Framboid formation

Framboid formation through iron monosulfide conversion to pyrite appears to boost the total pyrite content of the sediments, presumably beyond levels attained through direct pyrite precipitation. 

Sweeney, R.E., and Kaplan, I.R. (1973) Pyrite framboid formation laboratory synthesis and marine sediments. Econ. Geol. 68, 618-634. 

Type II mineralization is replacement type mineralization occurring in the fragmental tops of mafic flows, within the Sunnyside Formation near its lower contact with the Archibald Settlement Formation. Type II Veins and veinlets are commonly intricately banded with zones of early rhythmically-layered amorphous silica and later sulfide commonly displaying framboidal textures. 

Pyrite is formed by two mechanisms in freshwater sediments. Fram-boidal pyrite results from reaction of iron monosulfides with S° (15), a slow reaction leading to gradual conversion of iron monosulfides to pyrite. In contrast, single crystals of pyrite are formed rapidly through reaction of Fe2+ and poly sulfides (161). Framboidal pyrite has been reported in lake sediments (37, 189), where it appears to form in microenvironments of plant or animal skeletons (cf. 35, 36). Rapid formation of pyrite has been observed in short-term measurements of sulfate reduction with SO/-. Up to 90% of reduced has been observed in pyrite after incubations of 1-24 h (72, 79, 98). A large fraction of inorganic S in the form of pyrite in surface sediments also has been interpreted to indicate rapid formation (112, 190). As discussed later, there is little evidence for extensive conversion of monosulfides to pyrite. 

In marine and lacustrine muds, the initial sulfide phase precipitated during early diagenesis is mackinawite (FeS09) which is subsequently converted to greigite (Fe3S4) and pyrite (FeS2) (85-89). This reaction path leads to the formation of framboidal pyrite (88.90). However, in salt marsh sediments under low pH and low sulfide ion activity conditions, direct precipitation of pyrite by reaction of ferrous iron with elemental sulfur without the formation of iron monosulfides as intermediates has been reported (85-87.89.91.92). This reaction is one possible pathway for the precipitation of pyrite as single crystals (89). 

Organic sulfur is the dominant form in peats described in these studies. Pyrite, however, is abundant in brackish and marine peats, occurring in void spaces in or between plant debris (3). In a study of pyrite formation in freshwater peats, Altschuler et al. (5) determined parallel decline in ester sulfate with increases in pyrite as depth increased and concluded that pyrite formed at the expense of organic sulfur. In general, framboidal morphology is present at all salinities. Altschuler et al. (5) and Lowe and Bustin (10) found monosulfides to be minor in peats. 

In these clastic sediments the dominant form of sulfur is pyritic, while organic sulfur is usually present only in trace amounts. For this reason, much work on sulfur in these sediments focuses on pyrite formation and its crystallization has been studied in detail by Berner (IT), Sweeney and Kaplan (12). Rickard (13). Rickard (14) and others. Under saline and hypersaline conditions precipitation of monosulfides may be the initial step. Sulfur is then added to these precipitates, converting them to pyrite. Laboratory studies indicate that if griegite is present in the original precipitate, sulfurization may produce framboidal aggregates (12). Conversion may depend on chemical factors such as H2S concentrations (9). In contrast, in conditions that are undersaturated with respect to monosulfides, but supersaturated with respect to pyrite, pyrite may form directly and rapidly from... 

Butler I. B. and Rickard D. (2000) Framboidal pyrite formation via the oxidation of iron(II) monosulfide by hydrogen sulphide. Geochim. Cosmochim. Acta 64(15), 2665-2672. 

Wilkin R. T. and Barnes H. L. (1997) Formation processes of framboidal pyrite. Geochim. Cosmochim. Acta 61(2), 323-339. 

The genesis of pyrite has claimed the attention of many workers. Pyrite has been synthesized chemically in the laboratory under a variety of temperatures and pressures (Berner, 1964a Roberts et al., 1969 Rickard, 1969). Pyrite framboids, so named because of their raspberry-like texture when viewed under a microscope, are found in clay sediments and silts, or as infillings of foram, diatom, or radiolarian tests. Biotic and abiotic mechanisms have been proposed for their formation (Schneiderhohn, 1923 Schou-ten, 1946 Love, 1965 Rickard, 1970). More recently, structures have been synthesized in the laboratory which resemble the pyrite framboids found in marine sediments (Berner, 1969 Farrand, 1970 Suna awa et al., 1971 Sweeney and Kaplan, 1973). On the basis of experiments with stable isotope... 

Direct evidence for the formation of authigenic metal sulfide comes from x-ray microprobe and light-microscope examination of the salt-marsh sediments. In the presence of sulfur, iron monosulfides react to form pyrite, FeS2, which is known to occur as distinctive, characteristic aggregates of octahedral microcrystals of FeSj (framboids Berner, 1970 Sweeney and Kaplan, 1973). In the Farm River samples, framboidal FeSj was found to be common within at least the upper 14 cm of core examined, either as discrete framboids of 10- im diam. (Fig. 7) or as ordered clusters of framboids. Inspection of polished thin sections reveals a frequent association with the organic matrix, which appears to act as a template for their formation (see Fig. 8). 

Direct precipitation of pyrite without intermediate iron sulfide precursors was reported for salt marsh sediments, where pore waters were undersaturated with respect to amorphous FeS but oversaturated with respect to pyrite (Howarth 1979 Giblin and Howarth 1984). In these sediments, the oxidizing activity of the roots favored the formation of elemental sulfur and polysulfides which were thought to react directly with Fe +. The direct reaction pathway may proceed within hours, resulting in the formation of single small, euhedral pyrite crystals (Rickard 1975 Luther et al. 1982). Framboidal pyrite, apart from that formed by the mechanism presented by Rickard (1997), is formed slowly (over years) via intermediate iron sulfides (Sweeney and Kaplan 1973 Raiswell... 

The formation of pyrite can occur following two mechanisms (i) single pyrite crystals are formed rapidly through direct precipitation of Fe(II) and polysulfides (S ) and (ii) framboidal pyrite is produced by a slower reaction of FeS with S° to produce a gregite intermediate (Equations 11.1 and 11.2). The direct precipitation of pyrite requires the prior oxidation of H2S to either S° or S for reactions like the following (Luther, 1991 Rickard and Luther, 1997) ... 

Framboidal pyrite is thought to be formed by the influence of bacteria. However, subhedral-euhedral pyrite forms by recrystallization of framboidal pyrite, precursor or direct formation by... 

---