Dear Greg and others,

There is no simple system to remove sulfides from carbonate rock to my
knowledge (I did not find any of such method by getting through the
literature either).
What you can do is remove as much sulfides as possible (eg handpicking) from
the carbonate before you apply the acid digestion to the carbonate. This
acid digestion will produce H2S from sulfides, besides the CO2 from the
carbonates. H2S can be removed from CO2 according to methods given below
(again part of my Chapter 4-5 on carbonates....).
In general acid attack on sulfides, used for silicate rock samples, is
obviously not a feasible method for getting rid of sulfides from carbonates.
Some 'non-acid' reactions removing (specific) sulfides are described in
literature - also included here (copy of one of my appendixes for my books).

***

4-5.1.6 Purification of CO2 from H2S
 Carbonate samples containing sulfide contamination produce H2S by the
reaction with phosphoric acid. It is desirable to remove this pollutant from
the CO2 gas before measurement on a MS.

 Ag3PO4, loaded in the side arm of a similar reaction vessel as used for
carbonate-acid digestion, is evacuated and degassed at elevated temperature.
The CO2-H2S mixture is collected in the other arm of the reaction vessel by
liq-N2 freezing. The reaction vessel is closed and the liq-N2 removed from
the vessel, liberating the CO2-H2S mixture. After some minutes the H2S is
completely removed from the gas by reaction with the Ag3PO4 and the CO2 can
be collected for MS measurement (Smith & Croxford, 1975; Charef & Sheppard,
1984; Lindblom, 1984; Z. Roksandic, ID).
Other chemical methods, based on Pb-acetate, Cd-acetate, Cd-formiate,
Pb-oxide, to remove H2S from carbonate-CO2 samples are described or
mentioned by Boast et al. (1981),  M. Boettcher (ID), and Z. Roksandic (ID).
A more complicated method, by N. Lifton (ID), passes CO2 - H2S gas mixture
over CuO at ‰ 600°C a few times. H2S is oxidized into SO2 (and H2O) and is
separated from CO2 cryogenically in the usual way (Chapter 8-1.x).
Hahn-Weinheimer et al. (1969) remove H2S by precipitation as CuS (solubility
product: 2 x 10-47 at 16° - 18°C) in a 10% Cu-acetate solution, with dilute
HCl added to create a low pH thus avoiding formation of CuCO3 by CO2.
G. St-Jean (ID) uses drager tubes as H2S traps, connected into a the vacuum
CO2 extraction system. The reaction in the drager tubes is:

H2S + Pb+2 -> PbS + 2H+  [4-5.5]

Physical methods, such as use of a GC or cryogenically with a n-pentane
liq-N2 trap to separate CO2 from H2S, are mentioned by Z. Roksandic (ID).

References:
Boast, M., M. L. Coleman, and C. Halls, 1981, Textural and stable isotopic
evidence for the genesis of the Tynagh base metal deposit, Ireland. Econ.
Geol., v. 76, p. 27-55.
Charef, A., and S. M. F. Sheppard, 1984, Carbon and oxygen isotope analysis
of calcite or dolomite associated with organic matter. Isotope Geosci., v.
2, p. 325-333.
Hahn-Weinheimer, P., G. Markl, and H. Raschka, 1969, Stable carbon isotope
compositions of graphite and marble in the deposit of Kropfmühl/NE Bavaria.,
in P. A. Schenck, and I. Havenaar, eds., Advances in organic geochemistry
1968. International Series of Monographs in Earth Sciences., Pergamon Press,
p. 517-533.
Lindblom, S., 1984, Calcite carbon and oxygen isotope evidence for ore
formation at Laisvall, Sweden. Bull. Minéral., v. 107, p. 241-253.
Smith, J. W., and R. J. W. Croxford, 1975, An isotopic investigation of the
environment of deposition of the McArthur mineralisation. Miner. Deposita,
v. 10, p. 269-276.

***

Lithium aluminium hydride (LAH)
reductive attack decomposes pyrite. Reaction time is relatively short and
the sulfur is recovered as Ag2S (Lawlor et al., 1963; Smith et al., 1964;
Cameron, 1983; Hall et al., 1988).

Lithium chloride
soluable sulfate from soil samples is leached by a LiCl solution
(solution:soil = 5:1) for several days. Before centrifugation and membrane
filtration (0.45 µm) to collect the sulfate, the sample is agitated in an
ultrsonic bath for 5 minutes (Mayer et al., 1995). Not all sulfate is
extracted this way, but no S- or O-isotopic fractionation occurs under these
conditions (Van Stempvoort et al., 1990; Mayer et al., 1995)

FeCl3 treatment
galena is preferentially removed by treatment with FeCl3 at room temperature
(Czamanske & Ingamells, 1970). Aqueous 10% FeCl3 solution is added to galena
containing samples and reacted for at least 24 hours. Sulfur is recovered by
vacuum filtration of the solution, drying of the filter at <60°C, and
dissolving of free S in boiling benzene in a flask with refluxing column.
Filter the benzene solution in a vessel and evaporate the benzene (Czamanske
& Ingamells, 1970).

References:
Cameron, E. M., 1983, Genesis of Proterozoic iron-formation: Sulphur isotope
evidence. Geochim. Cosmochim. Acta, v. 47, p. 1069-1074.
Czamanske, G. K., and C. O. Ingamells, 1970, Selective chemical dissolution
of sulfide minerals: a method of mineral separation. Am. Mineral., v. 55, p.
2131-2134.
Hall, G. E. M., J.-C. Pelchat, and J. Loop, 1988, Separation and recovery of
various sulphur species in sedimentary rocks for stable isotopic
determination. Chem. Geol., v. 67, p. 35-45.
Lawlor, D. L., J. L. Fester, and W. E. Robinson, 1963, Pyrite removal from
oil-shale concentrates using lithium aluminium hydroxide. Fuel, v. 42, p.
239-244.
Mayer, B., K. H. Feger, A. Giesemann, and H.-J. Jäger, 1995, Interpretation
of sulfur cycling in two catchments in the Black Forest (Germany) using
stable sulfur and oxygen isotope data. Biogeochemistry, v. 30, p. 31-58.
Smith, J. W., N. B. Young, and D. L. Lawlor, 1964, Direct determination of
sulphur forms in Green River oil shale. Anal. Chem., v. 36, p. 618-622.
van Stempvoort, D. R., E. J. Reardon, and P. Fritz, 1990, Fractionation of
sulfur and oxygen isotopes in sulfate by soil sorption. Geochim. Cosmochim.
Acta, v. 54, p. 2817-..


= NB - I do not know if there is any objection to use one of these last 3
methods for cabonates. At least they are no acid reactions, which in fact
were meant for 'sulfur' collection from samples, but might have some use in
removal of the sulfur compounds from the rock (carbonate) samples for other
purpose. This is just a 'spontaneous suggestion' - just a wild proposal....
if it can work it will be marvelous.

My two cents...
Best wishes,
Pier.
********************************************************************
Dr. Pier A. de Groot
Economic Geology Research Unit
University of the Witwatersrand
Private Bag 3
2050 Johannesburg
South Africa
Tel. +27 11 717 6582  Fax. +27 11 339 1697
E-mail: [log in to unmask]

WEB-site on "Handbook of Stable Isotope Analytical Techniques (2 volumes)":
http://www.icon.co.za/~pagroot/index_1.html
last update: November 8, 2000
********************************************************************
----------
>From: "Greg B. Arehart" <[log in to unmask]>
>To: [log in to unmask]
>Subject: Carbonates and sulfides
>Date: Wed, Nov 29, 2000, 12:24 am
>

> Dear listmembers,
>
> As long as the subject of contaminants in carbonates has arisen, I would
> like to know what folks are
> doing these days about sulfides in carbonate samples.  I know that there
> are a number of methods for
> removing sulfides off-line, but has anybody dealt with significant sulfide
> contamination (percent
> level) in automated devices, particularly at elevated (>30C) temperatures
> of reaction?
> --
> Dr. Greg B. Arehart
> Department of Geological Sciences, MS-172
> University of Nevada, Reno
> Reno, NV 89557-0138
> *****please note new area code*****
> phone: 775-784-6470
> fax: 775-784-1833
> email: [log in to unmask]