Dear Uwe,

I have investigated the temperature dependence for  H3PO4 preparation of
kutnahorite (ordered CaMn(CO3)2 with very minor Mg or Zn/Fe), assuming that
the fractionation factor at 25°C is positioned between calcite and
rhodochrosite, which works for the BaCO3-MgCO3 (Vennemann & Böttcher, in
prep) and CaCO3-MgCO3 systems.

The reaction was completed within hours even at temperatures below 50°C  so
much faster as for pure dolomite. I would expect a trend towards dolomite
behavior with increasing substitution of Mn (and/or Ca) by Mg.

Results have been published in 1996 in "Isotope in Environment and health
studies"
Vol 32, 299-305.

Enjoy kutnahorite, its a pretty interesting carbonate.

Michael

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Dr. Michael E. Böttcher
Department of Biogeochemistry
Max-Planck-Institute for Marine Microbiology
Celsiusstr.1
D-28359 Bremen
Voice:  (+49)421-2028-632
Fax:    (+49)421-2028-580
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