Dave, Hector, and others...
A few years ago Matt Kirby and I developed a
method for deriving the calcite values from
various artificial mixtures of calcite and
dolomite online using a Kiel-III coupled to a MAT
252. We mixed <63 micron powders of calcite and
dolomite at ratios of 90:10, 70:30, and 50:50. By
lowering the reaction temperature to 50 degrees
and reacting the mix for less than 2 minutes, we
got a consistent calcite-only value. We used that
temperature because 50 degrees is about as low as
you can go and still have 103% phosphoric move
through the system.

Because we weren't interested in the dolomite
value (Devonian contamination of our Holocene
marl), we didn't pursue that. However, you may
try reacting the mixes with a weak acid (like
dilute acetic acid) for several hours/days and
after washing and drying try to recover the
dolomite-only value?

Hope this helps?
Cheers,
Bill

>Hector,
>
>Coincidentally, I was about to send a similar email to the list.  I tried
>the same technique after reading of its use in a Precambrian Research
>article (Kimura et al, 2005).  To try it myself, I made a variety of
>mixtures of calcite and dolomite standards, reacting them at 25°C and then
>75°C.  I tried collecting CO2 from the calcite anywhere from 30 min after
>reaction to almost immediately.  In every case there was cross-
>contamination with reacting dolomite.
>
>However, what I was able to do was get the dolomite values by reacting the
>mixes at 25°C for 1 hour, pumping off the gas from the calcite (and some
>dolomite), and then reacting the remainder of the dolomite at a higher
>temp, assuming all the calcite had reacted.  There may be some
>fractionation due to less reactive crystals, but the values were very close
>to what they should have been for the dolomite std.
>
>I have not pursued this farther, but thought if you know the percentages of
>each mineral, the overall mixed isotopic value of the sample, and the
>dolomite values, should be able to get to the calcite value by a mass
>balance equation.
>
>Very interested to hear if you have any success with this.
>
>Dave Mrofka
>UC Riverside Earth Sciences


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