Reactions Forming Pyrite from Precipitated Amorphous Ferrous

Chapter 10

Reactions Forming Pyrite from Precipitated Amorphous Ferrous Sulfide 1

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 9, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0612.ch010

Yoko Furukawa and H. L. Barnes Ore Deposits Research Section, Pennsylvania State University, University Park, PA 16802

The replacement reactions that convert precipitated ferrous sulfide to pyrite were evaluated as possible kinetic paths by comparing solid molar volume changes of the reactions (∆V ). They may proceed by iron loss rather than by sulfur addition, the conventional conversion process: FeS(s) + S(s) --> FeS (py). Alternative conventional reactions include the addition of sulfur from aqueous species, such as polysulfides or thiosulfate. However, these solids

2

sulfur-addition reactions result in positive ∆V that slows the reaction to ineffective rates by causing armoring of precursor mineral surfaces. Instead, reactions that combine the loss of Fe and oxidation of precursor phase such as 2FeS(s) + l/2O + 2H --> FeS (py) + Fe + H O solids

2+

+

2

2+

2

2

cause a decrease in ∆V with resulting shrinkage cracks that promote solute mobility and speed further replacement reactions. solids

Authigenic pyrite is common in both modern and ancient marine sediments. Its remarkable post-depositional persistence in earth surface environments makes pyrite an ideal geochemical indicator. In many natural aquatic systems, such as water columns in anoxic basins and pore waters in marine sediments, authigenic pyrite formation correlates with specific combinations of oxidation state, sulfate concentration, and iron concentration of the systems (e.g., 1). Thus understanding the 1

Current address: Naval Research Laboratory, Code 7431, Stennis Space Center, MS 39529

0097-6156/95/0612-0194$12.00/0 © 1995 American Chemical Society In Geochemical Transformations of Sedimentary Sulfur; Vairavamurthy, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


10.

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Reactions Forming Pyrite

195

mechanisms of authigenic pyrite formation would allow pyrite to be used as an indicator of geochemical variables such as oxidation state, sulfate concentration and influx of detrital iron minerals. Authigenic iron sulfide formation is also critical to sedimentary paleomagnetic studies. Among the intermediate phases leading to sedimentary pyrite, greigite is ferrimagnetic and therefore its diagenetic formation in post-depositional environments has a strong effect in the interpretation of paleomagnetic data (e.g., 2). One of the major minerals to carry remanent magnetism in sediments, magnetite, is a major iron source for diagenetic iron sulfide formation and therefore its conversion by this process is a major cause of magnetic instability in marine sediments (3). Thus an understanding of the mechanisms of authigenic pyrite formation enables us to better understand the magnetization processes in sediments and to better interpret paleomagnetic data. Berner (7, 4) divided the authigenic pyrite formation process into three stages. In the first stage, aqueous sulfate (SO4 ") is reduced to sulfide (H2S or HS") by bacterial reduction that accompanies the biologic oxidation of organic matter. In the second stage aqueous sulfide, formed during the first stage, reacts with F e provided by the bacterial dissolution-reduction of detrital iron minerals in the sediments, or suspended in the water column, to form an amorphous ferrous sulfide precipitate. This amorphous ferrous sulfide precipitate subsequently reacts to increasingly ordered mackinawite (FeQSg) (5). The third stage is a series of replacement reactions that produce pyrite progressively through a series of sulfur-rich phases from mackinawite as the initial ordered phase. Experimental studies have shown that direct precipitation of pyrite nuclei from low temperature aqueous solutions is kinetically prohibited, although growth of pyrite nuclei is possible (6, 7). Schoonen and Barnes (6, 7) experimentally showed that direct pyrite precipitation does not occur in solutions undersaturated with respect to precursor phases such as amorphous ferrous sulfide unless pyrite nuclei are present. Although this three-stage process has been the topic of many previous studies (e.g., 1, 4-11), our knowledge of the reaction mechanisms of pyrite formation at low temperatures remains incomplete. Sulfate reduction is reviewed by Berner (7). Rickard (8) used T-tube experiments to study amorphous ferrous sulfide precipitation from F e and aqueous sulfide, and subsequent conversion to mackinawite was inferred by an EXAFS study (5). However, our understanding of the third stage, Fe-S mineral replacement processes, is far from complete. Experimental studies have indicated that elemental sulfur (S°), or aqueous sulfur species with intermediate oxidation states such as polysulfides (S "), are essential to form pyrite through replacement reactions (7, 4, 7) as seen in the reactions, 2

2+

2+

2

n

FeS(s) + S°(s) -> FeS (s) 2

amorph.

pyrite

In Geochemical Transformations of Sedimentary Sulfur; Vairavamurthy, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

0)

196

GEOCHEMICAL TRANSFORMATIONS OF SEDIMENTARY SULFUR

2

2

FeS(s) + S " -> FeS (s) + S ^ " n


amorph.

(2)

2

pyrite

However, sufficient elemental sulfur and adequate intermediate sulfur species concentrations are not found in many pyrite-forming environments. This paper will review previous experimental studies on the third stage, the replacement sequence from mackinawite to pyrite, in low temperature aqueous environments, and discuss those experiments in terms of possible alternative reaction mechanisms that do not require elemental sulfur or polysulfides as reactants. It is important to understand the mechanism of the replacement sequences because pyrite growth depends on the presence of initial pyrite nuclei, that must have formed by replacement because direct precipitation is kinetically prohibited. All sedimentary pyrite, both framboidal and euhedral, should originate as pyrite nuclei that are products of replacement reactions. What is presented here is a new interpretation of previous experimental studies pertinent to this process. Previous Studies Pyrite was synthesized by Berner (4) at 65°C by the replacement of freshly precipitated amorphous ferrous sulfide (described as FeS in the study) in aqueous solution with neutral pH. A suspension of solid elemental sulfur (S°) and its immediate products upon hydrolysis, hydrogen sulfide, aqueous polysulfides, and thiosulfate were present in the solution. Pyrite formation depended on the presence of excess elemental sulfur on the surface of which pyrite crystallized. No pyrite was recovered from runs in which elemental sulfur was completely dissolved to yield aqueous sulfur species. Due to the absence of pyrite in runs without elemental sulfur, the following reaction was suggested as dominant: (3)

This reaction can be modified to accommodate the more recent knowledge that mackinawite occurs as the first ordered phase in the replacement reaction following its recrystallization from the amorphous ferrous sulfide precipitate: Fe S (s) + 10S°(s) -> 9FeS (s). 9

8

2

mackinawite

(4)

pyrite

On the other hand, Schoonen and Barnes (7) proposed the overall reaction; 2

2

FeS(s) + S ~ -> FeS (s) + S ^ " , n

amorph.

2

pyrite

which also can be better written as:


(5)

10.

FURUKAWA & BARNES

Reactions Forming Pyrite 2

197

2

Fe S (s) + 10S " -> 9FeS (s) + l O S ^ " . 9

8

mackinawite

n

(6)

2

pyrite

In the study by Schoonen and Barnes (7), pyrite resulted when amorphous ferrous sulfide (described as FeS in the study) was aged either in a polysulfide solution (S4 " = 0.066 mol/liter) at 65°C with no elemental sulfur in suspension, or in a solution containing both aqueous intermediate sulfur species and suspended solid elemental sulfur. Mackinawite was the first ordered phase formed immediately after the amorphous ferrous sulfide in the chain of replacement reactions. Next, mackinawite was occasionally replaced by either greigite or marcasite, and always by pyrite as the final product. Greigite was favored as an intermediate product where the solution was slightly oxidized, and marcasite was favored if the solution was acidic (pH < 5). Aqueous sulfur species of intermediate oxidation states, such as polysulfides or thiosulfate, were essential in order to form pyrite in the experiments of Schoonen and Barnes (7). The intermediate aqueous sulfur species were considered to be the sulfur source for sulfidation of mackinawite in a reaction like (6). Reaction (6) (or (5)) was favored over reaction (4) (or (3)) because elemental sulfur is virtually inert in low temperature aqueous solutions. The reason why Berner (4) failed to obtain pyrite in experimental runs that contained aqueous intermediate sulfur species but no solid elemental sulfur is unknown. However, the surfaces of elemental sulfur tend to be saturated with polysulfides (12) and, therefore, can be considered as the most likely site for pyrite crystallization, as was described by Berner (4). Schoonen and Barnes (7) concluded that pyrite forms through sulfidation of mackinawite rather than by iron loss from mackinawite. Sulfidation reactions such as (6) were preferred over ironloss reactions such as


2

2+

2FeS(s) + 2H+ -> FeS (s) + F e + H (g) 2

amorph.

(7)

2

pyrite

because no pyrite was obtained from experimental runs that contained no sulfur species with intermediate oxidation states. Reaction (7) is an iron-loss reaction in which iron diffuses from the original, solid structure and dissolves into the aqueous solution in order to stoichiometrically balance the solid-phase conversion. Other experimental studies agree with the studies by Berner (4) and by Schoonen and Barnes (7) that sulfur species with intermediate oxidation states are necessary for converting mackinawite to pyrite. Freshly precipitated mackinawite was converted to greigite and then pyrite in 5 to 11 days at 60-85 °C in the presence of elemental sulfur in the experimental study by Sweeney and Kaplan (9), in which isotopic measurements indicated addition of sulfur in the mass balanced conversion of precursor phases to pyrite. Rickard (13) quantified the rate of pyrite formation in reaction (3). Luther (77) investigated pyrite formation by mixing F e and Na S (n=2, 4, 5) solutions at 25°C and neutral pH. The mixing resulted in the immediate black coloration of the solution which was described as due to the mixture of soluble F e - (HS") - S " complexes such as [Fe(SH)(S )]~ and an amorphous solid such as 2+

2

2+

2

n

n


n

198


Fe(HS) . The initial black mixture was aged to form pyrite after several hours to months in the presence of dissolved polysulfides. In summary, the previous experimental studies support the conclusion that replacement of amorphous ferrous sulfide or mackinawite proceeds when aqueous sulfur species of intermediate oxidation states, such as polysulfides or thiosulfate, are available. The replacement reaction is expressed as sulfidation of mackinawite as in reaction (6). The above authors presumed that pyrite did not form by iron loss reactions such as (7). However, the proposed sulfidation reactions have positive changes in the overall Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 9, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0612.ch010

2

a s

r o n a t o m s

molar volume of the solids (+AF solids) i ^e preserved in solids and additional sulfur is introduced to the solid structures. Generally, replacement m

or

reactions are favored by -AV solids der for efficient transport of atoms among participating solid phases and an aqueous solution, and replacement reactions among iron sulfide minerals should follow this principle. In addition, the scarcity of intermediate sulfur species in at least some pyrite-forming environments makes such sulfidation reactions unrealistic in geological environments especially at low temperature. The next sections discuss the AV \[r\ of replacement reactions, and consequently, alternative replacement mechanisms among iron sulfides will be so

proposed that lead to -AV

s

solids-

Theory of Replacement In a mineral replacement process, a pre-existing mineral reacts to become a successor mineral, as a result of changing chemical potential gradients (14-16). In a hydrothermal or low temperature aqueous process, the cause of replacement process is an aqueous solution which has concentrations in disequilibrium with the original solid phase, and the replacement process begins by surface reaction between the original phase and the aqueous solution. Replacement reactions require a physical or chemical link between the original phase and the replacing phase. The original phase and the replacing phase may share common elements (e.g., replacement of hydrothermal pyrrhotite by pyrite, calcite by dolomite) or share a similar crystal structure as well (e.g., replacement of calcite by magnesite). Replacement reactions are more likely to proceed efficiently when there is a net decrease of total solid volume (i.e., -AV solids)- The resulting void space in the solid promotes reactant transport by diffusion between the original solid phase and aqueous solution, and thus promotes further replacement reaction. This process is demonstrated in Figure 1(a), in which the replacing phase, shown in solid black, occupies a smaller volume than the original phase (shaded). Because of the net decrease in solid volume, there is an increase in porosity that is represented in the figure by cracks. The porosity promotes more rapid transport between the original phase and aqueous solution, and the replacement reaction may proceed to completion. If a replacement reaction leads to a net increase in solid volume (+AV solids)' secondary phase armors the initial phase with a reaction rim and limits mass transfer m


e


10.

FURUKAWA & BARNES

199


between the solution and the original phase. Figure 1(b) illustrates the later scenario. The replacing phase (solid black) has negligible surface porosity and permeability due to the net increase in solid volume and armors the original phase to prevent further direct interaction between the original phase (shaded) and aqueous solution. Once the armoring occurs, the exchange of chemical components between the original phase and solution necessary for the replacement reaction can proceed only by solid state diffusion through the rim of the replacement phase. This will decrease the efficiency of replacement reaction significantly because solid state diffusion is a much slower process than aqueous transport of elements especially at low temperature. Murrowchick (7 7) showed that the replacement of pyrrhotite by pyrite proceeds by conserving sulfur as in: 2+

Fe S (s) + 6H+ + 3/20 (aq) -> 4FeS (s) + 3Fe + 3H 0. 7

8

pyrrhotite

2

2

(8)

2

pyrite

When 4 moles of pyrite replace 1 mole of monoclinic pyrrhotite, the solid volume change is negative (-32%) and the reaction proceeds by iron loss rather than sulfur addition. Sulfur addition would have resulted in a net increase of solid volume. A -AV soling was apparent in photomicrographs of samples from Kutna Hora, Czechoslovakia where replacement of pyrrhotite by pyrite produced increased pore space. "Bird's eye" texture of pyrite and marcasite (14) is another example of -AV solids during mineral replacement. In summary, replacement reactions proceed much more rapidly in the direction of a net decrease in the volume of solids. Replacement reactions with -AV iids favors interaction between the original phase and aqueous solution, as well as among two or more solid phases including the original phase and replacing phases. so

AV

solids during Fe-S System Replacement Reactions Previous studies show that pyrite formation at low temperature is initiated by replacement of precursor phases because direct nucleation of pyrite is kinetically prohibited. Evidence for the replacement origin of pyrite in nature is the residual magnetic character from greigite cores within some pyrite framboids where the replacement has not gone to completion (2). The question here is which reactions provide a -AV solids which would permit fast replacement at low temperatures. Published experimental studies of authigenic pyrite formation support sulfidation rather than iron loss as the mechanism of the replacement reactions that convert mackinawite to pyrite (e.g., 1, 4,7 -11). Experimental conversion of iron monosulfides to pyrite always required the presence of a sulfur source, such as elemental sulfur, thiosulfate, or polysulfides, and Sweeney and Kaplan (9) showed that isotopic compositions of their experimental products implies the sulfur addition mechanism. Sulfidation reactions would proceed by the preservation of iron and


200


addition of sulfur to solids, whereas iron-loss reactions would proceed by the preservation of sulfur in solids and loss of iron to an aqueous solution. Aqueous sulfur species of intermediate oxidation states, such as polysulfides, must be the sulfur source. For example, the replacement of mackinawite by greigite would be written as; Fe S (s) + 9

8

4S 2-

3Fe S (s) +

n

3


mackinawite

4

4S . 2", n

(9)

1

greigite

and the replacement of mackinawite by pyrite is written as; 2

9FeS (s) + l O S ^ " .

mackinawite

pyrite

9

8

The solid volume change (AV AF

2

Fe S (s) + 10S "

s o l i d s

n

(6)

2

solids)

r

f° reaction (9) is given by

( % ) = (3 x F - F ) -sx 100 = (3 x 72.5 - 184.3) -s- 184.3 x 100= +18%, g

r

m

k

(10)

and is positive. (Molar volumes of iron sulfide minerals are calculated using density data in (18)). Similarly in the case of reaction (6), mackinawite to pyrite, AF

s o l i d s

( % ) = (9 x F - F ) * F x ioo = (9 x 23.9- 184.3)- 184.3 x 100 = +17% p

y

m k

m

k

(11)

and is again positive. In both examples, the +AV \\^ should make these reactions prohibitively slow at low temperatures where solid state diffusion rates are negligible. SQ

S

Figure 2 summarizes such AV i j for all of the Fe-S minerals in replacement reactions that have been suggested to occur based on experimental evidence. All but so

ds

one reaction would lead to a +AV i j when the replacement reactions proceed by sulfidation. The greigite to pyrite step is the only sulfidation reaction that has a so

ds

solids-

An alternative mechanism to sulfidation in the conversion of mackinawite to pyrite is replacement by iron loss to the solution. In such reactions, sulfur is conserved in the solid structure whereas iron is removed from the original solid structures and released into an aqueous solution. Such reactions result in a -AV iids> as shown by the example of pyrite replacing mackinawite below. A possible reaction for this process is the oxidation of mackinawite by combined generation and loss of hydrogen, and can be written as; so

2+

Fe S (s) + 10H+ -> 4FeS (s) + 5Fe + 5H (g) 9

8

mackinawite

2

2

pyrite


(12)


10. FURUKAWA & BARNES


201

Figure 1. Schematic diagrams of replacement reactions, (a): When solid volume change is negative; (b): When solid volume change is positive. [-47%] (+20%) mackinawite (Fe S ) 9

8

o/]2

+18

—l~ ] ( /«)

pyrrhotite (Fe,. S) x

smythite (Fe S„) 9

greigite (Fe S ) 3

'

4

[-32%] (+2%)^r [-30-310/0] ( 22~ 27o/ot [-32~-33Vo] (+i9~+23Vo) +

+

f - " * l

Reactions Forming Pyrite from Precipitated Amorphous Ferrous

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