NM-SCALE STRUCTURE OF KAOLINS, PALEOCENE OF EAST TEXAS, AND THE POTENTIAL ROLE OF BIOLOGICAL PRECIPITATION

Wilcox (mainly Simsboro) sands in East Texas contain abundant kaolin as large, discrete “books” and as reworked clay pellets, matrix, and authigenic pore-fill (McBride et al., this meeting). With the SEM up to 100,000X we have found many nm-scale features in these clays that suggest a microbiological origin.

1) Kaolin sheets with diameters <0.5-2 µm are single; larger ones stack into books (cf. Woodward, 1951).

2) Kaolin books show lateral grooves and salients that are propagated from sheet-to-sheet over distances of several hundred µm, even though sheets do not touch each other.

3) Individual kaolin sheets have amazingly uniform thicknesses of 45-60 nm (cf. Conley, 1966).

The above three points are important characteristics but their cause remains a mystery.

4) In plan view kaolin sheets range from perfect “hexagons” to those with scalloped edges, as if they were made of coalesced tiny lenticles.

5) Edges of kaolin sheets seen in profile are not planar crystal faces; instead they are semicircular like the end of a hot dog.

6) Clumps of small balls (mostly 50-100 nm) are common in most samples. They resemble nannobacteria (or bacterial byproducts) as seen in many other sedimentary minerals. Sausage-shaped objects are also common and their thickness equals the kaolin sheet thickness.

7) Some large kaolin books (up to 0.3 mm long) are covered with thin skins made largely of 40-100 nm balls. These probably formed within a clay bed and have been reworked into the sandstones.

The above four points are new observations and point toward the possibility of (nanno)biological precipitation.

Flux of acid, reducing waters rich in humics from lignite beds may have provided nutrients for prolific microbial growth, which resulted in the precipitation of kaolin (cf. Logan, 1919). Once precipitation was initiated by organic processes, then inorganic precipitation took over to form more euhedral crystal sheets.