Dear Héctor and all others interested,

a method for the reaction of carbonate mixtures is described by:

Al-Aasm, I. S., Taylor, B. E., and South, B. (1990). Stable isotope
analysis of multiple carbonate samples using selective acid
extraction. Chemical Geology, 80, 119-125.

I tried this selective acid extraction method myself and found that it
works well, especially using different temperatures. 1-2 hours at 25
degC give you a reliable value for for calcite; dolomite follows at 50
degC, but after a delay of either continued reaction at 25 degC
overnight or heating the reaction bottle with e.g. a hot air gun to drive
out all residue calcite. After pumping down, reaction for dolomite
continues at 50 deg for at least 4 hours or overnight.

This applies, however, to conventional extraction techniques using
good old Y-tubes and waterbaths. I have no experience as to the
application of modern devices auch as Kiel, gasbench etc. Would be
interested to learn if any ideas are out there how to employ such
machines for sequential carbonate extraction.

regards and cheers,
Uwe


On 25 Jul 2005 at 19:16, Héctor O. Panarello wrote:

> Dear list members
>
>
> We need to determine carbon-13 and oxygen-18 in mixtures of
carbonates,
> particularly calcite and dolomite.
> We used the method of separation that makes use of the variable
reactions
> rate of these minerals with phosphoric acid.
> To test the method we used different mix ratios of calcite and
dolomite of
> known isotopic composition, that were reacted at 25ºC. We
collected the CO2
> produced in the first hour as representative of the calcite, we
allowed the
> reaction to proceed for three more hours (pumping away the CO2)
and we made
> a new CO2 collection after 72 hs, that we assumed to represent
the dolomitic
> fraction.  However, we observed cross-contamination in the
isotopic signals
> of both minerals.
>
> Can anybody help us in telling what can be wrong or if you suggest
a
> different technique for this purpose?
>
> Thank you in advance,
>
>
> Héctor
>
>
> Héctor O. Panarello
> Director
> INSTITUTO DE GEOCRONOLOGÍA Y GEOLOGÍA ISOTÓPICA
(INGEIS)
> Pabellón INGEIS - Ciudad Universitaria
> 1428 Ciudad de Buenos Aires
> Tel: +54-11-4784 7798
> Fax  +54-11-47833024
>
>
>
>
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--
Dr. Uwe E. Horstmann
Council for Geoscience
0001 Pretoria, South Africa
Tel.: +27 12 841-1361, Fax: -1278
Cell: 082 4083212
e-mail: [log in to unmask]




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