What is a "gogotte" in geology ?

Gogotte : definition

In geology, a gogotte is a siliceous concretion, often sandstone, with harmoniously rounded shapes. It is mostly grains of sand cemented in place by silica deposited by water circulation within a sedimentary layer. Chemically speaking, a gogotte is made almost entirely of silica and therefore has roughly the same composition as quartz.

Gogottes appear very sporadically, sometimes in isolation but most often as horizontal slabs made up of interconnected gogottes. They are formed at the level of the piezometric surface (at the level of the water table) by dissolution and precipitation of silica, in the same way as opals.

Gogottes are most often white in color but are sometimes colored red or brown by iron oxides. On some breaks, it is not uncommon to observe alternations. Traces of manganese oxides can also be found in the form of fine black spots with a dendritic tendency. During their growth, they can exceptionally include elements present in the sedimentary layer : sandy calcite also called "Fontainebleau calcite" or "Bellecroix calcite", flints of all sizes or more rarely fossils.

Depending on the nature and grain size of what they encompass, their locality of extraction, the quantity and purity of silica they contain, several names are given to them : gogotte, menilite opal, fairy stone, etc... We find these formations in the sand pits near Fontainebleau (France), in the desert of Irhoud (Morocco), in Agramon (Spain), etc...

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