On 30 Oct 2020, at 04:50, Crowley, Stephen <[log in to unmask]> wrote:
I would urge you to read a paper by Yui & Gong (2003) Chemical Geology, 201, 359-368. Unfortunately most natural dolomites are relatively poorly crystalline, non-stoichiometric and fine-grained and are thus quite reactive, even with buffered acetic acid.
For chemical isolation of dolomite the use of Na-EDTA (pH 6.3) is more effective than acetic acid in removing calcite/aragonite while minimising dolomite decomposition (see Glover 1961, J. Sedimentary Petrology, 31, 622-626). This due to the fact that EDTA forms Ca complexes preferentially. Again, unfortunately this is a time-consuming process. All samples must be crushed to a standard grain size (<37 microns is a reasonable size) in order to expose “all” calcite in intergrowths of calcite and dolomite and to ensure that grain-size dependent effects on the rate of calcite decomposition are standardised. If you fail to crush to a consistent grain size then the occurrence of a few coarser calcite grains will significantly extend the reaction time required to remove all calcite.
Several messages have advocated the use of sequential phosphoric acid dissolution techniques which have the benefit of being relatively easy to use. In my opinion these methodologies are misleading. Authors reporting the development of these methods validate their procedures based on experiments using artificial mixtures of calcite and dolomite in which the dolomite component is well crystalline, near-stoichiometric and coarse-grained. This in no way reflects the reality of most natural (particularly sedimentary) samples where dolomite is much more reactive. These experiments generally lead to an over-optimistic impression of performance.
I’ve never tried to use a density separation technique. I would imagine that similar sample dependent (particularly textural) factors would greatly influence the effectiveness of this method.
A further consideration is the relative amounts (and the magnitude of the difference in isotopic composition) of calcite and dolomite in any mixture. Once the minor component occur in amounts below 20 wt.% there will be very little chance of obtaining a reliable isotopic composition. Obviously if you’re only interested in measuring the major component then your chances of obtaining a good outcome greatly increase as the minor component heads towards zero.
Would anyone have literature regarding the separation of Calcite and Dolomite in samples for analysis of stable isotopes of O and C? I recall doing something of the sort years ago using acetic acid to dissolve the calcite fraction first, then a stronger acid to react the dolomitic fraction.