Seen through

What is so fascinating about cherts? Various things: Mineralogists take interest in the silicification process, geologists in the stratum, palaeontologists in the fossils, and some person of refined taste may simply enjoy the polished sample. In my opinion, most fascinating is the 3D-preservation of tissue. Think about this: Would we be able, with all our advanced techniques, to preserve 3D-information of plant tissue for several hundred million years? Perhaps by freezing, plastination, or digitalisation and storing on CD? Seems ridiculous. Humbly we have to admit that the cherts are something unique, unequalled.

As the saying goes, every apple has its worm. There is no presently available technique for non-destructive 3D-scanning of the silicified tissue in tiny detail. Developing such technique would require multi-disciplinary research effort, and the outcome would be uncertain. One should not be discouraged but make use of the large variety of presently available techniques including those applied to other materials, as amber, or the so-called coal balls. The combination of techniques can yield remarkable results, as seen in the following.

						Fig.1: chert slab seen in reflected or transparent light
						Fig.2: axis with small thorn-like enations, seen in transparent light
						Fig.3: pinnules attached to the pinna seen in transparent light

The transparency of this slab is poor so that in reflected light one can see the fossil content only on the surface and in a rather thin layer below. Intense illumination from behind shows immediately that structures hidden in the depth can also be brought to light, as in Fig.1. In this way, connections of plant parts are revealed which could be only guessed under reflected light (Fig.2 & Fig.3). Compared to conventional thin sections, the slabs investigated here are very thick: about 7mm. This requires an intense light source, possibly cold light to avoid too high temperature of the sample. It is not easy to choose an optimal thickness: Thin slabs contain less structure information and thick slabs block the light. With the present material, the thickness of 7mm chosen here seems to be optimal for observation. For taking microscopic pictures, thinner slabs can be more suitable. Inspection of cut slabs with transparent light is also useful for judging the possible risk and expected result of polishing.

Thanks are due to GERT MUELLER for proposing this contribution and for kindly providing two slabs for investigation. These pictures are meant as an encouragement to look not only on the surface of the chert samples. Unconventional methods and experience from research on other materials might help to reveal more information still hidden within the samples.

text & photographs: R. KRETZSCHMAR/Chemnitz