Picture Page 1: Pyrite Ooids: Enigmatic Particles of Uncertain Origin

Pyrite ooids occur in several localities within the Chattanooga Shale of Tennessee. They are concentrically laminated pyrite grains of up to 2 mm diameter. Comparable occurrences are found in other sedimentary basins. How they form is not known, but a popular idea is that they formed as a replacement of precursor iron or carbonate ooids. The examples from the Chattanooga Shale shown on this page, however, suggest that they might be of primary origin. How a supposedly shallow high energy environment (envisioned for most ooids) can be compatible with the reducing conditions supposedly required for their growth is the mystery here. If you have seen comparable pyrite grains in other sedimentary sequences, send me a message ( ). I would be most interested to hear about it. Photos are low resolution for faster loading. Click on the image to get a larger, high resolution picture.

Figure 1/1: SEM images from broken pyrite ooid. Bottom photograph shows concentric nature of pyrite cortexes. The outer cortex is coarse crystalline with radiating pyrite crystals, and marked as R in the photograph (see also Fig. 1/3 for a polished section equivalent). The upper left image is a portion of the same ooid (marked by frame) but taken at higher magnification. The upper right image shows a portion of the upper left image (marked by frame) at even higher magnification and shows that the pyrite grains are of submicron size.

Figure 1/2: Pyrite ooid with pyritic core that contains two pyrite ooids (SEM image). This observation suggests that pyrite ooids in core formed essentially at the seafloor, and were pyritic prior to incorporation into ooid. Scale bar 0.1 mm long.

Figure 1/3: Py-ooid etched with HNO3. Inner cortexes (fine crystalline) stained brown due to etching. Outer cortex, coarse radial pyrite, not affected by HNO3, revealing overgrowth nature. Scale bar 0.5 mm long.
Figure 1/4: Py-ooid with multiple zones of "hard" polishing pyrite. Remnant laminae within "hard" pyrite intervals indicate that the latter is a replacement of finely crystalline "soft" pyrite cortexes. Scale bar 0.5 mm long.


More Pictures on Microbial Mat Features in Proterozoic Shales

More Pictures on Miscellaneous Sedimentary Features

More Pictures on Silica Filled Cysts in the Chattanooga Shale

More Pictures on Sculpting of Muddy Bottoms, Erosion Surfaces and Ripples

More Pictures on Bioturbation in the Chattanooga Shale


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© Jürgen Schieber, UTA Department of Geology
Last updated: March 12, 2001.