At 12:38 PM 3/24/96 +0000, Kolodny wroe:
:
 "...but an opal is an opal"

   Agreed, but there is the question of surface area.  Diatomite has one of
the highest surface areas of anything known.  Most of the OH and water is
probably on the surfaces and can be removed via the stepwise fluorination
method without affecting the tetrahedral oxygen very much..  In the case of
milky opal or precious opal the surface area is nothing special.  Here, the
hydrous components are buried within the structure and it is not clear how
effective the method is. 

  One thing to remember in using the stepwise fluorination method is to NOT
substitute bromine pentafluoride for fluorine gas.  Flourine appears to
yield the right delta-value on quartz or opal as long as you get more than
about 5% yield (a miracle!).  With bromine pentafluoride, a 95% or better
yield is required or your number can be several per mil off.   Cheers,


Paul Knauth
Department of Geology
Arizona State University
Box 871404
Tempe, Arizona 85287-1404
(602) 965-2867 office
(602) 831-1469 home
(602) 965-8102 fax