A rare and intriguing example of sandstone known as a Gogotte, was generously donated to the Museum recently by Daniel Eskenazi and family in honour of Sir David Attenborough’s 90th birthday.
Read on to find out more about how it formed, why we were presented it, why it is important and how we are using behind the scenes facilities to study it.
What is it?
The Gogotte is a sandstone concretion, a sedimentary rock consisting of quartz grains cemented together to form a hard rock.
How did it form?
This Gogotte formed within a rock that was deposited about 30 million years ago in northern France, when the area was covered by sand dunes leaving a thick deposit of pure sand. These sands have been mined to make quality glass.
Its contours and creases are the result of much later geological processes where water, rich in silica, gradually filtered through the loose, pure sand. The silica then cemented the sand together to form the Gogotte’s fluid lines.
How and why did we get it?
This was a very generous donation to the Museum by Daniel Eskenazi and family in honour of Sir David Attenborough’s 90th birthday. Eskenazi is a London dealer in Chinese art and artifacts. The Gogotte is now on permanent display in the Lasting Impressions gallery of the Museum.
Why are they important?
Today, Gogottes are prized as rare pieces of natural art – each one possessing its own unique and unusual shape. Because the layers within which they formed have been quarried away, they are extremely rare and are sometimes sold for high prices at auctions.
How are we studying it?
To mount the specimen, two cores were drilled in the base and we have been able to use these to carry out some preliminary investigations.
A tiny fragment of < 0.10g was scratched off a core sample and powdered in an agate mortar for analysis in an X-ray diffraction machine (XRD).
XRD is an analytical technique primarily used to identify the mineral composition of rocks.
The diffraction pattern acquired from the sample powder, shows us that the Gogotte is made almost exclusively of quartz. Some have suggested that Gogottes are made of chalk (calcium carbonate).
We also used one of our Scanning Electron Microscopes (SEMs). This microscope produces images of a sample by scanning the surface with a focused beam of electrons. The microscope measures the energy released as the electrons hit the sample. This allows us investigate the sample’s composition and produce a map of the element distribution on the rock surface.
The element distribution maps show that the sample is mainly crystals of quartz (in blue in the image above) confirming the results of the XRD. The maps also show small grains of limestone made of calcium and magnesium (in yellow and green respectively). It also shows that the rock is still slightly porous.
If you are in the area, why not come and see the gogotte in the Lasting Impressions gallery? If you are not able to visit then you can find results of further analysis on this new Curator of Petrology blog that will soon be written by Epi Vaccaro.
Fontainebleau sands are very famous for their gogottes and sand calcites ( Bellecroix). Have you any idea of the mineralogical composition of the silica cementing the grains of sand of a gogotte ? Would it be possible to explain the wonderful shape of the gogottes by the fact that gogottes would be “sand chalcedonia” ?
Hi Norbert.
Thank you for your interest in our blog. We are currently in the process of getting our labs to produce a thin section of one of the cores. This should tell us more about the silica composition and the history of cementation in the gogotte. We hope to publish another blog piece soon with more details of our analysis. There might be a bit of a wait though as we hope to carry out a CT scan too. In the meantime, we would suggest that the shape is more likely a result of the percolating groundwater than the composition of the gogotte.
All the best, Giles and Epi
Hi,
Thank you for your answer !
Almost the same process is involved for the genesis of Calcites of Bellecroix but with calcium carbonate instead of silica. It’s obvious than calcites of Bellecroix are calcite crystals but for gogottes the question is not answered at the moment. The results of your analysis will be very interesting !
Best regards
NJ