Dear Kathy,

Why not to try just a simple fractional extraction? Reaction of calcite
with phosphoric acid is much faster than that of dolomite of similar grain
size. We routinely do fractional extractions, and it works fine.

You should have a look to ...:

Walters, L.J.,Jr.,Claypool, G.E. and Choquette, P.W. (1972): "Reaction
rates and delta 18-O variation for the carbonate-phosphoric acid
preparation method". Geochim. Cosmochim. Acta, 36, 129-140.

Al-Aasm, I.S., Taylor, B.E. and South, B. (1990): "Stable isotope analysis
of multiple carbonate samples using selective acid extraction". Chem. Geol.
(Isotope Geosci. Sect.), 80, 119-125.

Non-carbonates in your sediment samples should not be much of a problem,
unless you have organic matter or sulphides/sulphates. We eliminate
organics by roasting under vacuum (400 deg C for two hours, although check
the more or less recent discussion about this matter). Sulphur can be
eliminated post-phosphoric acid reaction by means of silver phosphate (for
example; again, see the not so recent postings regarding the subject).

I hope this helps to get you started.

Cheers and good luck.

Clemente


>I am working on a masters project looking at secular variation of oxygen
>and carbon isotopes in Holocene lake calcite (laminated sediment). The
>sediment carbonate content varies from 5-40%.  This carbonate consists of
>calcite and dolomite(XRD), dolomite varying from 5-30% of the total
>carbonate.  I only want stable isotope data on the calcite.  How do I
>avoid this dolomite in my sample preparation?  Can I attempt
>sedimentological-type physical separations, or will wet-seiving or pipette
>analysis allow for the water used to engage in isotopic exchange with the
>calcite?  Can I use heavy liquids, or will this also set up an isotope
>exchange risk?
>
>Should I alternatively try to solve the problem in the CO2 extraction
>process?  Does anyone know of a stable isotope lab that has experience
>with this procedure that can do the carbon and oxygen isotope analyses for
>$20 per sample (130 samples total)?
>
>Thanks!
>Kathy Guiles
>Syracuse University

-----------------------------------------------
Dr. Clemente Recio
Stable Isotope Laboratory
Fac. de Ciencias
Univ. de Salamanca
E 37008 - Salamanca
      Spain

Phone: (+34) 23 29 44 00 Ext. 1540
Fax:   (+34) 23 29 45 14
E-mail: [log in to unmask]
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