Steve N.,

As I have no experience with vacuum roasting I'm just going to talk off the top
of my head.  Ethan and Steve Crowley have referred to negative effects of
enhanced exchange between carbonate oxygen and other oxygen-bearing species at
higher temperatures.  The vacuum isolates the samples from the atmospheric
reservoir of oxygen and water vapor.  It also serves to remove water and other
oxygen-bearing species evolved from the organic matter during heating.

To those who have participated in this discussion:
I have found the points raised to be quite informative.  Clearly grain size is a
factor. But I had not previously considered the response of inorganically
precipitated carbonate relative to biogenic carbonate to the cleaning process.
Am I correct to assume that this has to do with crystal structure or porosity
and hence effective grain size?  Or, as Steve Crowley has mentioned, does it
have something to do with the thermodynamic stability of the different
carbonates?  I understand the concept of thermodynamic stability when talking
about aragonite relative to calcite, but is there a difference between the
thermodynamic stability of, for example, inorganic low-Mg calcite and organic
low-MG calcite?

Reed McEwan
Stable Isotope Lab
University of Minnesota



Responding to the message of  <v03007800aff13059d8d5@[128.187.66.7]>
from Stable Isotope Geochemistry <[log in to unmask]>:
>
> I have generally not worked with organic-rich materials up to this point.
> Thus, forgive a little ignorance on my part.  If one chooses to roast to
> remove organic matter, why should it be done in a vacuum?
>
> ********************************
> Steve Nelson
> Dept. of Geology
> 673 WIDB
> Brigham Young University
> Provo. Utah  84602
>
> voice:  1-801-378-8688
> FAX:  1-801-378-8143
>
> "INTEL INSIDE" is a warning label
> *********************************
>
> .