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Dear Marilyn and Pierre,

Thinking about all the methods mentioned for carbides oxidation, I can 
indicale two points:
1) In our case we have a long time of reaction - up to 60 min (which is 
not the case for EA)
and
2) We are removing the produced CO2 from the reactor volume with LN2 
trap during the reaction (which is not the case for sealed-tube method).

AND we never succeed with direct oxidation of SiC. I think Pierre is 
right about the SiO2 covering issue. However, people from the University 
of Lausanne have claim that they did the job with Carlo Erba EA-1500 
(Trumbull et al., The carbon isotope composition of natural SiC 
(moissanite) from the Earth's mantle:New discoveries from ophiolites. 
Lithos 113 (2009) 612–620 doi:10.1016/j.lithos.2009.06.033). I have used 
that reference material (GFZ-S113) and the data looks reliable.

I think the Fe3C is easier material to oxidize using a fine powdered 
material.

Vadim

Pierre Cartigny писал 2016-01-15 21:17:
> Hi Marilyn,
> 
> We had the same difficulty combusting carbide (silicon carbide, iron
> carbide). There are two reasons, I think. First, a kinetic issue.
> Carbide are like diamond, they can be slow to combust and so not
> really adapted to EA.
> 
> But, as opposed to diamond, carbide did not fully combust either in a
> sealed-tube (or a reactor) at high T (up to 1100°C) even it was as a
> very fine powder. I assume, this is because any grain gets covered by
> its oxide (e.g. SiO2) preventing the remaining carbide in the center
> to combust.
> 
> I would also be interested to hear if some of you succeeded in getting
> 100% yield.
> 
> Pierre.
> 
> Le 15 janv. 2016 à 15:36, Marilyn Fogel a écrit :
> 
>> Dear Colleagues,
>> 
>> I have been asked to analyze some cohenite, which is iron carbide,
>> to get a carbon isotope value. The iron carbide is a reactant in
>> high pressure mineral physics experiments and will most likely be
>> analyzed in very small samples via the ion microprobe. I have tried
>> to analyze iron carbide before, just with standard elemental
>> analysis techniques (around 1000°C, oxygen pulse) with little
>> success. Incomplete combustion was the major symptom, but I did not
>> try adding any of the tungsten or other oxides to the mix.
>> 
>> If anyone has tried this and succeeded, or tried and failed, please
>> let me know what worked and what did not work. It is always a good
>> idea to “ground truth” samples used as ion probe standards, so I
>> am interested in trying to figure this out.
>> 
>> Marilyn
>> 
>> Dr. Marilyn L. Fogel, Professor of Ecology
>> Chair, Life and Environmental Sciences Unit
>> Faculty Director, Environmental Analytical Laboratory
>> President, AGU Biogeosciences
>> School of Natural Sciences
>> University of California at Merced
>> 5200 Lake Rd., Merced California 95343
>> Phone: 209-205-6743
> 
> Pierre Cartigny
> Laboratoire de Géochimie des Isotopes Stables,
> Institut de Physique du Globe de Paris
> 1 rue Jussieu, 75005 Paris, France
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