Crazy cracks
deutsche Version

enigmatic_crackA small sample of apparent silicified wood has got quite uncommon cracks inside which  deserve closer consideration. One would hardly believe that the long yellow lines in these pictures represent cracks on the cut and polished face if they were not really seen as narrow gaps filled with brittle yellow matter (Figs.1-4).

Fig.1: Crack of enigmatic shape seen as a yellow line. Width of every image 7mm.

Crack propagation is governed by fracture mechanics. Homogeneous stress in homogenous materials makes straight cracks. Other crack shapes can often be explained by locally variable fracture toughness of the material. The propagating crack tip follows a path of low fracture toughness. This is conspicuous with some plant fossils, where the waxy cuticle on the epidermis is often poorly silicified so that cracks preferably follow plant surfaces, as seen in Rhynie Chert News 1Rhynie Chert News 36 , Permian Chert News 15 . Low fracture toughness of interfaces between wood cells can make an apparently undulating crack path:  Fossil Wood News 37
peculiar crackNothing of the like applies to the cracks in this sample. A bleaching process seems to have produced pale spots and clouds in a dark matrix so that in some places an opposite aspect of a pale matrix with dark clouds with enigmatic yellow cracks appears. Some crack fragment may form a distinct boundary between the clouds which means that the bleaching did not cross the gap.

Fig.2: Two cracks of erratic shape in the apparently structureless interior of a dark cloud.

Another phenomenon is seen in Fig.3 where two former wide cracks, now filled with pale substance, had torn asunder the dark cloud together with the yellow crack inside. The different types of cracks, narrow and yellow or wide and pale, indicate that the sample had undergone a sequence of stages with changing material properties.

peculiar crackFig.3 (right): Dark cloud with narrow yellow crack, torn asunder by two wide pale cracks on the right.

Fig.4 (below): Narrow cracks with enigmatic erratic shapes.
peculiar crack

Most probably the crack shapes do not represent an early pre-existing structure but are brought about by some kind of spontaneous structure formation in the course of silicification. Phenomea of this kind can be highly complex, as may be guessed from Fig.4, where the extremely erratic crack paths seem to defy any explanation.

H.-J. Weiss     2021
quartz crystal with wood inside
Fossil Wood News  39
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