999 USA

205-349-2852

fax 205-349-2861

www.gsa.state.al.us



Admin note: 


We later received the following further information from David
Kopaska-Merkel:  


I received comments from two geologists indicating that quartz does
exhibit piezoelectricity in the manner described in question 986728206.Ph.
These comments do not shed any further light on the causes of the
phenomenon.

I have a collection of orthoquartzites collected from the Ordovician
Eureka Quartzite in Nevada that I use to illustrate piezoelectricity,
similar to the description noted here. By increasing the stress between
stones (i.e. sharp point on flat surface, or greater application of
pressure), the brightness of the "spark" increases.  This sparking
phenomenon is not surficial, it occurs below the surface of the stones,
and occurs just as readily underwater.  It is associated with the odor of
ozone.  Cool phenomenon to observe around a campfire or at night.

Britt
I've also observed (in a poorly lit rock preparation lab) during the
slabbing of the Eureka Quartzite and other granular siliceous rocks that a
"glow" is visible within the interior of the rock adjacent to the surface
being cut.  The first time I noticed this effect, I assumed that the rock
was being overheated by my forcing it against the diamond saw blade.  I
tried to compensate by turning up the coolant (water) flow and using less
force against the blade.  The effect persisted.  It seems to be more
visible in lighter coloured rock and is only apparent in the interior of
the rock sample to a distance of 2 - 3 mm away from the cut surface.  It
does not continue after stress is removed from the rock so I believe that
it is a piezoelectric phenomenon.

Ron McDowell