FMF - Friends of Minerals Forum, discussion and message board

[Amir Akhavan]

Hi,

Jordi and many other dealers offer these odd looking brown chalcedony nodules from Smara/Es Semara in Morocco. Sometimes you read that they are silicified sponges.
@ Jordi: Is there any evidence for this or where does this explanation come from?
Do we know anything about the geology of their locality?

I have prepared a thin section and can tell it is radially grown chalcedony with a single interspersed layer of radially grown (mega-)quartz crystals (the white stuff) which are again overgrown by a thin layer of chalcedony. So they are essentially intergrown spherulites.

In one respect they are rather unusual, though: They don't look like the typical chalcedony spherulitic and botryoidal specimens because they are not composed of the common length-fast chalcedony, but of quartzine (length-slow quartzine) which has the c axes of the crystallites in the chalcedony fibers aligned in the direction of fiber growth.
I have so far only seen quartzine intergrown with length-fast chalcedony, these are the first "pure" quartzine specimens in my collection.

In sedimentary petrology, quartzine is seen as an indicator of evaporites. So it would be interesting to know the geological setting of these nodules.

Amir

[Amir Akhavan]

A photo of a specimen cut in half. The central part was used to prepare a thin section.
The section on the photo is still 2.1mm thick and 32mm wide, and was reduced to 30µm (no photo of that yet).
When still intact, the specimen was 64mm wide.

You can see a concentric, kidney-like internal structure. The bright part is quartz, the rest quartzine colored brown by inclusions. Growth direction is radial from inside out, obviously not a pseudomorph or fossil.

[Jordi Fabre]

Amir,

The specimen of your photo is obviously not a former sponge. What we call sponges are well rounded spheres frequently with traces of what seems to be spicules. At Shows like Ste. Marie there are thousands and thousands of similar material and only a few seems to be former sponges, so, a possibility could be that among these thousands of silica radially grown just some former sponges replaced by Quartz been there and that the rest are just radially grown Quartz.

I requested to a friend a search of some Spanish papers about them and the geology of that area. If we have success we'll let you know.

Best

Jordi

[Amir Akhavan]

Hi Jordi,

thanks for the response!
I have to try harder at Sainte Marie to find one with traces of fossils in them. :-)

It's odd that there is so little information on specimens like these that are offered in such large quantities.

Amir

[Bob Morgan]

There are fossil sponges claimed for our local Savannah River agate. It's only agate an agate lover could love, but it does have some features that may be like filled holes in a sponge. The shape is anything but spherical, and it is found in what looks to be kaolin.