Pyrite framboids

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.

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

Wignall P. B. and Newton R. (1998) Pyrite framboid diameter as a measure of oxygen deficiency in ancient mudrocks. Am. J. Sci. 298, 537-552. 

Wilkin R. T., Arthur M. A., and Dean W. E. (1997) History of water-column anoxia in the Black Sea indicated by pyrite framboid size distributions. Earth Planet. Sci. Lett. 148, 517-525. 

Figure 13 A cluster of syngenetic pyrite framboids in a bituminous coal. Micrometer-sized bright grains dispersed in the cluster are crystals of clausthalite (PbSe). Scanning Electron photomicrograph, back-scattered electron image (scale bar = 10 p,m).

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... 

Figure 18. Wavelength-dispersive electron microprobe image of a cluster of pyrite framboids within coal from the Black Warrior basin Alabama. Bright colored areas show the presence of arsenic, nickel, and sulfur. Arsenic is concentrated in epigenetic overgrowths and cement surrounding the framboids, while the diagenetic framboid interiors them.selves contain little or no arsenic. This indicates that the arsenic was added after earliest diagenesis.

Fig. 7. A single pyrite framboid about 20-p.m diam. Specimen current image (a) shows the subround cross section of the framboid. X-Ray images of sulfur (b) and iron (c) are shown for the same specimen. The scan for copper (d), initiated before the phosphor completely faded (faint grey background), failed to detect a significant concentration within the framboid.

Fig. 8. Ordered distribution of iron sulfide. The photomicrograph (a) shows individual pyrite grains dispersed throughout the matrix and clustered inside an organic ( ) envelope, 70 X. The graininess typical of pyrite framboids is evident at 320 x (b). X-Ray images of sulfur (c) and iron (d) portray the tendency of pyrite to conform to organic templates.

Fixation of metals as their insoluble sulfides is clearly evident for Fe and is considered the likely method for fixing the other trace metals, except for Mn. The blackened sediment beneath the thin-surface-oxidized layer, the lower SO4/CI ratio than that of marine water, and the presence of pyrite framboids (FeS2> show that sulfate is actively reduced to sulfide in the salt marsh. No discrete metal sulfides, besides pyrite framboids, were detected by microprobe examination however, the simultaneous appearance of S04 and trace metals in the interstitial water of cores stored in air suggests that other trace metals may be present as finely dispersed insoluble sulfides. 

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