Dear all,

There was a fair amount of interest (both within and without the group)
after I enquired about Ba in  calcites, which perhaps warrants this
follow-up mail.

Max Coleman said:

>>In some cases we can be fairly sure that the Ba is in the carbonate -
>>measurements on samples from exceptionally-preserved Jurassic fauna - still
>>aragonite and uncompacted - give values over 1000 ppm (and a negative
>>correlation with delta 13-C)

I said:

>Trace barite will of course dissolve in a limited way if present. However,
>I have observed no changes in Ba concentration or Sr/Ba ratio between HCl,
>HNO3, acetic acid or buffered solution (pH=4.9) leachates of >98% pure
>limestones, which indicates that Ba is really in the carbonate. The next
>test is SEM and microprobe analysis.

Well, the electron microscope analysis was quite conclusive, at least for
the carbonate sample most enriched in barium (2000 ppm Ba). Ba was only
found in detectable quantities in tiny (<2 microns) crystals of
disseminated barite, and so does not exist in significant amounts within
the calcite structure. So, although barite should not dissolve in
appreciable amounts during acid leacing, it may well dissolve in geological
samples. It would be hard to imagine barite crystals in Max's Jurassic
example, but the existence of tiny amounts of trace minerals such as
apatite and barite can cause havoc to the interpretation of carbonate rock
leaching experiments!

Graham





Graham Shields
Centre de Géochimie de la surface (CNRS)
1 rue Blessig
F-67084 Strasbourg
France

Tel: 0033 3 88 35 86 27
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