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The characterization of iron sulfide minerals in anoxic marine sediments
Marine Chemistry, 22 (1987) 193 206 Elsevier Science Publishers B.V., Amsterdam
193 Printed in the Netherlands
THE CHARACTERIZATION OF IRON S U L F I D E MINERALS IN ANOXIC MARINE S E D I M E N T S *
JEFFREY C. CORNWELL** and JOHN W. MORSE Department of Oceanography, Texas A & M University, College Station, TX 77843-3146(U.S.A.)
(Received July 7, 1986; revision accepted June 15, 1987)
ABSTRACT Cornwell, J.C. and Morse, J.W., 1987. The characterization of iron sulfide minerals in anoxic marine sediments. Mar. Chem., 22: 19~206. The performance of extractants commonly used in the determination of acid volatile sulfide minerals (e.g., 'amorphous-FeS', mackinawite and greigite) and pyrite has been evaluated using pure mineral phases and anoxic sediments. 'Amorphous-FeS' and mackinawite are quantitatively recovered by most cold acid extractions, but greigite recovery is incomplete. Harsher extractants with reducing agents are necessary for the complete recovery of greigite, but dissolution of fine-grained synthetic pyrite occurs under such conditions. A quantitative separation between greigite and pyrite is not possible using these techniques, but the use of 'acid volatile sulfide' and 'pyrite' as operational categories is adequate for most field studies. INTRODUCTION
Sedimentological studies generally separate iron sulfides into two ~operational' categories: acid volatile sulfide (AVS) and pyrite (Berner, 1964a; Berner et al., 1979; Howarth and Jorgensen, 1984; Davison et al., 1985; Morse et al., 1986). Acid volatile sulfides are evolved via acid distillation and generally are considered to include 'amorphous-FeS', mackinawite, greigite and pyrrhotite. Pyrite often is the most abundant iron sulfide. It is presumed to resist dissolution by acid attack and is generally determined by oxidative (e.g., Goldhaber et al., 1977) or reductive dissolution (Zhabina and Volkov, 1978; Canfield et al., 1986). T h e s e t w o c a t e g o r i e s o f i r o n s u l f i d e m i n e r a l s h a v e b e e n u s e d t o e x a m i n e s e d i m e n t i r o n s u l f i d e p o o l s i z e s a n d 3~S-derived S f l u x e s ( G o l d h a b e r e t al., 1977; H o w a r t h , 1979; G o l d h a b e r a n d K a p l a n , 1980; H o w a r t h a n d J o r g e n s e n , 1984). However, recent reports suggest that in some instances the AVS determination may under- or overestimate the 'true' concentration of iron monosulfides ( W e s t r i c h , 1983; C h a n t o n a n d M a r t e n s , 1985). D e s p i t e t h e c o n t i n u e d u s e o f different AVS techniques, no systematic examination of the recovery of the various pure iron sulfide minerals using AVS techniques has been carried out. * Presented at the IX International Symposium 'Chemistry of the Mediterranean', May 1986, Primo~ten, Yugoslavia. ** Current address: University of Maryland, CEES Hornpoint Environmental Laboratory, P.O. Box 775, Cambridge, MD 21613, U.S.A. 0304-4203/87/$03.50
~ 1987 Elsevier Science Publishers B.V.
194
In this paper we present the results of AVS determination of synthetic ~amorphous-FeS', mackinawite, greigite and fine-grained pyrite, and coarse grained natural pyrite under a variety of acid solution compositions and temperatures. Pyrrhotite was not examined in detail because it is not commonly identified in most anoxic marine sediments. Our purpose is to examine how well different methods separate the ~amorphous-FeS', mackinawite and greigite from pyrite, and to determine the potential for development of methods which might lead to the separate determination of all mineral forms. TECHNIQUES FOR IRON SULFIDE MINERAL IDENTIFICATION
Despite the importance of iron sulfide mineral formation to global iron and sulfur fluxes, iron sulfide minerals are usually found in marine sediments in very low concentrations. In general, sediments contain only a few percent or less of FeS2 and FeS. Pyrite often occurs as distinct crystals or in framboids (Sweeney and Kaplan, 1973) while the black coating of sulfide in many reducing sediments is presumed to be iron monosulfides. In general, pyrite is the most abundant iron sulfide mineral in marine sediments (Berner et al., 1979), and non-pyritic forms of iron sulfide may be finely dispersed. The difficulty in direct determination of iron sulfide mineralogy and quantity arises from several factors: their mass and volume are small relative to other sediment components and in the case ofmonosulfides they may occur in a finely dispersed manner. Thus, bulk sediment analyses by X-ray diffraction, examination via microscope and surface chemical techniques are generally not applied to the quantification of iron sulfide minerals. Despite these difficulties, iron monosulfides have been identified in some sedimentary environments (Berner, 1974; Goldhaber and Kaplan, 1974). Identification, and perhaps quantification of sedimentary pyrite may be possible using SEM, but other sulfide minerals are not generally accessible by this technique (Morse and Cornwetl, 1987). The reaction conditions used for chemical extraction of iron sulfides are diverse (see Table I for summary). The AVS techniques involve acidification of wet sediment with non-oxidizing acids followed by active or passive transport of H2S out of solution into an appropriate trapping reagent. Generally, glass distillation apparatus are used, but adaptations for syringes (Davison and Lishman, 1983) and examining total S content before and after acidification (Sheu and Presley, 1986) have been used. The addition of SnC12 to reaction flasks has been used to inhibit the interference of sedimentary Fe(III) with H2S evolution (Pruden and Bloomfield, 1968) 2FeOOH + 4H + + H2S -
2Fe 2~ + S + 4H20
Reducing agents such as SnC12 and TIC13 (Pruden and Bloomfield, 1968; Albert, 1984) reduce this interference. The trapped H2S may be analyzed via iodometric titration (Kolthoff and Sandell, 1952), gravimetry as AgS or BaSO4, or via potentiometric titration (Orion Research, 1977). The effectiveness of the different reaction conditions on recovery of iron sulfides has not been clearly defined. The most fundamental separation
195 TABLE I Extraction methods for iron sulfide digestion Method
Reference
Acid volatile sulfide (H2S emanation)
Cold HC1 (1-12 N) Hot HC1 (6-12 N)
Goldhaber et al. (1977), Jorgensen (1977), Aller (1980) Kolthoff and Sandell (1952), Berner (1974), Zhabina and Volkoff (1978), Wieder et al. (1985) Cold HC1 (6 N) + SnC12 Westrich (1983) Hot HC1 (6 N) + SnC12 Berner et al. (1979), Westrich (1983) Cold H2SO4 (1N) + TiCl~ Albert (1984) Total reduced S (H2S emanation)
Cr(II) reduction
Zhabina and Volkov (1978), Canfield et al. (1986)
Total S
Aqua-regia digestion Combustion Pyrite
Goldhaber et al. (1977) Berner (1974)
via Fe content
HNO:~digestion ~
Lord (1982)
Oxides and silicates removed in pretreatment. r e q u i r e d is b e t w e e n p y r i t e a n d all of the less s t a b l e iron sulfide m i n e r a l s . O p t i m a l l y , t h e m o s t h a r s h AVS t e c h n i q u e w h i c h r e c o v e r s all t h e m o n o s u l f i d e m i n e r a l s a n d g r e i g i t e a n d n o n of the p y r i t e s h o u l d be used. W e s t r i c h (1983) a n d B e r n e r et al. (1979) used a h o t HC1-SnC12 p r o c e d u r e to this end b u t C h a n t o n a n d M a r t e n s (1985) h a v e d e m o n s t r a t e d t h a t p y r i t e is n o t i m p e r v i o u s to a t t a c k by this r e a g e n t . O t h e r r e a g e n t s for sulfide d e t e r m i n a t i o n h a v e n o t b e e n e x a m i n e d for selectivity. T h e c h r o m i u m r e d u c t i o n p r o c e d u r e ( Z h a b i n a a n d Volkov, 1978; Canfield et al., 1986) a p p e a r s to r e c o v e r t o t a l r e d u c e d sulfur (TRS) w i t h o u t s i g n i f i c a n t l y a f f e c t i n g o r g a n i c sulfur c o m p o u n d s . T h e p y r i t e c o n t e n t of s e d i m e n t is c o n s i d e r e d to be t h e t o t a l r e d u c e d sulfur c o n t e n t m i n u s the c o n t r i b u t i o n of AVS a n d e l e m e n t a l sulfur. In m o s t sediments, e l e m e n t a l sulfur is a v e r y small p r o p o r t i o n of t o t a l sulfur, a n d t h e p y r i t e c o n t e n t m a y be under- or o v e r e s t i m a t e d b e c a u s e of p y r i t e d e c o m p o s i t i o n or i n c o m p l e t e r e c o v e r y of AVS. T h e e s t i m a t i o n of iron sulfide d i s t r i b u t i o n u s i n g e x t r a c t i o n t e c h n i q u e s h a s s t r e n g t h s a n d w e a k n e s s e s s i m i l a r to t h o s e of t r a c e m e t a l d e t e r m i n a t i o n with ~selective' e x t r a c t a n t s (e.g., v a n V a l i n a n d Morse, 1982). B o t h k i n d s of a n a l y s e s suffer from a l a c k of a b s o l u t e specificity in m i n e r a l (or phase) s e p a r a t i o n . T h e r e c a n be significant o v e r l a p s b e t w e e n o p e r a t i o n a l c a t e g o r i e s , a n d t h e s e o v e r l a p s m a y n o t be c o n s t a n t b e t w e e n different s e d i m e n t a r y e n v i r o n m e n t s . The s t r e n g t h of e x t r a c t i o n t e c h n i q u e s lies in t h e i r ease of use on l a r g e n u m b e r s of i n d i v i d u a l s a m p l e s a n d t h e g e n u i n e l a c k of m o r e definitive s e p a r a t i o n s u s i n g o t h e r t e c h n i q u e s . Thus, e x t r a c t i o n s r e m a i n t h e m o s t a t t r a c t i v e t e c h n i q u e for s e p a r a t i n g different c a t e g o r i e s of i r o n sulfide m i n e r a l s . D e t a i l e d u n d e r s t a n d i n g of w h i c h m i n e r a l s t h e s e o p e r a t i o n a l c a t e g o r i e s include is sorely needed.
196 METHODS
Mineral syntheses All techniques for mineral synthesis in this study involve the use of aqueous solutions. Various reactants were introduced into a multiple-neck 500ml reaction flask containing deoxygenated water. A separate Cr(II) solution (Shriver, 1969) was used to scrub pre-purified N2 gas of residual 02, and the reaction solution was bubbled overnight with N2 to remove all dissolved oxygen. A constant overpressure of N 2 or H2S was used to maintain anoxic conditions. Continual stirring of the solution via a paddle type stirrer ensured complete mixing. The synthesis of 'amorphous-FeS', greigite and pyrite all started with 6.5 g Fe(NH4)2(SO4)2"6H20 and 7.0 g Na2S" 9H20 in 400 ml of distilled water under N2. ~Amorphous-FeS' is the initial room temperature product of the synthesis. Greigite is formed by the overnight boiling of the above mixture with 5 ml of a polysulfide solution, following the procedure of Wada (1977). Pyrite was formed by the addition of 10 ml of polysulfide solution and 3.0 g of elemental S to the amorphous FeS mixture and boiling for several days. Mackinawite was synthesized by the reaction of Fe metal with H2S (Berner, 1964b; Shoesmith et al., 1980). Hydrogen sulfide gas was bubbled overnight in a reaction vessel containing 20 g of Fe plate. Constant stirring resulted in the suspension of the mackinawite into the solution above the Fe metal. The formation of pyrrhotite was noted in instances when the synthesis continued for prolonged periods (e.g., Shoesmith et al., 1980). The characterization of the synthetic minerals was performed by X-ray diffraction (XRD) analysis of a portion of the precipitate collected on glass fiber filters. A Phillips-Norelco XRD was used with Cu K~ radiation and the diffraction patterns were compared to reference patterns for each mineral. The XRD analyses were carried out as quickly after filtration as possible, with the synthetic greigite showing short-term evidence of air oxidation on limited occasions. The material used for estimation of digestion efficiency was sampled from the flask (generally 3.0ml was taken), washed with water, and in the cases of greigite and pyrite, washed with carbon disulfide or acetone to remove any remaining elemental S or polysulfides. Synthetic pyrite was dried before digestion. The 'commercial' pyrite used (MCB Reagents, IX02060-1) was < 50 mesh and guaranteed to be > 85% pyrite.
Digestion techniques The digestion and acid volatilization of iron sulfides utilized 125ml Erlenmeyer flasks, capped with rubber stoppers, with ports for the input of N2 gas, the exit of N2 and H2S gases and the introduction of digesting solutions. Prepurified N2 was used for purging the apparatus and the H2S evolved was trapped in 20 ml of sulfide anti-oxidant buffer (Orion Research, 1977) in a 25 ml
197
TABLE II Synthetic mineral results (% recovery)a
CH3COOH H3PO4 1.0 N HC1 6.0 N HC1 6.0 N HC1 + SnC12 Hot 6.0 N HC1 Hot 6.0NHC1 + SnC12 1.0NH2SO4 + Ti Cr reduction
Amorphous
Mackinawite
Greigite
Synthetic pyrite
Pyrite
59 90 100 100 100 100 100 100 100
7~3 89 92 98-102 100-102 96-97 100-101 101 100
11 17 40 40~7 60~9 63-75 66~84 93-100 96-100 100
0 0 0 0 4~10 5~ 97-100 48-82 99-100
0 0 0 0 0 0 2 0 87
When ranges are presented, they represent results from different batches of synthetic iron sulfide minerals. test tube. W a t e r - c o o l e d West c o n d e n s e r s were used w h e n s o l u t i o n s were boiled. Gas flow rates were kept b e t w e e n 40 and 7 0 m l m i n '. T r a p p i n g efficiencies exceeded 99% at these flow rates. The solutions used to digest the m i n e r a l s are given in Table II. All were added in liquid form except for r e a g e n t s with SnC12, in w h i c h cases 5.0 g of SnC12"2H20 was added in solid form directly to the r e a c t i o n flask before acidification. The Cr(II) digestion used 10 ml Of e t h a n o l , 40 ml of a 1.0 M CrCl~0.5 N HC1 s o l u t i o n w h i c h was passed t h r o u g h a J o n e s r e d u c t o r and 20ml of conc. HC1 (Canfield et al., 1986). The TIC13 H2SO4 r e a g e n t consisted of 32 ml of 20% TIC13 s o l u t i o n (Baker V884-7) plus 28 ml conc. H2SO 4 made to 500 ml and passed t h r o u g h a J o n e s reductor. S y n t h e t i c minerals, except pyrite, were added to the flask as a wet precipitate on a glass fiber filter. ' A m o r p h o u s - F e S ' , m a c k i n a w i t e and greigite were e x t r a c t e d at p o o r e r efficiencies after d r y i n g u n d e r v a c u u m . The sulfide r e c o v e r y from e a c h m i n e r a l was c a l c u l a t e d by c o m p a r i s o n with the complete r e c o v e r y u s i n g the Cr(II) digestion procedure. We f o u n d t h a t the sampling of suspended m i n e r a l s from a well-stirred r e a c t i o n vessel i n t r o d u c e s < 3% e r r o r in the e s t i m a t i o n of m i n e r a l recoveries. Sediment samples were weighed in the wet state directly into digestion flasks. P e r c e n t a g e w a t e r was d e t e r m i n e d by w e i g h t loss u p o n d r y i n g o v e r n i g h t at 65°C for c o r r e c t i o n of the d a t a to a dry w e i g h t basis. The e q u i v a l e n t dry w e i g h t of most samples was g e n e r a l l y b e t w e e n one and two grams. Digestion times were the same for b o t h s y n t h e t i c m i n e r a l s and sediment samples. Cold digestion times were 45min, h o t digestion times were 60 min and c h r o m i u m r e d u c t i o n times were 90 min, all timed from the a d d i t i o n of reagents. Boiling of digestion m i x t u r e s c o m m e n c e d ~ 12 min after r e a g e n t addition. Time-series e x p e r i m e n t s were c a r r i e d out on sediments and s y n t h e t i c minerals. A 50 ml digestion flask with 30 ml of r e a g e n t was stripped at the rate
198
of 46 + 2 m l m i n 1 and the H2S was trapped in 30ml of 50% (v/v) SAOBII. The SAOBII was maintained at 25°C with a thermostatted water jacket and the cumulative sulfide content of the trap was monitored at short intervals using a AgS electrode. Millivolt readings were converted to concentrations after standardization with Na2S. Total sulfide at the end of the digestion was measured with a potentiometric titration.
Sulfide analysis The content of sulfur in SAOBII was determined in a potentiometric titration with AgS (Orion Research, 1977) and double junction reference electrodes used for endpoint detection. An automatic titration apparatus (Metrohm E526; 655) was used to add Pb(C1Q) 2 (~0.035M) to the SAOB mixture which was diluted to twice its volume with deionized water prior to titration. The location of the endpoint (approximately -720mV) was determined empirically using Na2S standards. The final approach to the endpoint (from 790 to - 7 2 0 mV) was done manually to avoid overshooting the endpoint. Standardization of the Pb solution was via atomic absorption spectroscopy using certified Pb standards. RESULTS
A major concern throughout this study was the mineratogic purity and grain size of the synthetic minerals. X-ray diffraction of the wet minerals demonstrated that they had no detectable quantities of other sulfide minerals. Sharp diffraction peaks were found for mackinawite, greigite and pyrite, with ~amorphous-FeS' showing only a broad diffraction hump characteristic of this mineral (Berner, 1967). Upon aging, this amorphous phase converts to mackinawite and exhibits the mackinawite solubility product (Cornwell, unpublished data), in accordance with Rickard's (1969) observations. Its existence as a phase separate from mackinawite has not been established. The grain sizes of ~amorphous-FeS' and greigite are quite small with incomplete retention on 0.45gm filters. The mackinawite is completely retained by 1.0#m filters and unlike ~amorphous-FeS' and greigite, settled to the bottom of the reaction vessel within several min of cessation of stirring. Scanning electron microscopy of the ~amorphous-FeS', mackinawite and greigite all showed fine grain size, with greigite and ~amorphous-FeS' forming aggregates. Higher ionic strengths in solution enhanced the aggregation of these minerals. The synthetic pyrite was 1-2 pm in diameter, did not aggregate to the same degree as the other minerals and was not framboidal. The precision of the AVS determinations was usually better than + 5% for both synthetic minerals and sediments. Exceptions generally occurred with acetic acid and phosphoric acid digestants, where standard deviations often exceeded 20%. The Cr digestion precision was better than _+2%. Analytical variance of AVS concentrations in sediments was excessive when less than 10 gmol S was recovered.
199
The results of the AVS digestions are presented in Table II. 'Amorphous-FeS' was completely recovered under most dissolution conditions, although acetic acid dissolution was incomplete. Mackinawite results are very similar with poor recoveries for acetic acid. Greigite behaves quite differently from 'amorphous-FeS' and mackinawite. Acetic acid and phosphoric acid recoveries of greigite are low and complete recovery of greigite was found only with the hot 6NHC1 + SnCl 2 and H2SO4 + TIC13 procedures. On the basis of these greigite results, it would appear t hat a strong r e d u c t a n t is necessary for complete recovery of greigite. The fraction of S recovered with weaker conditions is close enough to 0.75 to suggest t ha t one mole of greigite produces three moles of H2S and one mole of elemental S. Visually complete dissolution of greigite with cold 1 N HC1, cold 6 N HC1, cold 6 N HC1 + SnC12 and hot 6 N HC1 suggests t h a t the mineral dissolves completely but produces elemental S. Indeed, filtration, of the digestion solution and its redigestion with the Cr reduction reagent recovers all of the ~lost' sulfur. P y r r h o t i t e digestion was not examined in detail, but complete dissolution was visually apparent using 6 N HC1. The results of synthetic pyrite digestion show that the harsher AVS techniques can recover pyrite. Two separate batches of synthetic pyrite were used and poorer recoveries were found with pyrite refluxed 7 days than with a batch refluxed 3 days during synthesis. The hot 6 N HC1 + SnC12 digestion is capable of complete dissolution of synthetic pyrite. The TIC13 technique has variable recoveries but a high proportion of synthetic pyrite was dissolved. Even the cold 6N HC1 + SnCl~ and hot 6 N HC1 digestions result in significant recoveries of synthetic pyrite. The coarser, commercial pyrite dissolved only in the Cr digestion, with a small amount dissolving under hot 6 N HC1 + SnC12 conditions. The 87% recovery of this pyrite using Cr reduction is good considering its stated minimum purity of 85%. Because of the overlap found in the dissolution of the various iron sulfide minerals, time-course experiments were carried out to see if variations in dissolution rate could be used to identify iron sulfide minerals. Stripping times for > 90% recovery of Na2S standards were ~ 7 _+ 3min for cold digestion conditions and < 1.5 min in boiling solutions. For all the digestion techniques except acetic acid, most mineral recovery occurs in < 15min (Fig. 1). The separation of minerals on the basis of different dissolution rates is not possible. In general, the cumulative recovery curve levels off before 40 min. Important exceptions to this are acetic acid with mackinawite and cold 6 N HC1 + SnCl 2 with both greigite and synthetic pyrite. The poor recovery by acetic acid appears to be a function of slower dissolution r a t h e r t han a lack of dissolution at a given point in time. However, the poor recovery of greigite does not improve with increased dissolution time. The slope of the cold 6NHC1 + SnC12 recovery line indicates that for greigite and synthetic pyrite, sulfur recovery increases with time. Thus, the cold 6NHC1 + SnC12 may operate in fundamentally the same way as this reagent under hot conditions but the rate is slower. These recovery vs. time
2oo I00 --
_
50
~
'
~
, - / /
o,, HCr
/ /"
6 N HCr + SnCI2
MacMnawlte OJ ~>' >@ O o IT
50 t
o~
rl
0 1013
/ ~
]
~
.~
1 N H2SO4 TICh
6NHCI hot 6 N HCt
E
°
__
GrelgJte
6N HCr+SnCI~ hot 6 N HC[ + SnCl~
'
~
-
"
~
"
CH~COOH
I
hot 6 N HCI + SnCI~
Synthettc Pyrite
50
! 0
40
20
60
Time (min)
Fig. 1. Time-course digestions of mackinawite, greigite and synthetic pyrite. A hot 6NHCI digestion of synthetic pyrite (not shown) overlies the time-course of 6 N HC1 + SnC12. curves indicate that the differences in greigite and synthetic pyrite recoveries are related to the presence of a reducing agent. A small, non-increasing recovery of synthetic pyrite occurs with hot 6 N HC1 digestion. Two sediment samples were digested using the various AVS reagents (Fig. 2). The Saanich Inlet sample has a very low percentage of AVS while the Cape Lookout Bight sample has approximately one-third of total S in the AVS pools. In general, the AVS extractants gave similar results, except for the hot 6 N HC1 + SnC12 which gave much higher concentrations, most likely via the dissolution of pyrite. Pyrite concentrations may be estimated from the difference between TRS and AVS. Elemental S, estimated using the procedure of Troelson and Jorgensen (1982), makes up less than 0.5% of TRS in these sediments (Morse and Cornwell, 1987). The pyrite S estimate, taken as the difference between TRS and cold 6 N HC1 + SnC12 values, is plotted in Fig. 2. A pyrite S estimate was made using the pyrite Fe technique of Lord (1982). This technique involves the dissolution of Fe oxides and silicate Fe leaving only the pyrite Fe which is dissolved in concentrated HNO3. Agreement between the two techniques is fair, with the Fe-based measurement recovering 81 and 91% of the sulfur-based
201
100[
~! Saanich I n l e t TRS= 320 pmolg ~
rr
i
0
_
E
100 13._
Cape Lookout Bight TRS~246,umol.g 1
0
~
(D
o
:E
Z ~
Z ~D
o3 +
~
Z c0
+
0.
"r Z
Q-
~ Z
I
5
~
Fig. 2. AVS and pyrite digestions on two sediment samples. The Saanieh Inlet sample is a surficial sediment taken from approximately 225 m depth in an anoxie fjord (Murray et al., 1978; Devol et al., 1984). The Cape Lookout Bight sample is from 8.5 m water depth in a rapidly sedimenting coastal lagoon and encompasses 1 10 cm sediment depth (Martens and Klump, 1984). The S-pyrite S is the difference between total reduced S and the cold 6 N HC1 + SnC12 AVS determination. The Fe-pyrite S is the pyrite S content inferred from the pyrite Fe technique of Lord (1982).
values for Saanich Inlet and Cape Lookout Bight, respectively. Recoveries of synthetic and commercial pyrite using the Lord technique were 65 and 93% of the sulfur-based estimates and account for the differences between S- and Fe-based pyrite estimates. Less than 2% of greigite Fe is recovered as pyrite Fe. Canfield et al. (1986) also noted agreement between these two pyrite techniques. Time-series digestions of Saanich Inlet and Cape Lookout Bight sediments (Fig. 3) show that digestion and stripping for most acid conditions is nearly complete after 20 min. In Saanich Inlet, the rate of recovery of AVS with 6 N HC1 + SnC12 leveled off at 30 min, then it increased again. A more complete series of digestions on the Cape Lookout Bight sample showed that all digestion techniques reach a maximum recovery in a time similar to the synthetic minerals. As in the case of synthetic minerals, the use of time-series provided little insight into the identification of iron sulfide minerals.
202 15
S~{arllcP met TRS 320 prl~()l c]
cr
Z
E Q
o
7 5"
(1) G_
~
~
6 N HCI - SrlCI2
/
6 N HC!
0
• 30
0
Time 50"
6~0
(min)
Cape Lookout Bight TRS 2781mlor g ~ / ~ -
hot6N HCr + SnCI
i I
CO P¢
6NHCI
~--
/
/
~
+ SnCI
hot6NHC,
~_
CH:ICOOH
0~ j 0
30
60
Time (min)
Fig. 3. Time-course digestion of Saanich Inlet and Cape Lookout Bight sediments. The recovery is expressed as a percentage of total reduced sulfur. The Cape Lookout Bight sample is from 20 to 30 cm sediment depth, deeper than the sample presented in Fig. 2.
DISCUSSION
The complete separation of different synthetic iron sulfide minerals by varying the extraction conditions does not appear feasible. Significant overlap occurs between the 'true' monosulfides ('amorphous-FeS' and mackinawite) and greigite. Most greigite is recovered with HC1 treatments, but complete recovery requires a hot HC1 + SnC12 or H2SO4 + TIC13 extractant, both of which extract pyrite. A difficulty in interpreting the meaning of our results with synthetic minerals for sediment extractions is the potential difference in reactivity of synthetic and naturally occurring sulfide minerals. Our mineral syntheses result in very small particle sizes, and their fine grained nature may enhance their dissolution. A decreased reactivity of synthetic pyrite occurs upon aging and it may not be as resistant to acid attack as pyrite formed more slowly at
203
lower temperatures in sediments. Similarly, aging of greigite for one week decreases its recovery in weaker acids, though partial oxidation to pyrite could account for the difference. In an examination of sulfide mineral occurrence using SEM we have identified greigite in sediment from only one of fourteen anoxic sediments (Morse and Cornwell, 1987), and in t hat ease it was poorly crystalline. It would appear t hat greigite and mackinawite occur as very finely dispersed minerals with similar sizes and reactivities as our synthetic minerals. The fundamental process of dissolution of 'amorphous-FeS', mackinawite and greigite in sediments appears to be similar to that in synthetic minerals. Stannous chloride has two effects in the determination of AVS. The original purpose of SnC12 addition was to reduce Fe(III) to Fe(II), thus eliminating poor recoveries of H2S via oxidation (Purden and Bloomfield, 1968). The addition of SnC12 can also be used to enhance dissolution of iron sulfide minerals which have a mean S valence greater than 2.0 (assuming all Fe is Fe(II); Morice et al., 1969). In separating 'amorphous-FeS', mackinawite and greigite from pyrite, the presence or absence of SnClz makes only a small difference (Fig. 2). Using cold 6 N HC1 + SnClz digestions on pure minerals, it would appear that we recover all amorphous FeS and mackinawite, ~ 75% of greigite, and a small amount of synthetic pyrite. In a survey of sediment samples from a wide variety of locations, 77 _+ 11% (n = 14) of cold 6 N HC1 + SnC12 AVS was recovered with 6 N HC1 (Morse and Cornwell, 1987). Enhanced recovery using SnCL may be a result of elimination of oxidation artifacts or increased dissolution of greigite and/or pyrite. Pruden and Bloomfield (1968) reported > 5% losses of H2S at concentrations of Fe203 above 60mg in 25ml of 50% HC1. Addition of very fine synthetic goethite (45.5m2g 1 surface area) to 6NHC1 digestions showed no effect at a goethite addition of ~ 2 mg (Cornwell, unpublished data). A 20% loss of H2S occurred with 16mg of goethite added. Stannous chloride eliminated Fe(III) interferences at additions of goethite exceeding 160mg. Citrate-dithionate extraction of 14 sulfidic sediments to determine Fe20:~ yielded Fe concentrations equivalent to between 2 and 19mgg 1 goethite, though we believe much of this may be non-oxide Fe (Morse and Cornwell, 1987). The extent of Fe(III) interference will be influenced by the concent rat i on and reactivity of the Fe oxide and the time interval during which the Fe(III) and H2S are in contact. Indeed, when small amounts of sediment are digested and the H2S stripping rate is rapid, there appears to be little influence of Fe(III) on H2S recovery (G. Cutter, Old Dominion University, personal communication). Making a choice of a 'best' AVS technique involves several considerations. Acetic acid and phosphoric acid are unsuitable because of poor recoveries and hot 6NHC1 + SnC12 and 1NH2SO4 + TIC13 recover too much pyrite. Hot 6 N HC1 recoveries are generally not much different from cold acid digestions. The cold 6NHC1 and cold 6NHC1 + SnC12 ext ract ant s are probably as suitable as any others for these digestions. The 6 N HC1 is susceptible to low recoveries because of Fe(III), but it is not certain t hat this effect occurs in all anoxic sediments. Alternatively, the 6NHC1 + SnC12 may overestimate the
204 AVS concentration via partial dissolution of pyrite. In most marine sediments, pyrite-S is the predominant form of reduced S, so the estimation of pyrite concentration is somewhat insensitive to the small differences in recovery between cold 6 N HC1 and cold 6 N HC1 + SnC12 digestants. Slight dissolution of pyrite in pyrite-rich sediments could significantly increase the AVS estimate, and cold 6 N HC1 could be a good extractant. With both extractants, ~ 25% of greigite-S would recovered as pyrite-S. The differences between extractants may be even larger in the recovery of H235S after short-term 3~SO4 incubations. Recent studies have attempted to separate the reduced 35S into AVS, pyrite and elemental sulfur fractions (Howarth and Jorgensen, 1984; King et al., 1985). Carbon disulfide was used to extract elemental S, non-reducing acid solutions were used for AVS determination, and Cr(II) was used for TRS. The formation of pyrite has been alternatively suggested as a multi-step, slow process (Berner, 1970) or by rapid formation from solution (Howarth, 1979). The overlaps found in our determination of mineral pool sizes suggest that oxidation of the H2S during AVS extraction could result in an overestimate of pyrite formation rates (via recovery of AVS-produced elemental sulfur) and that harsher AVS techniques could result in an underestimation of pyritic 3~S. Howarth and Jorgensen (1984) corrected for the oxidation of H2S by Fe(III) by extracting the elemental 3~S produced via oxidation into CS2. Westrich (1983) suggested that enhanced recovery of 35S with hot 6 N HC1 + SnCl2 over that recovered with cold 6 NHC1 + SnC12 in coastal sediment incubations, indicates greigite formation. Dissolution of the outer layers of pyrite is a more likely cause. Inorganic isotope exchange between reduced S pools may render the distinction of 35S in elemental S, AVS and pyrite impossible (Jorgensen et al., 1984). CONCLUSION The reactivity of synthetic iron sulfide minerals in different acid solutions is variable, but a complete separation of these minerals is not feasible using standard extractants. Extractions using hot 6NHC1 + SnC12 and H2SOt + TIC13 completely recover all forms of AVS, but also digest a large proportion of synthetic pyrite. Weaker AVS digestions have an incomplete recovery of greigite. We conclude that it is not possible to completely separate greigite from pyrite, but cold 6NHC1 and cold 6NHC1 + SnC12 appear acceptable for AVS determinations. Despite these difficulties, the AVS and pyrite operational categories provide useful information on the reactivity of iron sulfide minerals in sediments. ACKNOWLEDGMENTS This work was supported by the National Science Foundation Marine Chemistry Program (NSF OCE-8309540). We especially thank Saulwood Lin for valuable technical assistance and Drs. S. Emerson, J. Chanton and C.S.
205
Martens for supplying sediment samples. Dr. D. Albert provided the information necessary for making Ti reagents. REFERENCES Albert, D., 1984. Improved techniques for measurement of sulfate reduction and pyrite formation rates in sediments. Trans. Am. Geophys. Union, 45: 906. Aller, R.C., 1980. Diagenetic processes near the sedimen~water interface of Long Island Sound. II: Fe and Mn. Adv. Geophys., 22: 351~415. Berner, R.A., 1964a. Iron sulfides formed from aqueous solution at low temperatures and atmospheric pressure. J. Geol., 72: 293-306. Berner, R.A., 1964b. Distribution and diagenesis of sulfur in some sediments from the Gulf of California. Mar. Geol., 1: 117-140. Berner, R.A., 1967. Thermodynamic stability of sedimentary iron sulfides. Am. J. Sci., 265: 773-785. Berner, R.A., 1970. Sedimentary pyrite formation. Am. J. Sci., 268: 1-23. Berner, R.A., 1974. Iron sulfides in Pleistocene deep Black Sea sediments and their paleo-oceanographic significance. In: E.T. Degens and D.A. Ross (Editors), The Black Sea: Geology, Chemistry and Biology. American Association of Petroleum Geologists Memoir, 20: 52~534. Berner, R.A., Baldwin, T. and Holdren, G.R., Jr., 1979. Authigenic iron sulfides as paleosalinity indicators. J. Sediment. Petrol., 49: 1345-1350. Canfield, D.E., Raiswell, R., Westrich, J.T., Reaves, C.M. and Berner, R.A., 1986. The use of chromium reduction in the analysis of reduced inorganic sulfur in sediments and shales. Chem. Geol., 54: 149-155. Chanton, J.P. and Martens, C.S., 1985. The effects of heat and stannous chloride addition on the active distillation of acid volatile sulfide from pyrite-rich marine sediment samples. Biogeochemistry, 1: 375-383. Davison, W. and Lishman, J.P., 1983. Rapid colorimetric procedure for the determination of acid volatile sulphide in sediments. Analyst (London), 108: 1235-1239. Davison, W., Lishman, J.P. and Hilton, J., 1985. Formation of pyrite in freshwater sediments: Implications for C/S ratios. Geochim. Cosmochim. Acta, 49: 1615-1620. Devol, A.H., Anderson, J.J., Kuivila, K. and Murray, J.W., 1984. A model for coupled sulfate reduction and methane oxidation in the sediments of Saanich Inlet. Geochim. Cosmochim. Acta, 48: 993-1004. Goldhaber, M.B. and Kaplan, I.R., 1974. The sulfur cycle. In: E.D. Goldberg (Editor), The Sea, Vol. 5. Wiley, New York, pp. 56~655. Goldhaber, M.B. and Kaplan, I.R. 1980. Mechanisms of sulfur incorporation and isotope fractiontion during early diagneses in sediments of the Gulf of California. Mar. Chem., 9: 95-143. Goldhaber, M.B., Aller, R.C., Cochran, J.K., Rosenfeld, J.K., Martens, C.S. and Berner, R.A., 1977. Sulfate reduction, diffusion and bioturbation in Long Island Sound sediments: Report of the FOAM Group. Am. J. Sci., 277: 193-237. Howarth, R.W., 1979. Pyrite: its rapid formation in a salt marsh and its importance to ecosystem metabolism. Science, 203: 49-51. Howarth, R.W. and Jorgensen, B.B., 1984. Formation of 35S-labelled elemental sulfur and pyrite in coastal marine sediments (Limfjorden and Kysing Fjord, Denmark) during short-term 35SO~reduction measurements. Geochim. Cosmochim. Acta, 48:1807 1818. Jorgensen, B.B., 1977. The sulfur cycle of a coastal marine sediment (Limfjorden, Denmark). Limnol. Oceanogr., 22: 814-832. Jorgensen, B.B., Fossing, H. and Thode-Anderson, S., 1984. Radiotracer studies of pyrite and elemental sulfur in coastal sediments. Trans. Am. Geophys. Union, 45: 906. King, G.M., Howes, B.L. and Dacey, J.W.H., 1985. Short-term endproducts of sulfate reduction in a salt marsh: Formation of acid volatile sulfides, elemental sulfur, and pyrite. Geochim. Cosmochim. Acta, 49: 1561-1566. Kolthoff, I.M. and Sandell, E.B., 1952. Textbook of Quantitative Inorganic Analysis: MacMillan, New York, 759 pp.
206 Lord, C.J., III, 1982. A selective and precise method for pyrite determination in sedimentary materials. J. Sediment. Petrol., 52: 664666. Martens, C.S. and Klump, J.V., 1984. Biogeochemical cycling in an organic-rich coastal basin 4. An organic carbon budget for sediments dominated by sulfate reduction and methanogenesis. Geochim. Cosmochim. Acta, 48: 1987-2004. Morice, J.A., Rees, L.V.C. and Rickard, D.T., 1969. Mossbauer analysis of the iron sulfides. J. Inorg. Nucl. Chem., 31:3797 3802. Morse, J.W. and Cornwell, J.C., 1987. Acid volatile sulfides and pyrite in anoxic marine sediments. Mar. Chem., 22: 55-71. Morse, J.W., Millero, F.J., Cornwell, J.C. and Rickard, D., 1986. The chemistry of the hydrogen sulfide and iron sulfide systems in n a t u r a l water. E a r t h Sci. Rev., 24: 142. Murray, J.W., Grundmanis, V. and Smethie, W.M., Jr., 1978. Interstitial water chemistry in the sediments of Saanich Inlet. Geochim. Cosmochim. Acta, 42: 1011-1026. Orion Research, 1977. I n s t r u m e n t M a n u a l for Sulfide Ion Electrode and Silver Ion Electrode, Model 94-16. Orion Research Inc., Cambridge, MA. Pruden, G. and Bloomfield, C., 1968. The determination of ferrous sulfide in soil in the presence of ferric oxide: Analyst (London), 93:532 534. Rickard, D.T., 1969. The chemistry of iron sulfide formation at low temperatures. Stockholm Contrib. Geol., 20: 67-95. Sheu, D.D. and Presley, B.J., 1986. Variations of calcium carbonate, organic carbon and iron sulfides in anoxic sediment from the Orca Basin, Gulf of Mexico. Mar. Geol., 70:103 118. Shoesmith, D.W., Taylor, P., Bailey, M.G. and Owen, D.F., 1980. The formation of ferrous sulfide polymorphs during the corrosion of iron by aqueous hydrogen sulfide at 21°C. Electrochem. Sci. Technol., 12:100.7 1015. Shriver, D.F., 1969. The Manipulation of Air Sensitive Compounds. McGraw Hill, New York, pp. 175 201. Sweeney, R.E. and Kaplan, I.R., 1973. Pyrite framboid formation: laboratory synthesis and marine sediments. Econ. Geol., 68: 618-634. Troelson, H. and Jorgensen, B.B., 1982. Seasonal dynamics of elemental sulfur in two coastal environments. Estuarine Coastal Shelf Sci., 15: 255-266. Van Valin, R. and Morse, J.W., 1982. An investigation of methods commonly used for the selective removal and characterization of trace metals in sediments. Mar. Chem., 11: 535-564. Wada, H., 1977. The synthesis of greigite from polysulfide solution at about 100°C. Bull. Chem. Soc. Jpn., 50: 2615-2617. Westrich, J.T., 1983. The consequences and controls of bacterial sulfate reduction in marine sediments. Ph.D. Dissertation, Yale University, New Haven, CT, 530 pp. Wieder, R.K., Lang, G.E. and Granus, V.A., 1985. An evaluation of wet chemical methods for quantifying sulfur fractions in freshwater wetland peat. Limnol. Oceanogr., 30: 1109-1115. Zhabina, N.N. and Volkov, I.I., 1978. A method of determination of various sulfur compounds in sea sediments and rocks. In: W.E. Krumbein (Editor), Environmental Biogeochemistry and Geomicrobiology, Vol. 3: Methods, Metals and Assessment. Ann Arbor Sci. Publ. MI, pp. 735 746.
193 Printed in the Netherlands
THE CHARACTERIZATION OF IRON S U L F I D E MINERALS IN ANOXIC MARINE S E D I M E N T S *
JEFFREY C. CORNWELL** and JOHN W. MORSE Department of Oceanography, Texas A & M University, College Station, TX 77843-3146(U.S.A.)
(Received July 7, 1986; revision accepted June 15, 1987)
ABSTRACT Cornwell, J.C. and Morse, J.W., 1987. The characterization of iron sulfide minerals in anoxic marine sediments. Mar. Chem., 22: 19~206. The performance of extractants commonly used in the determination of acid volatile sulfide minerals (e.g., 'amorphous-FeS', mackinawite and greigite) and pyrite has been evaluated using pure mineral phases and anoxic sediments. 'Amorphous-FeS' and mackinawite are quantitatively recovered by most cold acid extractions, but greigite recovery is incomplete. Harsher extractants with reducing agents are necessary for the complete recovery of greigite, but dissolution of fine-grained synthetic pyrite occurs under such conditions. A quantitative separation between greigite and pyrite is not possible using these techniques, but the use of 'acid volatile sulfide' and 'pyrite' as operational categories is adequate for most field studies. INTRODUCTION
Sedimentological studies generally separate iron sulfides into two ~operational' categories: acid volatile sulfide (AVS) and pyrite (Berner, 1964a; Berner et al., 1979; Howarth and Jorgensen, 1984; Davison et al., 1985; Morse et al., 1986). Acid volatile sulfides are evolved via acid distillation and generally are considered to include 'amorphous-FeS', mackinawite, greigite and pyrrhotite. Pyrite often is the most abundant iron sulfide. It is presumed to resist dissolution by acid attack and is generally determined by oxidative (e.g., Goldhaber et al., 1977) or reductive dissolution (Zhabina and Volkov, 1978; Canfield et al., 1986). T h e s e t w o c a t e g o r i e s o f i r o n s u l f i d e m i n e r a l s h a v e b e e n u s e d t o e x a m i n e s e d i m e n t i r o n s u l f i d e p o o l s i z e s a n d 3~S-derived S f l u x e s ( G o l d h a b e r e t al., 1977; H o w a r t h , 1979; G o l d h a b e r a n d K a p l a n , 1980; H o w a r t h a n d J o r g e n s e n , 1984). However, recent reports suggest that in some instances the AVS determination may under- or overestimate the 'true' concentration of iron monosulfides ( W e s t r i c h , 1983; C h a n t o n a n d M a r t e n s , 1985). D e s p i t e t h e c o n t i n u e d u s e o f different AVS techniques, no systematic examination of the recovery of the various pure iron sulfide minerals using AVS techniques has been carried out. * Presented at the IX International Symposium 'Chemistry of the Mediterranean', May 1986, Primo~ten, Yugoslavia. ** Current address: University of Maryland, CEES Hornpoint Environmental Laboratory, P.O. Box 775, Cambridge, MD 21613, U.S.A. 0304-4203/87/$03.50
~ 1987 Elsevier Science Publishers B.V.
194
In this paper we present the results of AVS determination of synthetic ~amorphous-FeS', mackinawite, greigite and fine-grained pyrite, and coarse grained natural pyrite under a variety of acid solution compositions and temperatures. Pyrrhotite was not examined in detail because it is not commonly identified in most anoxic marine sediments. Our purpose is to examine how well different methods separate the ~amorphous-FeS', mackinawite and greigite from pyrite, and to determine the potential for development of methods which might lead to the separate determination of all mineral forms. TECHNIQUES FOR IRON SULFIDE MINERAL IDENTIFICATION
Despite the importance of iron sulfide mineral formation to global iron and sulfur fluxes, iron sulfide minerals are usually found in marine sediments in very low concentrations. In general, sediments contain only a few percent or less of FeS2 and FeS. Pyrite often occurs as distinct crystals or in framboids (Sweeney and Kaplan, 1973) while the black coating of sulfide in many reducing sediments is presumed to be iron monosulfides. In general, pyrite is the most abundant iron sulfide mineral in marine sediments (Berner et al., 1979), and non-pyritic forms of iron sulfide may be finely dispersed. The difficulty in direct determination of iron sulfide mineralogy and quantity arises from several factors: their mass and volume are small relative to other sediment components and in the case ofmonosulfides they may occur in a finely dispersed manner. Thus, bulk sediment analyses by X-ray diffraction, examination via microscope and surface chemical techniques are generally not applied to the quantification of iron sulfide minerals. Despite these difficulties, iron monosulfides have been identified in some sedimentary environments (Berner, 1974; Goldhaber and Kaplan, 1974). Identification, and perhaps quantification of sedimentary pyrite may be possible using SEM, but other sulfide minerals are not generally accessible by this technique (Morse and Cornwetl, 1987). The reaction conditions used for chemical extraction of iron sulfides are diverse (see Table I for summary). The AVS techniques involve acidification of wet sediment with non-oxidizing acids followed by active or passive transport of H2S out of solution into an appropriate trapping reagent. Generally, glass distillation apparatus are used, but adaptations for syringes (Davison and Lishman, 1983) and examining total S content before and after acidification (Sheu and Presley, 1986) have been used. The addition of SnC12 to reaction flasks has been used to inhibit the interference of sedimentary Fe(III) with H2S evolution (Pruden and Bloomfield, 1968) 2FeOOH + 4H + + H2S -
2Fe 2~ + S + 4H20
Reducing agents such as SnC12 and TIC13 (Pruden and Bloomfield, 1968; Albert, 1984) reduce this interference. The trapped H2S may be analyzed via iodometric titration (Kolthoff and Sandell, 1952), gravimetry as AgS or BaSO4, or via potentiometric titration (Orion Research, 1977). The effectiveness of the different reaction conditions on recovery of iron sulfides has not been clearly defined. The most fundamental separation
195 TABLE I Extraction methods for iron sulfide digestion Method
Reference
Acid volatile sulfide (H2S emanation)
Cold HC1 (1-12 N) Hot HC1 (6-12 N)
Goldhaber et al. (1977), Jorgensen (1977), Aller (1980) Kolthoff and Sandell (1952), Berner (1974), Zhabina and Volkoff (1978), Wieder et al. (1985) Cold HC1 (6 N) + SnC12 Westrich (1983) Hot HC1 (6 N) + SnC12 Berner et al. (1979), Westrich (1983) Cold H2SO4 (1N) + TiCl~ Albert (1984) Total reduced S (H2S emanation)
Cr(II) reduction
Zhabina and Volkov (1978), Canfield et al. (1986)
Total S
Aqua-regia digestion Combustion Pyrite
Goldhaber et al. (1977) Berner (1974)
via Fe content
HNO:~digestion ~
Lord (1982)
Oxides and silicates removed in pretreatment. r e q u i r e d is b e t w e e n p y r i t e a n d all of the less s t a b l e iron sulfide m i n e r a l s . O p t i m a l l y , t h e m o s t h a r s h AVS t e c h n i q u e w h i c h r e c o v e r s all t h e m o n o s u l f i d e m i n e r a l s a n d g r e i g i t e a n d n o n of the p y r i t e s h o u l d be used. W e s t r i c h (1983) a n d B e r n e r et al. (1979) used a h o t HC1-SnC12 p r o c e d u r e to this end b u t C h a n t o n a n d M a r t e n s (1985) h a v e d e m o n s t r a t e d t h a t p y r i t e is n o t i m p e r v i o u s to a t t a c k by this r e a g e n t . O t h e r r e a g e n t s for sulfide d e t e r m i n a t i o n h a v e n o t b e e n e x a m i n e d for selectivity. T h e c h r o m i u m r e d u c t i o n p r o c e d u r e ( Z h a b i n a a n d Volkov, 1978; Canfield et al., 1986) a p p e a r s to r e c o v e r t o t a l r e d u c e d sulfur (TRS) w i t h o u t s i g n i f i c a n t l y a f f e c t i n g o r g a n i c sulfur c o m p o u n d s . T h e p y r i t e c o n t e n t of s e d i m e n t is c o n s i d e r e d to be t h e t o t a l r e d u c e d sulfur c o n t e n t m i n u s the c o n t r i b u t i o n of AVS a n d e l e m e n t a l sulfur. In m o s t sediments, e l e m e n t a l sulfur is a v e r y small p r o p o r t i o n of t o t a l sulfur, a n d t h e p y r i t e c o n t e n t m a y be under- or o v e r e s t i m a t e d b e c a u s e of p y r i t e d e c o m p o s i t i o n or i n c o m p l e t e r e c o v e r y of AVS. T h e e s t i m a t i o n of iron sulfide d i s t r i b u t i o n u s i n g e x t r a c t i o n t e c h n i q u e s h a s s t r e n g t h s a n d w e a k n e s s e s s i m i l a r to t h o s e of t r a c e m e t a l d e t e r m i n a t i o n with ~selective' e x t r a c t a n t s (e.g., v a n V a l i n a n d Morse, 1982). B o t h k i n d s of a n a l y s e s suffer from a l a c k of a b s o l u t e specificity in m i n e r a l (or phase) s e p a r a t i o n . T h e r e c a n be significant o v e r l a p s b e t w e e n o p e r a t i o n a l c a t e g o r i e s , a n d t h e s e o v e r l a p s m a y n o t be c o n s t a n t b e t w e e n different s e d i m e n t a r y e n v i r o n m e n t s . The s t r e n g t h of e x t r a c t i o n t e c h n i q u e s lies in t h e i r ease of use on l a r g e n u m b e r s of i n d i v i d u a l s a m p l e s a n d t h e g e n u i n e l a c k of m o r e definitive s e p a r a t i o n s u s i n g o t h e r t e c h n i q u e s . Thus, e x t r a c t i o n s r e m a i n t h e m o s t a t t r a c t i v e t e c h n i q u e for s e p a r a t i n g different c a t e g o r i e s of i r o n sulfide m i n e r a l s . D e t a i l e d u n d e r s t a n d i n g of w h i c h m i n e r a l s t h e s e o p e r a t i o n a l c a t e g o r i e s include is sorely needed.
196 METHODS
Mineral syntheses All techniques for mineral synthesis in this study involve the use of aqueous solutions. Various reactants were introduced into a multiple-neck 500ml reaction flask containing deoxygenated water. A separate Cr(II) solution (Shriver, 1969) was used to scrub pre-purified N2 gas of residual 02, and the reaction solution was bubbled overnight with N2 to remove all dissolved oxygen. A constant overpressure of N 2 or H2S was used to maintain anoxic conditions. Continual stirring of the solution via a paddle type stirrer ensured complete mixing. The synthesis of 'amorphous-FeS', greigite and pyrite all started with 6.5 g Fe(NH4)2(SO4)2"6H20 and 7.0 g Na2S" 9H20 in 400 ml of distilled water under N2. ~Amorphous-FeS' is the initial room temperature product of the synthesis. Greigite is formed by the overnight boiling of the above mixture with 5 ml of a polysulfide solution, following the procedure of Wada (1977). Pyrite was formed by the addition of 10 ml of polysulfide solution and 3.0 g of elemental S to the amorphous FeS mixture and boiling for several days. Mackinawite was synthesized by the reaction of Fe metal with H2S (Berner, 1964b; Shoesmith et al., 1980). Hydrogen sulfide gas was bubbled overnight in a reaction vessel containing 20 g of Fe plate. Constant stirring resulted in the suspension of the mackinawite into the solution above the Fe metal. The formation of pyrrhotite was noted in instances when the synthesis continued for prolonged periods (e.g., Shoesmith et al., 1980). The characterization of the synthetic minerals was performed by X-ray diffraction (XRD) analysis of a portion of the precipitate collected on glass fiber filters. A Phillips-Norelco XRD was used with Cu K~ radiation and the diffraction patterns were compared to reference patterns for each mineral. The XRD analyses were carried out as quickly after filtration as possible, with the synthetic greigite showing short-term evidence of air oxidation on limited occasions. The material used for estimation of digestion efficiency was sampled from the flask (generally 3.0ml was taken), washed with water, and in the cases of greigite and pyrite, washed with carbon disulfide or acetone to remove any remaining elemental S or polysulfides. Synthetic pyrite was dried before digestion. The 'commercial' pyrite used (MCB Reagents, IX02060-1) was < 50 mesh and guaranteed to be > 85% pyrite.
Digestion techniques The digestion and acid volatilization of iron sulfides utilized 125ml Erlenmeyer flasks, capped with rubber stoppers, with ports for the input of N2 gas, the exit of N2 and H2S gases and the introduction of digesting solutions. Prepurified N2 was used for purging the apparatus and the H2S evolved was trapped in 20 ml of sulfide anti-oxidant buffer (Orion Research, 1977) in a 25 ml
197
TABLE II Synthetic mineral results (% recovery)a
CH3COOH H3PO4 1.0 N HC1 6.0 N HC1 6.0 N HC1 + SnC12 Hot 6.0 N HC1 Hot 6.0NHC1 + SnC12 1.0NH2SO4 + Ti Cr reduction
Amorphous
Mackinawite
Greigite
Synthetic pyrite
Pyrite
59 90 100 100 100 100 100 100 100
7~3 89 92 98-102 100-102 96-97 100-101 101 100
11 17 40 40~7 60~9 63-75 66~84 93-100 96-100 100
0 0 0 0 4~10 5~ 97-100 48-82 99-100
0 0 0 0 0 0 2 0 87
When ranges are presented, they represent results from different batches of synthetic iron sulfide minerals. test tube. W a t e r - c o o l e d West c o n d e n s e r s were used w h e n s o l u t i o n s were boiled. Gas flow rates were kept b e t w e e n 40 and 7 0 m l m i n '. T r a p p i n g efficiencies exceeded 99% at these flow rates. The solutions used to digest the m i n e r a l s are given in Table II. All were added in liquid form except for r e a g e n t s with SnC12, in w h i c h cases 5.0 g of SnC12"2H20 was added in solid form directly to the r e a c t i o n flask before acidification. The Cr(II) digestion used 10 ml Of e t h a n o l , 40 ml of a 1.0 M CrCl~0.5 N HC1 s o l u t i o n w h i c h was passed t h r o u g h a J o n e s r e d u c t o r and 20ml of conc. HC1 (Canfield et al., 1986). The TIC13 H2SO4 r e a g e n t consisted of 32 ml of 20% TIC13 s o l u t i o n (Baker V884-7) plus 28 ml conc. H2SO 4 made to 500 ml and passed t h r o u g h a J o n e s reductor. S y n t h e t i c minerals, except pyrite, were added to the flask as a wet precipitate on a glass fiber filter. ' A m o r p h o u s - F e S ' , m a c k i n a w i t e and greigite were e x t r a c t e d at p o o r e r efficiencies after d r y i n g u n d e r v a c u u m . The sulfide r e c o v e r y from e a c h m i n e r a l was c a l c u l a t e d by c o m p a r i s o n with the complete r e c o v e r y u s i n g the Cr(II) digestion procedure. We f o u n d t h a t the sampling of suspended m i n e r a l s from a well-stirred r e a c t i o n vessel i n t r o d u c e s < 3% e r r o r in the e s t i m a t i o n of m i n e r a l recoveries. Sediment samples were weighed in the wet state directly into digestion flasks. P e r c e n t a g e w a t e r was d e t e r m i n e d by w e i g h t loss u p o n d r y i n g o v e r n i g h t at 65°C for c o r r e c t i o n of the d a t a to a dry w e i g h t basis. The e q u i v a l e n t dry w e i g h t of most samples was g e n e r a l l y b e t w e e n one and two grams. Digestion times were the same for b o t h s y n t h e t i c m i n e r a l s and sediment samples. Cold digestion times were 45min, h o t digestion times were 60 min and c h r o m i u m r e d u c t i o n times were 90 min, all timed from the a d d i t i o n of reagents. Boiling of digestion m i x t u r e s c o m m e n c e d ~ 12 min after r e a g e n t addition. Time-series e x p e r i m e n t s were c a r r i e d out on sediments and s y n t h e t i c minerals. A 50 ml digestion flask with 30 ml of r e a g e n t was stripped at the rate
198
of 46 + 2 m l m i n 1 and the H2S was trapped in 30ml of 50% (v/v) SAOBII. The SAOBII was maintained at 25°C with a thermostatted water jacket and the cumulative sulfide content of the trap was monitored at short intervals using a AgS electrode. Millivolt readings were converted to concentrations after standardization with Na2S. Total sulfide at the end of the digestion was measured with a potentiometric titration.
Sulfide analysis The content of sulfur in SAOBII was determined in a potentiometric titration with AgS (Orion Research, 1977) and double junction reference electrodes used for endpoint detection. An automatic titration apparatus (Metrohm E526; 655) was used to add Pb(C1Q) 2 (~0.035M) to the SAOB mixture which was diluted to twice its volume with deionized water prior to titration. The location of the endpoint (approximately -720mV) was determined empirically using Na2S standards. The final approach to the endpoint (from 790 to - 7 2 0 mV) was done manually to avoid overshooting the endpoint. Standardization of the Pb solution was via atomic absorption spectroscopy using certified Pb standards. RESULTS
A major concern throughout this study was the mineratogic purity and grain size of the synthetic minerals. X-ray diffraction of the wet minerals demonstrated that they had no detectable quantities of other sulfide minerals. Sharp diffraction peaks were found for mackinawite, greigite and pyrite, with ~amorphous-FeS' showing only a broad diffraction hump characteristic of this mineral (Berner, 1967). Upon aging, this amorphous phase converts to mackinawite and exhibits the mackinawite solubility product (Cornwell, unpublished data), in accordance with Rickard's (1969) observations. Its existence as a phase separate from mackinawite has not been established. The grain sizes of ~amorphous-FeS' and greigite are quite small with incomplete retention on 0.45gm filters. The mackinawite is completely retained by 1.0#m filters and unlike ~amorphous-FeS' and greigite, settled to the bottom of the reaction vessel within several min of cessation of stirring. Scanning electron microscopy of the ~amorphous-FeS', mackinawite and greigite all showed fine grain size, with greigite and ~amorphous-FeS' forming aggregates. Higher ionic strengths in solution enhanced the aggregation of these minerals. The synthetic pyrite was 1-2 pm in diameter, did not aggregate to the same degree as the other minerals and was not framboidal. The precision of the AVS determinations was usually better than + 5% for both synthetic minerals and sediments. Exceptions generally occurred with acetic acid and phosphoric acid digestants, where standard deviations often exceeded 20%. The Cr digestion precision was better than _+2%. Analytical variance of AVS concentrations in sediments was excessive when less than 10 gmol S was recovered.
199
The results of the AVS digestions are presented in Table II. 'Amorphous-FeS' was completely recovered under most dissolution conditions, although acetic acid dissolution was incomplete. Mackinawite results are very similar with poor recoveries for acetic acid. Greigite behaves quite differently from 'amorphous-FeS' and mackinawite. Acetic acid and phosphoric acid recoveries of greigite are low and complete recovery of greigite was found only with the hot 6NHC1 + SnCl 2 and H2SO4 + TIC13 procedures. On the basis of these greigite results, it would appear t hat a strong r e d u c t a n t is necessary for complete recovery of greigite. The fraction of S recovered with weaker conditions is close enough to 0.75 to suggest t ha t one mole of greigite produces three moles of H2S and one mole of elemental S. Visually complete dissolution of greigite with cold 1 N HC1, cold 6 N HC1, cold 6 N HC1 + SnC12 and hot 6 N HC1 suggests t h a t the mineral dissolves completely but produces elemental S. Indeed, filtration, of the digestion solution and its redigestion with the Cr reduction reagent recovers all of the ~lost' sulfur. P y r r h o t i t e digestion was not examined in detail, but complete dissolution was visually apparent using 6 N HC1. The results of synthetic pyrite digestion show that the harsher AVS techniques can recover pyrite. Two separate batches of synthetic pyrite were used and poorer recoveries were found with pyrite refluxed 7 days than with a batch refluxed 3 days during synthesis. The hot 6 N HC1 + SnC12 digestion is capable of complete dissolution of synthetic pyrite. The TIC13 technique has variable recoveries but a high proportion of synthetic pyrite was dissolved. Even the cold 6N HC1 + SnCl~ and hot 6 N HC1 digestions result in significant recoveries of synthetic pyrite. The coarser, commercial pyrite dissolved only in the Cr digestion, with a small amount dissolving under hot 6 N HC1 + SnC12 conditions. The 87% recovery of this pyrite using Cr reduction is good considering its stated minimum purity of 85%. Because of the overlap found in the dissolution of the various iron sulfide minerals, time-course experiments were carried out to see if variations in dissolution rate could be used to identify iron sulfide minerals. Stripping times for > 90% recovery of Na2S standards were ~ 7 _+ 3min for cold digestion conditions and < 1.5 min in boiling solutions. For all the digestion techniques except acetic acid, most mineral recovery occurs in < 15min (Fig. 1). The separation of minerals on the basis of different dissolution rates is not possible. In general, the cumulative recovery curve levels off before 40 min. Important exceptions to this are acetic acid with mackinawite and cold 6 N HC1 + SnCl 2 with both greigite and synthetic pyrite. The poor recovery by acetic acid appears to be a function of slower dissolution r a t h e r t han a lack of dissolution at a given point in time. However, the poor recovery of greigite does not improve with increased dissolution time. The slope of the cold 6NHC1 + SnC12 recovery line indicates that for greigite and synthetic pyrite, sulfur recovery increases with time. Thus, the cold 6NHC1 + SnC12 may operate in fundamentally the same way as this reagent under hot conditions but the rate is slower. These recovery vs. time
2oo I00 --
_
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~
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6 N HCr + SnCI2
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rl
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]
~
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-
"
~
"
CH~COOH
I
hot 6 N HCI + SnCI~
Synthettc Pyrite
50
! 0
40
20
60
Time (min)
Fig. 1. Time-course digestions of mackinawite, greigite and synthetic pyrite. A hot 6NHCI digestion of synthetic pyrite (not shown) overlies the time-course of 6 N HC1 + SnC12. curves indicate that the differences in greigite and synthetic pyrite recoveries are related to the presence of a reducing agent. A small, non-increasing recovery of synthetic pyrite occurs with hot 6 N HC1 digestion. Two sediment samples were digested using the various AVS reagents (Fig. 2). The Saanich Inlet sample has a very low percentage of AVS while the Cape Lookout Bight sample has approximately one-third of total S in the AVS pools. In general, the AVS extractants gave similar results, except for the hot 6 N HC1 + SnC12 which gave much higher concentrations, most likely via the dissolution of pyrite. Pyrite concentrations may be estimated from the difference between TRS and AVS. Elemental S, estimated using the procedure of Troelson and Jorgensen (1982), makes up less than 0.5% of TRS in these sediments (Morse and Cornwell, 1987). The pyrite S estimate, taken as the difference between TRS and cold 6 N HC1 + SnC12 values, is plotted in Fig. 2. A pyrite S estimate was made using the pyrite Fe technique of Lord (1982). This technique involves the dissolution of Fe oxides and silicate Fe leaving only the pyrite Fe which is dissolved in concentrated HNO3. Agreement between the two techniques is fair, with the Fe-based measurement recovering 81 and 91% of the sulfur-based
201
100[
~! Saanich I n l e t TRS= 320 pmolg ~
rr
i
0
_
E
100 13._
Cape Lookout Bight TRS~246,umol.g 1
0
~
(D
o
:E
Z ~
Z ~D
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~
Z c0
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0.
"r Z
Q-
~ Z
I
5
~
Fig. 2. AVS and pyrite digestions on two sediment samples. The Saanieh Inlet sample is a surficial sediment taken from approximately 225 m depth in an anoxie fjord (Murray et al., 1978; Devol et al., 1984). The Cape Lookout Bight sample is from 8.5 m water depth in a rapidly sedimenting coastal lagoon and encompasses 1 10 cm sediment depth (Martens and Klump, 1984). The S-pyrite S is the difference between total reduced S and the cold 6 N HC1 + SnC12 AVS determination. The Fe-pyrite S is the pyrite S content inferred from the pyrite Fe technique of Lord (1982).
values for Saanich Inlet and Cape Lookout Bight, respectively. Recoveries of synthetic and commercial pyrite using the Lord technique were 65 and 93% of the sulfur-based estimates and account for the differences between S- and Fe-based pyrite estimates. Less than 2% of greigite Fe is recovered as pyrite Fe. Canfield et al. (1986) also noted agreement between these two pyrite techniques. Time-series digestions of Saanich Inlet and Cape Lookout Bight sediments (Fig. 3) show that digestion and stripping for most acid conditions is nearly complete after 20 min. In Saanich Inlet, the rate of recovery of AVS with 6 N HC1 + SnC12 leveled off at 30 min, then it increased again. A more complete series of digestions on the Cape Lookout Bight sample showed that all digestion techniques reach a maximum recovery in a time similar to the synthetic minerals. As in the case of synthetic minerals, the use of time-series provided little insight into the identification of iron sulfide minerals.
202 15
S~{arllcP met TRS 320 prl~()l c]
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o
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~
~
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Time 50"
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~--
/
/
~
+ SnCI
hot6NHC,
~_
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30
60
Time (min)
Fig. 3. Time-course digestion of Saanich Inlet and Cape Lookout Bight sediments. The recovery is expressed as a percentage of total reduced sulfur. The Cape Lookout Bight sample is from 20 to 30 cm sediment depth, deeper than the sample presented in Fig. 2.
DISCUSSION
The complete separation of different synthetic iron sulfide minerals by varying the extraction conditions does not appear feasible. Significant overlap occurs between the 'true' monosulfides ('amorphous-FeS' and mackinawite) and greigite. Most greigite is recovered with HC1 treatments, but complete recovery requires a hot HC1 + SnC12 or H2SO4 + TIC13 extractant, both of which extract pyrite. A difficulty in interpreting the meaning of our results with synthetic minerals for sediment extractions is the potential difference in reactivity of synthetic and naturally occurring sulfide minerals. Our mineral syntheses result in very small particle sizes, and their fine grained nature may enhance their dissolution. A decreased reactivity of synthetic pyrite occurs upon aging and it may not be as resistant to acid attack as pyrite formed more slowly at
203
lower temperatures in sediments. Similarly, aging of greigite for one week decreases its recovery in weaker acids, though partial oxidation to pyrite could account for the difference. In an examination of sulfide mineral occurrence using SEM we have identified greigite in sediment from only one of fourteen anoxic sediments (Morse and Cornwell, 1987), and in t hat ease it was poorly crystalline. It would appear t hat greigite and mackinawite occur as very finely dispersed minerals with similar sizes and reactivities as our synthetic minerals. The fundamental process of dissolution of 'amorphous-FeS', mackinawite and greigite in sediments appears to be similar to that in synthetic minerals. Stannous chloride has two effects in the determination of AVS. The original purpose of SnC12 addition was to reduce Fe(III) to Fe(II), thus eliminating poor recoveries of H2S via oxidation (Purden and Bloomfield, 1968). The addition of SnC12 can also be used to enhance dissolution of iron sulfide minerals which have a mean S valence greater than 2.0 (assuming all Fe is Fe(II); Morice et al., 1969). In separating 'amorphous-FeS', mackinawite and greigite from pyrite, the presence or absence of SnClz makes only a small difference (Fig. 2). Using cold 6 N HC1 + SnClz digestions on pure minerals, it would appear that we recover all amorphous FeS and mackinawite, ~ 75% of greigite, and a small amount of synthetic pyrite. In a survey of sediment samples from a wide variety of locations, 77 _+ 11% (n = 14) of cold 6 N HC1 + SnC12 AVS was recovered with 6 N HC1 (Morse and Cornwell, 1987). Enhanced recovery using SnCL may be a result of elimination of oxidation artifacts or increased dissolution of greigite and/or pyrite. Pruden and Bloomfield (1968) reported > 5% losses of H2S at concentrations of Fe203 above 60mg in 25ml of 50% HC1. Addition of very fine synthetic goethite (45.5m2g 1 surface area) to 6NHC1 digestions showed no effect at a goethite addition of ~ 2 mg (Cornwell, unpublished data). A 20% loss of H2S occurred with 16mg of goethite added. Stannous chloride eliminated Fe(III) interferences at additions of goethite exceeding 160mg. Citrate-dithionate extraction of 14 sulfidic sediments to determine Fe20:~ yielded Fe concentrations equivalent to between 2 and 19mgg 1 goethite, though we believe much of this may be non-oxide Fe (Morse and Cornwell, 1987). The extent of Fe(III) interference will be influenced by the concent rat i on and reactivity of the Fe oxide and the time interval during which the Fe(III) and H2S are in contact. Indeed, when small amounts of sediment are digested and the H2S stripping rate is rapid, there appears to be little influence of Fe(III) on H2S recovery (G. Cutter, Old Dominion University, personal communication). Making a choice of a 'best' AVS technique involves several considerations. Acetic acid and phosphoric acid are unsuitable because of poor recoveries and hot 6NHC1 + SnC12 and 1NH2SO4 + TIC13 recover too much pyrite. Hot 6 N HC1 recoveries are generally not much different from cold acid digestions. The cold 6NHC1 and cold 6NHC1 + SnC12 ext ract ant s are probably as suitable as any others for these digestions. The 6 N HC1 is susceptible to low recoveries because of Fe(III), but it is not certain t hat this effect occurs in all anoxic sediments. Alternatively, the 6NHC1 + SnC12 may overestimate the
204 AVS concentration via partial dissolution of pyrite. In most marine sediments, pyrite-S is the predominant form of reduced S, so the estimation of pyrite concentration is somewhat insensitive to the small differences in recovery between cold 6 N HC1 and cold 6 N HC1 + SnC12 digestants. Slight dissolution of pyrite in pyrite-rich sediments could significantly increase the AVS estimate, and cold 6 N HC1 could be a good extractant. With both extractants, ~ 25% of greigite-S would recovered as pyrite-S. The differences between extractants may be even larger in the recovery of H235S after short-term 3~SO4 incubations. Recent studies have attempted to separate the reduced 35S into AVS, pyrite and elemental sulfur fractions (Howarth and Jorgensen, 1984; King et al., 1985). Carbon disulfide was used to extract elemental S, non-reducing acid solutions were used for AVS determination, and Cr(II) was used for TRS. The formation of pyrite has been alternatively suggested as a multi-step, slow process (Berner, 1970) or by rapid formation from solution (Howarth, 1979). The overlaps found in our determination of mineral pool sizes suggest that oxidation of the H2S during AVS extraction could result in an overestimate of pyrite formation rates (via recovery of AVS-produced elemental sulfur) and that harsher AVS techniques could result in an underestimation of pyritic 3~S. Howarth and Jorgensen (1984) corrected for the oxidation of H2S by Fe(III) by extracting the elemental 3~S produced via oxidation into CS2. Westrich (1983) suggested that enhanced recovery of 35S with hot 6 N HC1 + SnCl2 over that recovered with cold 6 NHC1 + SnC12 in coastal sediment incubations, indicates greigite formation. Dissolution of the outer layers of pyrite is a more likely cause. Inorganic isotope exchange between reduced S pools may render the distinction of 35S in elemental S, AVS and pyrite impossible (Jorgensen et al., 1984). CONCLUSION The reactivity of synthetic iron sulfide minerals in different acid solutions is variable, but a complete separation of these minerals is not feasible using standard extractants. Extractions using hot 6NHC1 + SnC12 and H2SOt + TIC13 completely recover all forms of AVS, but also digest a large proportion of synthetic pyrite. Weaker AVS digestions have an incomplete recovery of greigite. We conclude that it is not possible to completely separate greigite from pyrite, but cold 6NHC1 and cold 6NHC1 + SnC12 appear acceptable for AVS determinations. Despite these difficulties, the AVS and pyrite operational categories provide useful information on the reactivity of iron sulfide minerals in sediments. ACKNOWLEDGMENTS This work was supported by the National Science Foundation Marine Chemistry Program (NSF OCE-8309540). We especially thank Saulwood Lin for valuable technical assistance and Drs. S. Emerson, J. Chanton and C.S.
205
Martens for supplying sediment samples. Dr. D. Albert provided the information necessary for making Ti reagents. REFERENCES Albert, D., 1984. Improved techniques for measurement of sulfate reduction and pyrite formation rates in sediments. Trans. Am. Geophys. Union, 45: 906. Aller, R.C., 1980. Diagenetic processes near the sedimen~water interface of Long Island Sound. II: Fe and Mn. Adv. Geophys., 22: 351~415. Berner, R.A., 1964a. Iron sulfides formed from aqueous solution at low temperatures and atmospheric pressure. J. Geol., 72: 293-306. Berner, R.A., 1964b. Distribution and diagenesis of sulfur in some sediments from the Gulf of California. Mar. Geol., 1: 117-140. Berner, R.A., 1967. Thermodynamic stability of sedimentary iron sulfides. Am. J. Sci., 265: 773-785. Berner, R.A., 1970. Sedimentary pyrite formation. Am. J. Sci., 268: 1-23. Berner, R.A., 1974. Iron sulfides in Pleistocene deep Black Sea sediments and their paleo-oceanographic significance. In: E.T. Degens and D.A. Ross (Editors), The Black Sea: Geology, Chemistry and Biology. American Association of Petroleum Geologists Memoir, 20: 52~534. Berner, R.A., Baldwin, T. and Holdren, G.R., Jr., 1979. Authigenic iron sulfides as paleosalinity indicators. J. Sediment. Petrol., 49: 1345-1350. Canfield, D.E., Raiswell, R., Westrich, J.T., Reaves, C.M. and Berner, R.A., 1986. The use of chromium reduction in the analysis of reduced inorganic sulfur in sediments and shales. Chem. Geol., 54: 149-155. Chanton, J.P. and Martens, C.S., 1985. The effects of heat and stannous chloride addition on the active distillation of acid volatile sulfide from pyrite-rich marine sediment samples. Biogeochemistry, 1: 375-383. Davison, W. and Lishman, J.P., 1983. Rapid colorimetric procedure for the determination of acid volatile sulphide in sediments. Analyst (London), 108: 1235-1239. Davison, W., Lishman, J.P. and Hilton, J., 1985. Formation of pyrite in freshwater sediments: Implications for C/S ratios. Geochim. Cosmochim. Acta, 49: 1615-1620. Devol, A.H., Anderson, J.J., Kuivila, K. and Murray, J.W., 1984. A model for coupled sulfate reduction and methane oxidation in the sediments of Saanich Inlet. Geochim. Cosmochim. Acta, 48: 993-1004. Goldhaber, M.B. and Kaplan, I.R., 1974. The sulfur cycle. In: E.D. Goldberg (Editor), The Sea, Vol. 5. Wiley, New York, pp. 56~655. Goldhaber, M.B. and Kaplan, I.R. 1980. Mechanisms of sulfur incorporation and isotope fractiontion during early diagneses in sediments of the Gulf of California. Mar. Chem., 9: 95-143. Goldhaber, M.B., Aller, R.C., Cochran, J.K., Rosenfeld, J.K., Martens, C.S. and Berner, R.A., 1977. Sulfate reduction, diffusion and bioturbation in Long Island Sound sediments: Report of the FOAM Group. Am. J. Sci., 277: 193-237. Howarth, R.W., 1979. Pyrite: its rapid formation in a salt marsh and its importance to ecosystem metabolism. Science, 203: 49-51. Howarth, R.W. and Jorgensen, B.B., 1984. Formation of 35S-labelled elemental sulfur and pyrite in coastal marine sediments (Limfjorden and Kysing Fjord, Denmark) during short-term 35SO~reduction measurements. Geochim. Cosmochim. Acta, 48:1807 1818. Jorgensen, B.B., 1977. The sulfur cycle of a coastal marine sediment (Limfjorden, Denmark). Limnol. Oceanogr., 22: 814-832. Jorgensen, B.B., Fossing, H. and Thode-Anderson, S., 1984. Radiotracer studies of pyrite and elemental sulfur in coastal sediments. Trans. Am. Geophys. Union, 45: 906. King, G.M., Howes, B.L. and Dacey, J.W.H., 1985. Short-term endproducts of sulfate reduction in a salt marsh: Formation of acid volatile sulfides, elemental sulfur, and pyrite. Geochim. Cosmochim. Acta, 49: 1561-1566. Kolthoff, I.M. and Sandell, E.B., 1952. Textbook of Quantitative Inorganic Analysis: MacMillan, New York, 759 pp.
206 Lord, C.J., III, 1982. A selective and precise method for pyrite determination in sedimentary materials. J. Sediment. Petrol., 52: 664666. Martens, C.S. and Klump, J.V., 1984. Biogeochemical cycling in an organic-rich coastal basin 4. An organic carbon budget for sediments dominated by sulfate reduction and methanogenesis. Geochim. Cosmochim. Acta, 48: 1987-2004. Morice, J.A., Rees, L.V.C. and Rickard, D.T., 1969. Mossbauer analysis of the iron sulfides. J. Inorg. Nucl. Chem., 31:3797 3802. Morse, J.W. and Cornwell, J.C., 1987. Acid volatile sulfides and pyrite in anoxic marine sediments. Mar. Chem., 22: 55-71. Morse, J.W., Millero, F.J., Cornwell, J.C. and Rickard, D., 1986. The chemistry of the hydrogen sulfide and iron sulfide systems in n a t u r a l water. E a r t h Sci. Rev., 24: 142. Murray, J.W., Grundmanis, V. and Smethie, W.M., Jr., 1978. Interstitial water chemistry in the sediments of Saanich Inlet. Geochim. Cosmochim. Acta, 42: 1011-1026. Orion Research, 1977. I n s t r u m e n t M a n u a l for Sulfide Ion Electrode and Silver Ion Electrode, Model 94-16. Orion Research Inc., Cambridge, MA. Pruden, G. and Bloomfield, C., 1968. The determination of ferrous sulfide in soil in the presence of ferric oxide: Analyst (London), 93:532 534. Rickard, D.T., 1969. The chemistry of iron sulfide formation at low temperatures. Stockholm Contrib. Geol., 20: 67-95. Sheu, D.D. and Presley, B.J., 1986. Variations of calcium carbonate, organic carbon and iron sulfides in anoxic sediment from the Orca Basin, Gulf of Mexico. Mar. Geol., 70:103 118. Shoesmith, D.W., Taylor, P., Bailey, M.G. and Owen, D.F., 1980. The formation of ferrous sulfide polymorphs during the corrosion of iron by aqueous hydrogen sulfide at 21°C. Electrochem. Sci. Technol., 12:100.7 1015. Shriver, D.F., 1969. The Manipulation of Air Sensitive Compounds. McGraw Hill, New York, pp. 175 201. Sweeney, R.E. and Kaplan, I.R., 1973. Pyrite framboid formation: laboratory synthesis and marine sediments. Econ. Geol., 68: 618-634. Troelson, H. and Jorgensen, B.B., 1982. Seasonal dynamics of elemental sulfur in two coastal environments. Estuarine Coastal Shelf Sci., 15: 255-266. Van Valin, R. and Morse, J.W., 1982. An investigation of methods commonly used for the selective removal and characterization of trace metals in sediments. Mar. Chem., 11: 535-564. Wada, H., 1977. The synthesis of greigite from polysulfide solution at about 100°C. Bull. Chem. Soc. Jpn., 50: 2615-2617. Westrich, J.T., 1983. The consequences and controls of bacterial sulfate reduction in marine sediments. Ph.D. Dissertation, Yale University, New Haven, CT, 530 pp. Wieder, R.K., Lang, G.E. and Granus, V.A., 1985. An evaluation of wet chemical methods for quantifying sulfur fractions in freshwater wetland peat. Limnol. Oceanogr., 30: 1109-1115. Zhabina, N.N. and Volkov, I.I., 1978. A method of determination of various sulfur compounds in sea sediments and rocks. In: W.E. Krumbein (Editor), Environmental Biogeochemistry and Geomicrobiology, Vol. 3: Methods, Metals and Assessment. Ann Arbor Sci. Publ. MI, pp. 735 746.