Dear Tatjana,

Angular fabric is quite common in sandstone and sandy siltstone, provided that you have differences in the contraction rate (or expansion coefficient) between very thin pelitic layers and sandy siltstone. Alternating heating and cooling (desert like) consequently produces angular alligatoring network, that gives way to interstitial water and alteration tracing along fractures.

Please compare very similar pattern, that I found in the internet.

It is reasonable answer by Grundmann.....

Dear Tatjana,

in principle similar, nearly rectangular cracks are very common in layered silt-/sandstones, due to different contraction and expansion of the different layers. But in your special case it could be an artificial man-made type of "inscription", because several "cracks" are starting and ending in rectangular fiels surrounded by "cracks".

Primary sedimentary structures e.g. desiccation cracks are always polygonal, therefore it should be an result of mechanical stress...........

Are this linear elements really cracks or engravings??? I can´t make a clear decision on the basic of these photographs.........

best regards

Andreas

Well, as other contributors like Grundmann and Rohatsch mentioned above, it is common to see the rectangular cracks in sandstone and siltstone. Such rectangular cracks usually occur in the top of an anticline experiencing a planar extension to all the directions. It also can happen by rapid uplifting and unloading.

But, having been to many fieldworks in sedimentary environments, I haven’t seen such a feature in a small scale like this. Could be an artifact as Rohatsch said or could be a sediment stamped by a meteorite:).

Dear Tatjana,

Dear Andreas,

The lower photograph clearly shows that the rectangular pattern of "cracks" and/or "fractures" do not cut through the subjacent layer. In this case we are probably dealing with a combination of a limited number of major rectangular cracks that give way for salty water followed by evaporation and subsequent development of crystalline salt pattern probably created by bacteria.

Please compare the photo by J.M. Gómez-Gómez of "crystalline salt pattern created by E.coli., from the Courtesy of Science Daily. I see striking similarities.

I think this may be an additional explanation for that peculiar pattern.

Dear Guenter,

thank you very much for this really interesting additional explanation. At first sight it looks very similar indeed, except the triangular shape of many of these "engravings". Do you have  a scale-bar for this picture?

kind regards

Andreas

Dear Andreas,

Sorry, a scale-bar is not given, but it ranges on a microscopical scale as mentioned in the text. However, if you have a closer look at the "engravings" of Tatjana's specimen, you will find a lot of very small teeth, that reminds me on growth pattern of cubic crystals (halite?) sticking out of the main axis. What do you think?

Kind regards

Guenter

Dear colleagues,

many of the opinions put forward might be true or not. It is hard to take a decision. I will not add another idea but shift your thoughts to the methodology used in such a case. There will be no other choice than cutting a small rock slap from the specimen, perpendicular to the riffles and see how the grain size, texture and mineralogy changes off the surface and the riffles. It may be a thin section examined under the petrographic microscope or a scan using SEM/EDX or SEM/WDX which can reduce the field of speculation. The specimen is kept preserved like a gemstone which is investigated from the side not exposed to the observer. It should be a minimal invasive method. I guess after having obtained clear information on the microtexture and on the mineralogy in and adjacent to the riffles the enigma will be brought closer to a solution.

I hope this technical piece of advice will be of help.

Best regards

Harald G. Dill

here is the edge, and the pattern appears on both sides of the slab. The piece is on loan to me but I will ask about creating a fresher break, as this one looks somewhat rounded.

I also tested it with a weak hydrochloric acid and it does fizz, although its too hard to tell if the actual matrix reacted, or if was the fine yellowish mineralization that appears to be built up in the etched lines.

Dear Harald G. Dill,

thank you very much for your clear statement, I agree completely with you. The classical microscopical analysis of thinsections supplemented by SEM/EDX is the only rational way to interpretate this riffles seriously.

Best regards

Andreas Rohatsch

Dear Mrs. Mulligan,

you are true, the edge looks beveled, obviously due to the handling in the antique store. I have already mentioned the handling of the specimen and compared it with gemological investigations. You cannot avoid having a small part near the critical zone, where the riffles evolve, chopped from the specimen. As I see the problem you may be faced with sparking some discussion with the dealer. I suggest an analysis with SEM-EDX/WDX and skip the thin section. By means of the method recommended you can use material which needs not to be sputtered. You might see the texture, get a quick look at the chemical composition and the sample is still available for further studies.

This is what I would suggest at the moment. There are more highly-sophisticated methods but it is also important cast the economic part of the investigation not aside. I looked at the cross section at a higher magnification. There is a faint grain size variation attesting to bedding. Silty sandstones and siltstones in red bed-type sediments often very vaguely show their bedding. It is a very homogeneous grain size, so I suspect some kind of a lacustrine sediment (ephemeral or perennial). According to my knowledge of the geological setting in AZ, this would not be anything out of the ordinary.  But I hesitate to proceed any further with my interpretation of this picture at the moment.

Best regards

Harald G. Dill

Dear Tatjana,

thank you very much for your additional information. The hydrochloric acid test clearly points to calcite and/or aragonite as accessory minerals. The view of the edge shows the

joints of the sedimentary structure traced by dark brown and yellowish pigments.

Do we learn just from Mona Lisa?

The figure caption quotation from the link given below: 'Craquelure is usually hierarchical. Cracks accumulate over time, forming progressively finer lines. The subdivisions tend to be rectangular rather than hexagonal.'

No comment.

Kind regards,

Guenter

www.darkheaven.com/tech.html

I have nothing to offer; except, you have all added an intriguing phenomenon to my knowledge.  Thank you.

yes, a micochemical SEM analysis would be absolutely neccessary.

Other observations to be interpreted are:

• the horizontal plane of the slab seems "fresher" than its edge
• there are two crack regimes (coarse; fine)
• the fine cracks are filled with light (carbonaceous?) material; they are restricted to the top layer of the specimen
• the coarse cracks are empty
• some cracks exibit dark margins (weathering? infiltration of (manganese?) solutions?)

Best, Stephan

Hello

I find such structures in volcanic ash have fallen hot. The structures shrinkage occurs. photos included are the first two of silt size Miocene volcanic ash and the other a Permian volcanic ash