Dear Kathy,

        Rather than trying to "solve the problem in the CO2 extraction
process," your best (and most accurate) bet is to try to separate the
calcite and dolomite phases either physically or chemically.  There are no
techniques, that I'm aware of, anyway, which would allow for the selective
removal of dolomite from sediments consisting of admixtures of low-Mg
calcite and dolomite.  However, calcite can be selectively removed using
the leaching method of Glover (1961), which utilizes the ability of EDTA to
complex Ca2+.  Using a combination of techniques -- XRD and ICP analyses of
bulk samples followed by carbon and oxygen isotope analyses of bulk and
leached samples (calcite removed) -- you should be able to back-calculate
the isotopic composition of the calcite phase in your samples.  Such a
technique was used by Drummond et al. (1993) on lacustrine micrites made up
of dolomite and low-Mg calcite.  Below is a brief, general outline of the
technique and a list of references.

Calcite and dolomite stoichiometries are calculated for each sample using
XRD analysis and equations of Goldsmith et al. (1961).  Ca and Mg
concentrations, determined by ICP analysis, are used to calculate phase
abundances in each sample.  Each sample is then split into two aliquots.
On one aliquot, carbon and oxygen isotope compositions are determined
directly.  The other sample is leached using Glover's method to remove
low-Mg calcite.  The residual sample, consisting of dolomite, is then
analyzed to determine the isotope composition of the pure dolomite phase.
(XRD analysis of residual, leached powders has shown that Glover's leaching
technique will not alter the stoichiometry of dolomite).  Knowing the
relative abundances of calcite and dolomite in each sample and the carbon
and oxygen isotope composition of the dolomite should allow you to then
mathematically determine the isotope composition of the calcite in your
samples.

This is just one approach -- I'm sure someone out there has an alternative
method.  A drawback is that the method requires the use of rather large
samples.  In addition, the leaching technique will only remove low-Mg
calcite.  Any high-Mg calcite, if present, will remain in your sample.

Good luck!
Tracy Frank


References:

Drummond, Wilkinson and Lohmann (1993) Rock-dominated diagenesis of
lacustrine magnesian calcite micrite.  Carbonates and Evaporites, v. 8, p.
214-223.

Glover (1961) Method of solution of calcareous materials using the
complexing agent, EDTA. Journal of Sedimentary Petrology, v. 31, p.
622-626.

Goldsmith, Graf and Heard (1961) Lattice constants of the calcium-magnesium
carbonates.  American Mineralogist, v. 46, p. 453457.

Tracy D. Frank
507 Deike Bldg.
Dept. Geosciences
Penn State University
University Park, PA 16802
tel: (814) 863-2474