We've been using thermal analysis attached to an IRMS system for just
this problem (intimately intergrown calcite and soil organic matter).
This approach exploits the fact that generally SOM degrades at lower
temperatures than carbonates decompose, and we can 'easily' distinguish
del13C for calcite from that of cellulosic material and lignin-derived
material.

The only snag would be that siderite's thermal decomposition is at
lower temperatures than calcite, so the job ecomes a bit harder.

Also, we can't do N at the present time - but that's the next step in
the development of this system.

Get in touch if you'd like to send us a couple of samples to try.

David Manning



On 6 Jul 2005, at 19:00, tlarson wrote:

> Folks,
>
> We're trying to separate siderite from soil/clay samples so that we may
> analyze the organic components of the soil for d13C, d15N and CN
> ratios.
> The refs I've come across focus on 'carbonate' removal and indicate
> that
> siderite is less soluble. I found I can dissolve siderite in 6 molar
> HCl at
> 50°C overnite, or soaking in 6 molar HCl at 25°C for several days. To
> remove excess HCl I plan to freeze dry the samples and not pour out
> any of
> the acid in case some of the organics have also gone into solution (I
> think
> this is the same idea as vapour removal).
>
> Any comments or suggestions regarding possible effects others have
> observed
> using this method, or better methods before I follow the tedious path
> of
> sample preparation testing? The first line of testing I was going to
> do was
> to dose IAEA-C5 (wood) with siderite and see if THIS pretreatment
> affected
> its d13C values.
>
> Thanks in advance for saving me time,
>
> Toti.
>
>
> Toti Larson Ph.D.
> Los Alamos National Laboratory
> Earth and Environmental Sciences                505.667.9894 (lab)
> Los Alamos, New Mexico 87545            550.667.8006 (office)
>
>
Professor of Soil Science
School of Civil Engineering and Geosciences
University of Newcastle
Newcastle upon Tyne
NE1 7RU

telephone (+44) 0191 222 7893