THE ANALYSIS OF FORMS OF SULFUR IN ANCIENT SEDIMENTS AND SEDIMENTARY-ROCKS - COMMENTS AND CAUTIONS

Assumptions commonly made during analysis of the amount of monosulfide s [acid-volatile sulfides (AVS)] and disulfides in modern sediments, m ay not be valid for ancient sedimentary rocks. It is known that ferric iron can oxidize H2S during AVS analysis unless a reducing agent such as stannous chloride is added to the treatment. In addition, some mon osulfides such as greigite and pyrrhotite require heat during the AVS analysis in order to dissolve completely. However, the use of heat and /or stannous chloride in the AVS treatment may partially dissolve disu lfides and it is generally recommended that stannous chloride not be u sed in the AVS treatment for modern sediments. Most of the monosulfide s are assumed to be recovered as AVS without the addition of stannous chloride. This study investigates the recovery of monosulfides during sulfur speciation analysis with application to ancient sedimentary roc ks. Sulfur in samples containing naturally occurring greigite and mack inawite or pyrite was measured using variations of a common sulfur-spe ciation scheme. The sulfur-speciation scheme analyzes for monosulfide sulfur, disulfide sulfur, elemental sulfur, inorganic sulfate and orga nically bound sulfur. The effects of heat, stannous chloride and ferri c iron on the amounts of acid-volatile sulfide and disulfide recovered during treatment for AVS were investigated. Isotopic compositions of the recovered sulfur species along with yields from an extended sulfur -speciation scheme were used to quantify the effects. Hot 6 N HCl AVS treatment recovers > 60% of the monosulfides as AVS in samples contain ing pure greigite and mackinawite. The remaining monosulfide sulfur is recovered in a subsequent elemental sulfur extraction. Hot 6 N HCl pl us stannous chloride recovers 100% of the monosulfides as AVS. The add ition of ferric iron to pure greigite and mackinawite samples during A VS treatment without stannous chloride decreased the amount of monosul fides recovered as AVS and, if present in great enough concentration, oxidized some of the AVS to a form not recovered in later treatments. The hot stannous chloride AVS treatments dissolve < 5% of well-crystal lized pyrite in this study. The amount of pyrite dissolved depends on grain size and crystallinity. Greigite in ancient sedimentary rocks wa s quantitatively recovered as AVS only with hot 6 N HCl plus stannous chloride. Hot 6 N HCl AVS treatment of these rocks did not detect any monosulfides in most samples. A subsequent elemental sulfur extraction did not completely recover the oxidized monosulfides. Therefore, the use of stannous chloride plus heat is recommended in the AVS treatment of ancient sedimentary rocks if monosulfides are present and of inter est. All assumptions about the amount of monosulfides and disulfides r ecovered with the sulfur-speciation scheme used should be verified by extended sulfur-speciation and/or isotopic analysis of the species rec overed.