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24th Apr 2023 21:42 UTCDana Slaughter 🌟 Expert

Chalcedony

Bu Craa (Bou Craa; Boukraa), Laâyoune Cercle, Laâyoune Province, Laâyoune-Sakia El Hamra Region, Morocco

I recently obtained a few of these and am largely unfamiliar with them despite seeing hoards of them (generally not as attractive as these pieces) at Tucson. I've noticed that most of the notable and reputable dealers describe them as chalcedony replacing fossil sponges. Some fossil dealers specializing in Moroccan items simply offer them as chalcedony nodules, etc. I recently sold one at the Minerals of Arizona symposium and had two very experienced dealers tell me that these are not after fossil sponges. Any ideas? Thanks.

24th Apr 2023 21:48 UTCDavid Carter 🌟 Expert

I also had a few similar specimens (here is one) a while ago described as a result of sponge silica fossilisation where chalcedony had replaced a sculptural globular colonial sponge. It was reportedly from Laâyoune-Sakia El Hamra Region, Morocco.

24th Apr 2023 22:22 UTCDavid Carter 🌟 Expert

Dana Slaughter 🌟 Expert ✉️

… two very experienced dealers tell me that these are not after fossil sponges.

They said what they weren’t, but did they say what they were? In this paper (link below) regarding the Ouled Abdoun Basin in Morocco it mentions that “silicification occurs in nature from the deep sea to the surface (Millot, 1960, 1964)” and Millot “emphasises that the silicification (chert, chalcedony...) is the product of crystal growth from solutions that contain natural silica”. The report below also states “the relationship between SiO2 and major and trace elements in the siliceous and silicified facies reveal that these elements were precipitated by direct syngenetic, epigenetic and authigenetic processes. The silicification has taken place in the reduced and slightly oxidized environment, due to the rising of upwelling currents which allow to the proliferation of some organisms as radiolarians and sponges”.

https://www.researchgate.net/publication/271212309_SILICIFICATION_IN_THE_PHOSPHATE_DEPOSITS_OF_MAASTRICHTIAN-LUTETIAN_OF_THE_OULED_ABDOUN_BASIN_MOROCCO_SEDIMENTOLOGY_MINERALOGY_AND_GEOCHEMISTRY

25th Apr 2023 04:32 UTCDana Slaughter 🌟 Expert

Thank you David---this is an excellent article and reference. The two dealers thought that they were akin to chalcedony roses and that no replacement of any organic material had taken place in these specimens.