Framboids

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 Colloform pyrite contains appreciable amounts of As and Cu (Nakata and Shikazono, unpublished), whereas these contents of euhedral and framboidal pyrite are less than the detection limit of an electron microprobe analyzer.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. 

In contrast to the of hydrothermal solution for the vein, that of pyrite in hydrothermally altered rocks (Shimanto Shale) varies very widely, ranging from —5%o to - -15%o. Based on the microscopic observation, pyrite with low values less than 0%o is usually framboidal in form, suggesting that low 8 S was caused by bacterial reduction of seawater sulfate. There are two possible interpretations of high 8 " S values (+10%o to - -15%o). One is the reduction of seawater sulfate in a relatively closed system. The other one is a contribution of volcanic SO2 gas. As noted already, volcanic SO2 gas interacts with H2O to form H2SO4 and H2S. value of SO formed by... 

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. 

Framboidal textures which are common in Kuroko deposits are found in the deposits in less metamorphosed rocks (Watanabe et al., 1993). 

Three textural types of pyrite occur in all samples examined 1) early framboids or irregular masses, 2) isolated euhedra, and 3) pyrites with massive cores and spongy or porous rims (Fig. 1). Types 2 and 3 occur in both shale and cross-cutting to bedding parallel veins, whereas type 1 occurs in shale only. 

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. 

Fig. 4.20 Synthetic hematites grown from ferrihydrite at temperatures <100°C (Schwertmann, unpubl.) a) Hexagonal plates grown at pH 7 and RT acicular crystals are goethite b) Laths grown at pH 11 and 80 °C in the presence of 2.5 10 M citrate (see Schwertmann et al., 1968). The fine granular material is unreacted ferrihydrite c) Framboids grown at pH 6 and 70°C in the presence of 2 10 M oxalate (see Fischer, ...

Framboidal polycrystalline aggregation 157 Table 8.1 Terms used to describe textures of polycrystalline aggregates... 

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... 

Figure 1. Scanning electron photomicrographs of minerals from coals. The minerals were studied and photographed by a Cambridge Stereoscan microscope with an accessory energy-dispersive x-ray spectrometer at the Center for Electron Microscopy, University of Illinois. A. Pyrite framboids from the low-temperature ash of a sample from the DeKoven Coal Member. B. Pyrite cast of plant cells from the low-temperature ash of a sample from the Colchester (No. 2) Coal Member. C. Kaolinite (left) and sphalerite (right) in minerals from a cleat (vertical fracture), Herrin (No. 6) Coal Member. D. Calcite from a cleat in the Herrin (No. 6) Coal Member. E. Kaolinite books from a cleat in the Herrin (No. 6) Coal Member. F. Galena small crystals in the low-temperature ash of a sample from the DeKoven Coal Member.

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. 

Framboid A microscopic cluster of spheroidal minerals, usually pyrite (FeS2). 

Pyrite occurs in sediments in the form of single crystals, crystal clusters, spheres, framboids or as replacement for organic structures. Miroprobe analysis of pyritic aggregates often show the presence of appreciable carbon, and some coarser carbonaceous matter is visible microscopically. Microcrystals of pyrites are frequently found in organic particles when examined in the TEM. Sometimes a thin bright rim occurs around each crystal, which indicates that it is enclosed within a carbonaceous shell (Oberlin et al., 1980)19). 

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... 

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. 

The iron sulfides found intimately associated with fresh plant tissues in near surface sediments were in most cases classified as framboidal. In addition, irregular pyrite morphologies were identified deeper in the cores, infilling organic tissues that would be classified by coal petrographers as inertinite. 

Pyrite Morphology. Pyrite formed during early diagenesis in marsh sediments nas essentially two modes of occurrence, as small single crystals and as framboids (18). The processes that lead to one form or the other remain unclear. Euhedra may form by direct... 

---