George,

I have never tried epidote by BrF5 and externally heated bombs, but I have
analyzed a variety of other minerals with the technique.  My observations
might help.

I found as have many others that some Al-rich silicates did not easily
react with the BrF5.  I think it has to do with the Taylor and Epstein
(1962) explanation that there are volatile or partly volatile fluorides at
reaction temperatures (e.g., SiF4, TiF4, FeF2, ZrF4) and involatile
fluorides (e.g. AlF3, MgF2, CaF2,...), and that that the involatile
fluorides armor grains.

To get around that I used two approaches.  One was to increase the
temperature of the reaction.  I ran most fluorinations at 575-600 C, but
used 625 C for garnet and fayalite and even went as high as 700 C for
kyanite, sillimanite, and sapphirine.  If you decide to take this route,
the trick is to precondition the reaction vessels at a temperature 20-30 C
higher than you expect to run.  I also felt that it was pushing the
equipment to go higher than 650 C, but I have seen studies in the chemistry
litterature where fluorination reactions with BrF5 were done at 800 C for
48 hours.  I controlled temperature with thermocouples and temperature
controllers.   For high temperature runs I added double the usual amount of
BrF5. A very important check is the excess reagent after the run.  If the
Ni reaction vessels consume all reagent then all bets were off with the
d18O.

The second approach was to grind the material to a very fine grain size. My
thesis advisor showed me the way experimental petrologists grind charges.
You could always ask one in your department for advice.  I used a small
mortar and pestle.  The smaller the mortar, the smaller the grain size.  I
gently ground the sample with ethanol for about 20-30 minutes or until the
powder felt smooth.  (I read papers during this time.)  Also, agate may not
be the material of choice for your mortar.  You decide.  This gave 10-20
micron grains for garnet, coarser for kyanite because of the cleavage
fragments would roll, and finer for minerals like sapphirine.  One
difficulty is drying these powders after grinding.  They can hold a lot of
water.  Because I inferred that they were unreactive with BrF5 I
prefluorinated them without changing the oxygen yield.  I also checked this
by outgasing them in vaccuo at 300-350 C.

I found that the grinding was the most important step with the increased
temperature being the second most important step.  I suspect that epidote
is not as extreme as these minerals and know that you will succeed.
Samples that I analyzed this way gave 90-100 % yields and precise d18O.

I am not sure that this is the problem that you have encountered (partial
reaction).  If it is, some of these steps might help.  If, on the other
hand, your epidote is reacting and being pumped away in some way during
your pretreatment, this will not work.

Best,

James


James Farquhar
Geophysical Laboratory, Carnegie Institution of Washington
5251 Broad Branch Rd NW
Washington DC 20015
(202) 686-2410 ext. 2484
(202) 686-2419 (Fax)
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