Dear All

I have always been under the impression that carbonate roasting is not done
to remove organic contaminants through combustion, is done to "remove
volatile components", i.e. anything that will be volatilized during the
reaction under vacuum and trapped by the CO2 trap during purification of the
evolved gas.  That will include any "lightly" surface bound materials (H20,
CO2, some organics, inert gases, etc).  Roasting to combust organics will
result in decarbonation of calcite and aragonite, and most carbonate
minerals.

Way back in the early stages of my PhD (~mid 80's) , I conducted a series of
tests with Dave Dettman at KC Lohmann's lab at the University of Michigan.
We tested coral (Porites) and gastropod (Strombus) aragonite, high
Mg-calcite  echinoderm spines (~18 % MgCO3) and red algae (12-15 mol %
MgCO3), and reagent grade calcite as a control.  Reacted samples with no
roasting and roasted in vacuo from 100 to 500°C and found that:

1. Roasting made no difference in the average value of calcite and high-Mg
calcite.  
2.  Roasting decreased the variability (standard deviation) of the results,
with best results at 200°C and no significant improvement using temperatures
in excess of 200°C.
3. Aragonite transforms to 100% calcite when roasted at 500°C for 1 hour.
The transformation is accompanied by a negative shift in both carbon (>-1
per mil) and oxygen (>-0.5 per mil).  We suspected decarbonation of
aragonite with CO2 and CaO formation.  We did trap significant amounts of
CO2 during roasting, but never completed tests to see if hypothesis was
right.
4. That transformation of aragonite to calcite is negligible when roasted at
200 for 1 hour.  However, longer roasting periods (>> 1 hour) do result in a
slight transformation of aragonite to calcite and a slight shift in isotopic
composition.
5. That aragonite transformation at temperatures over 200°is proportional to
temperature and time.
6. The magnitude of the isotopic shift was proportional to the degree of
aragonite transformation.


Since that time I have been roasting at 200°C and making sure we only do so
for 1 hour.  Roasting at 100° for about 2 hours will have the same effect.
We treat all carbonate the same way.

No, I never published any of this, and I don't have all of the data now.  So
it may be worth pursuing systematic testing and to adopt a standardized
treatment for carbonates.

What is clear from all of us who have done similar testing is that aragonite
should not be roasted at temperatures greater than 200°.  Some have
suggested that we do not roast samples.


Luis

On 12/4/06 1:26 AM, "Clemente Recio" <[log in to unmask]> wrote:

> Dear Steve, Bill and others interested,
> 
> Tricky thing!
> 
> But taking it positively, why not attempting to
> compare methods as far as reasonably possible? I
> understand the difficulty of selecting the proper
> sample to test, but in the absence of an ideal
> one, maybe your lake marls would make a good
> starting point (fine-grained, abundant and
> homogeneous enough???).
> 
> If any of you takes the trouble of organizing a
> little test, I can contribute facilities for
> plasma ashing and vacuum roasting, together with
> different carbonate prep systems: off-line
> extraction, with reaction temperatures from 25 to
> 150 deg C (+/-0.1 deg); ISOCARB (common acid bath
> at 90 deg C) and Multiflow (single drop at 70
> deg).
> 
> Any body else willing to have a go at it?
> 
> Cheers,
> 
> Clemente
> 
> 
> 
>> Dear Bill,
>> 
>> I raised this subject on ISOGEOCHEM many years ago. I still can't
>> understand the logic. If it's OK to heat a sample at 200oC to "remove"
>> organic matter then I don't see any reason to heat a sample to any
>> greater temperature because the objective has already been achieved. In
>> fact I would suggest that it's a pretty stupid thing to do, particularly
>> if you're working with poorly structured biogenic carbonate. Or are we
>> saying that organic matter in calcite is much 'tougher' than organic
>> matter in aragonite?
>> 
>> Conversely, if heating to 350oC (or whatever) is what's required then
>> "roasting" can't be suitable for aragonite. Then one is forced to
>> conclude that any aragonite only heated to 200oC may still contain
>> reactive organic matter and any resultant isotopic data may be
>> compromised.
>> 
>> However, I would take this organic question much further. How do you
>> know when a treatment has been successful? For instance, there has been
>> some ISOGEOCHEM discussion that suggests that organic matter in contact
>> with phosphoric acid does not cause any problems. If this were the case
>> then you shouldn't need to use any organic removal techniques. Of course
>> this might be OK at 25oC, but most labs now use autoprep systems with
>> hot acid. Not only can the hot acid hydrolyse the organics, but immature
>> organic matter must start to breakdown thermally regardless of any acid
>> hydolysis.
>> 
>> I have tried different procedures (chlorox, oxygen plasma ashing, vacuum
>> roasting), but I can't give a definitive answer for the best method. For
>> example, I've tried treating different types of biogenic carbonate with
>> each technique and have found no systematic affects. Sometimes chlorox
>> and plasma ashing would give similar results and roasting different
>> values, for another sample the outcomes would be entirely different.
>> Sometimes carbon would be affected and not oxygen, sometimes the reverse
>> would be the case. This lack of any consistent behaviour is difficult to
>> understand and means that it is impossible to decide which procedure is
>> best.
>> 
>> As far as I'm concerned, trying to test different methods using
>> artificial mixtures is problematic because you need to intimately mix
>> suitable samples of pure carbonate (poorly structured carbonate, not
>> nice highly crystalline inorganic carbonate) with the right sort of
>> organic molecules (pigments, proteins, lipids, carbohydrates etc).
>> Perhaps using otolith, echinoid carbonate or low temperature inorganic
>> carbonate precipitate as a source of "organic-free" calcite might be a
>> starting point - not Iceland spar. In addition, we've been running lake
>> marls at Liverpool for a number of years and it is evident that organic
>> contaminants exist on virtually a molecular scale, so it's difficult to
>> see how you could accurately mimic a natural sample by mechanical
>> mixing. One possibility would be to prepare a low temperature
>> precipitate in the presence of organic acid anions?
>> 
>> I'd also like to mention a problem that "roasters" don't seem to
>> mention. Biogenic carbonate contains variable amounts of
>> stucturally-bound water. So in addition to any possible exchange between
>> the thermal decomposition products of organic molecules (perhaps bound
>> on the crystallite scale), you also have to consider the effects of
>> exchange between hot "H2O" and carbonate where water is present on a
>> molecular scale. In fact it may turn out that water is a more serious
>> problem for "roasters" than organics, and that variable amounts of
>> structurally bound water (either as H2O or OH) may account for the
>> apparent varying affects seen in different types of biogenic carbonate.
>> 
>> To be honest it strikes me that not every one can be right every time
>> and that at least some published data, based on samples treated using
>> each of the common organic removal techniques, must be compromised. I
>> couldn't begin to say how one might attempt to decide which data are
>> acceptable. The fact that you can decide that a data set "makes more
>> sense if a correction is applied" seems risible. How is it possible to
>> arbitrate in these cases? The idea that some data might seem more
>> palatable than others must surely only be a personal opinion. Of course
>> some data will appear completely unacceptable (outside what is
>> scientifically possible), but then where do you draw the line between
>> what is acceptable and what is "wrong"? Perhaps we should build in an
>> uncertainty to account for this. The problem here is that many labs
>> claim such high levels of precision, and that carbonate data are used to
>> make extremely fine distinctions in interpretation, that many
>> researchers/reviewers won't accept "realistic" uncertainty estimates.
>> 
>> At is stage it strikes me that we may unwittingly risk creating a
>> pseudo-science. Unfortunately, a detailed investigation of this problem
>> is not the sort of thing that attracts research funding. We may have to
>> face the fact that we may never fully know either the consequences of
>> organic contamination on our isotopic data or the effects of techniques
>> that attempt to remove organics.
>> 
>> Finally, I'd like to apologise for the length of this message. Once I
>> started to reply to Bill's message I couldn't stop. I also realise that
>> much of the contents may be unacceptable to many List Members, so I
>> should also apologise for any offence I may have caused.
>> 
>> I look forward to the implications of my message being "shot down in
>> flames".
>> 
>> Regards,
>> 
>> Steve Crowley
>> 
>> 
>> Department of Earth & Ocean Sciences
>> University of Liverpool
>> 4 Brownlow Street
>> Liverpool
>> L69 3GP
>> UK
>> 
>> 0151 794 5163/5164
> 



*******************************************************
Luis A. González, Director
KECK Paleoenvironmental and Environmental Stable Isotope Laboratory
 
Department of Geology
University of Kansas
Multidisciplinary Research Building, Rm. 130
2030 Becker Drive
Lawrence, KS 66047

tel:  (785) 864 - 3977
fax: (785) 864 - 5276
lab: (785) 864 - 7750

http://people.ku.edu/~lgonzlez
******************************************************