Framboidal pyrite

Pyrite is the most abundant ore mineral. It occurs as euhedral, framboidal, and colloform forms. Abundance of framboidal pyrite increases stratigraphically upwards. Colloform pyrite contains appreciable amounts of As and Cu (Nakata and Shikazono, unpublished), whereas these contents of euhedral and framboidal pyrite are less than the detection limit of an electron microprobe analyzer. Ishizuka and Imai (1998) found that the As content increases toward outer rim and reaches up to 5 wt% in the rim of colloform pyrite from the Fukazawa deposit. 

Komuro, K. and Sasaki, A. (1985) Sulfur isotope ratio of framboidal pyrite in Kuroko ores from the Ezuri mine, Akita Prefecture, Japan. Mining Geology, 35, 289-293. 

There is a type of polycrystalline aggregate of pyrite crystals showing a framboidal appearance, known as framboidal pyrite. It occurs in sedimentary... 

P5U ite crystals exhibit a wide range of Tracht and Habitus, and also occur in unusual forms of polycrystalline aggregate, such as framboidal pyrite. Although numerous crystal faces have been reported, the most important ones are 100, 111, and 210. Calcite also exhibits a variety of Tracht and Habitus, such as platy, nail-head, prismatic, or dog-tooth forms, but 1011 is the only F face. In this chapter, we focus our attention on the factors controlling the observed variations in Tracht and Habitus of p3U ite and calcite. 

Framboidal pyrite occurs, for example, in sedimentary rocks, muddy sediments, and precipitates in hot springs two controversial origins have been suggested, one bacterial and the other relating to agitation in hydrothermal solution. Framboidal... 

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). 

A record of morphology classes for each pyrite occurrence was kept during petrographic analyses. Monocrystalline pyrite includes euhedral and subhedral pyrite crystals. This morphology class is always more prevalent than framboidal pyrite except at the top of core 1 and the bottom of core 3. 

Figure 6.1. Electron microscope photograph of (a) framboidal pyrite sample and (b) a close-up of the framboidal pyrite crystals.

Kribek B. The origin of framboidal pyrite as a surface effect of sulfur grains. Hineral. Deposita 10, 389-396 [1975]. 

Wilkin R. T., Barnes H. L., and Brantley S. L. (1996) The size distribution of framboidal pyrite in modern sediments an indicator of redox conditions. Geochim. Cosmochim. Acta 60, 3897-3912. 

Berner R. A. (1969) The synthesis of framboidal pyrite. Econ. Geol. and Bull. Soc. Econ. Geol. 64(4), 383—384. 

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. 

In permeable reactive barriers, chalcopyrite, bomite [Cu5FeS4], greigite [Fe " Fe2 S4], and abundant framboidal pyrite have been observed as secondary precipitates within the reactive medium emplaced to treat waters laden with... 

Sunagawa, I., Endo, Y. and Nakai, N., 1971. Hydrothermal synthesis of framboidal pyrite. Soc. Min. Geol. Japan, Spec. Issue, 2 10—14. 

A second important factor is the particle size of the FeS2. Caruccio et al. (1976) concluded that AMD production was a serious problem when exposed pyrite grains were about 0.25 fxm in diameter in framboidal pyrite. This corresponds to a surface area of 4.8 m /g. For crystal sizes in excess of 5 to 10 m (5 fxm s 0.24 m /g), AMD production was greatly diminished, and became unlikely when crystal sizes exceeded 400 fiva. Particle sizes were smallest in marine coals and became larger in lacustrine coals (Caruccio and Perm 1974). A third factor determining the risk of AMD is the proximity of carbonate rocks that can neutralize AMD acidity. 

Caruccio, F. T., G. Gbidel, and J. M. Sewell. 1976. The character of drainage as a function of the occurrence of framboidal pyrite and ground wafer quality in eastern Kentucky. Proc. 6th symp. on coal mine drainage, pp. 1-16. Washington, DC Natl. Coal Assoc. 

At the surface of some marine sediments, organic sulphur can comprise as much as 50% of the total sulphur present (Francois, 1987) due to biosynthesis which incorporates sulphur of all oxidation states, but also, because of the reactivity of sulphides and polysulphides, by chemical addition. There is usually an increasing S/C ratio with depth in sediments, partly associated with humic substances, and most of this increase occurs in the oxic and suboxic zones. This organic repository may be the source of the sulphur required to convert metastable iron sulphides, formed in the lower part of the sulphur reduction zone, to framboidal pyrite, which is often found closely associated with organic matter. 

Calcite cementation in bioclastic hybrid and lithic arenites of the Bismantova-Termina succession is pervasive along layers and concretionary horizons. Cementation in the hybrid shelf arenites was mostly precompactional and began with marine calcite rims, syntaxial overgrowths on echinoderms, K-feldspar and dolomite overgrowths, chloritic clay rims, framboidal pyrite and heulandite, followed by... 

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... 

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