Dear Kathy,

I agree with Clemente, try fractional extraction. Two more papers may
be of interest :

Rosenbaum J. and Sheppard S.M.F. (1986)  An isotopic study of siderites,
dolomites and ankerites at high temperatures, Geochim. Cosmochim.
Acta, 50, 1147 - 1150.

Szaran J., Niezgoda H. (1996) Acid fractionations of 18O and 13C in
CO2 preparation from dolomite - calcite mixture, Abstracts of Isotope
Workshop III,  Acta Geol. Hung. Suppl., 39, 180 - 181.


With best regards,

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Tomasz Durakiewicz
Mass Spectrometry Lab, Institute of Physics
Maria Curie-Sklodowska University
pl. M.C. Sklodowskiej 1
20-031 Lublin, POLAND
tel. (0-81) 537 61 78, fax (0-81) 533 36 69
http://tytan.umcs.lublin.pl/users/dudek/psm.htm
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